EEVblog #811 - How The Varta 15 Minute Battery Charger Works

[EEVblog]

A look inside the Varta 15 minute 8A NiMH fast charger.
A teardown and look at how it works and the circuit topology.
Can Sanyo Eneloops be charged this fast too?

Datasheets:
http://litel.com.pl/karty-produktow/Varta-AA-2100mah-56706-datasheet.pdf
http://www.din-tek.jp/Upload/Product%20Doc/Datasheet/DTM4410.pdf
http://www.ti.com/lit/ds/symlink/tl494.pdf

[IanB]

Charging fresh, new Eneloops at a 15 minute rate may work, although it probably stresses the batteries and shortens their life. It's something best kept for emergencies and maybe stopping after 10 minutes.

If you try to charge old, weary batteries like this they will pop and hiss. They can't recombine the charge gases fast enough to prevent over-pressuring and popping the seal.

[DanielS]

I had some Energized 15 minutes AA NiMH cells many years ago and they started showing significant degradation after maybe 50 cycles or so.

I ended up modding the charger so it would always be in AAA mode and not cause AA cells to become horribly hot.

[Yansi]

Hmmm. Isn't the silicon coefficient somwhere abotu 2.2mV/K instead of 10?  10 woul'd be too much I think... 

[nuno]

Maybe they're setting the DC-DC output current to a fixed value (I) then "PWM"ing the individual cells and displaying the average current (I x duty-cycle).

[AF6LJ]

One question I have.........
I wonder if the batteries are pulsed in charge mode.
Taking that a step further, could the batteries be receiving a charge at different times.
First one battery then the next and so on.....
This might also make measuring charg current easier since the micro controller knows which battery is being charged.
Time to bust out the scope and make some live measurements.   

[gorf]

What if the lower Fets are only shunts for missing batteries? Then the DC/DC can set the current for charge and discharge.

[retrolefty]

I assume that the datasheet C1 charge recommendation is based on heat rise during typical charging over some ambient temp range.

 So that fact that forced fan air cooling is being used most likely means that one can safely charge at higher 'C' rates, along with temperature sensing that must cut charge rate if some value temp is exceeded. So possibly the batteries are not being abused and will not cut lifetime charge cycle specs. Still charging at 4C is pretty cool.

 I know some of us R/C electric plane fans were charging older nicads at rates to get 15-20min charges, but boy would those cells get hot. 

[TheSteve]

4C charging is still for lightweights. As previously mentioned the RC crowd has been heavily abusing Nicad and NiMH cells for years. Back in the day I routinely charged Sanyo 1200 mAh cells at 10 amps. When we were feeling more adventurous we went as high as 40 amps. The ends of the cells would actually bubble out which increased the capacity of the cell. This came at a major sacrifice to the total # of cycles. The same charger would also discharge cells at up to 40 amps.
A key to this was that it used constant current - no PWM of any kind. If you want major punch from your batteries PWM sucks. Yes it had a decent heatsink on it - it had to!
The same device could also delivery pulses of current in the hundreds of amps at up to 99 volts to revive old cells or give battery packs a little more punch just before a race. Ahh the good day, had a few cells pop and fizzle but nothing like a modern day LiPo fire.

[Warhawk]

Hmmm. Isn't the silicon coefficient somwhere abotu 2.2mV/K instead of 10?  10 woul'd be too much I think... 

I also wanted to point it out. Rule of thumb is -2 mV / K.
A good number where to start.

[barnacle2k]

On reading the video title i thought the day had finally come you did a video on a topic you already covered. (As predicted on the Amp Hour)

Anyway what i wanted to comment on:

Sensefets do not contain a sense resistor.

Current senseFET technology depends on the close matching of transistor cells within the PowerMOS.
A TrenchMOS device comprises many thousands of transistor cells in parallel.
Elements within the device are identical and the DRAIN current is shared equally between them.
The more cells that are in parallel for a given MOSFET chip area, the lower its on-state resistance will be.
It is possible to isolate the SOURCE connections of several cells from those of the majority and bring them out onto a separate SENSE pin.
The PowerMOS can now be thought of as two transistors in parallel with a common GATE and DRAIN but separate SOURCE pins.
When the devices are turned on, the load current will be shared as a ratio of their on-state resistances.

[Helix70]

Although not the same class of device, I was inspired, and here is the inside of my Sanyo NC-MQR06W, with a miserly 560mAx4/1120mAx2/1680mAx1 (AA) charge rate.

I do like the isolation, and high quality soldering.

[hamdi.tn]

i don't understand the use of 1R resistor to link AAA battery terminal to AA battery terminal , of curse it doesn't matter but it will not be the first value to had in mind, why not a 0R. The only reason i see , is that those resistor can measure somehow the charge / discharge current for the AAA battery at least according to the dave's schematics.

i think the current shown on charge mode is just a saved number not really what it is, the fact that they are 8.000 and 4.000 is odd to me, but hey still can measure discharge current by linking the bottom shunt resistor to one particular battery using low side mosfet. one at a time, if done fast enough you will not notice.

[SteveyG]

The "tabs" on the AC adaptor are to meet the UK regulations on the minimum distance from the phase and neutral pins to the edge of the plug

[rgawron]

One can always keep an additional battery in his/her charger, so there's no need to wait until a battery is charged (not even 15 minutes). Charging speed is not an issue then.

[hamdi.tn]

There were several resistors in parallel so the actual resistance was much less than 1 Ohm.

yap am aware of that, still it's made to just link both terminal , it make more sens to use 0R

[Zbig]

One question I have.........
I wonder if the batteries are pulsed in charge mode.
Taking that a step further, could the batteries be receiving a charge at different times.
First one battery then the next and so on.....
This might also make measuring charg current easier since the micro controller knows which battery is being charged.
Time to bust out the scope and make some live measurements.   

Exactly that. The "-dV" detection method implies that anyway: you send a current for a fixed period of time down the cell, then you let go and measure the open circuit voltage. The moment the voltage actually gets lower than after the previous current spike indicates the full charge state fairly precisely for this type of chemistry. So the charger is "multiplexing" the cells. This also means that 8A is a mean current and that the actual instantaneous current is much higher probably.
Not true, as it turns out. IanB has explained it properly later (thanks).


EDIT:
The UI of this device looks awfully like the BC700, BC900 and their later derivatives (like NC-1000, etc.) that are quite popular these sides. That means it's most probably a rebranded unit with its roots pointing at some Chinese OEM somewhere.

[EEVblog]

Anyway what i wanted to comment on:
Sensefets do not contain a sense resistor.

Yes, I showed the equivalent circuit in the video.

[rr100]

I was multitasking during the video and I probably missed most of it ... from the instructions it seems that the charger has just "one speed" (i.e. you can't set lower currents)?!

I don't see any warning about using other cells - yes Dave mentioned that Eneloops might be working just as well but how does it know what cells it has?

[EEVblog]

I was multitasking during the video and I probably missed most of it ... from the instructions it seems that the charger has just "one speed" (i.e. you can't set lower currents)?!
I don't see any warning about using other cells - yes Dave mentioned that Eneloops might be working just as well but how does it know what cells it has?

Correct, and it doesn't know.
Yet no warnings about other brands etc.

[mux]

All questions you have in this video can be answered pretty easily, but probably not very satisfactorily.

All NiMHs (like has been said before already) can be abused a whole lot. They don't have nearly as low of a thermal runaway temperature (and have much less hotspotting) than lithium chemistries. I think the thermal runaway temp is like 175-200C? It's really high. They use these things on satellites for that reason. As a general rule, the failure mode of these cells at high current is dendrite formation (internal short-circuiting), which scales roughly as the square of the current. So 1500 cycles at 2A means 100ish cycles at 8A. Although I've seen RC enthusiasts pump 20A into cells and have them survive 25-50 cycles, so YMMV.

That being said: consumers rarely use their batteries for more than 50 cycles. Especially stuff like eneloops; it gets used maybe 5 or 10 times, then it falls between the pillows of the couch to be forgotten.

As for sensing; they're obviously only doing FET sensing, and very roughly so. FETs aren't good resistive elements for many reasons, especially when used in a switched mode like they are. The only reasonably accurate current sensing that is going on is on charging. Current sensing is very expensive so the trick in these low-cost devices is always to just do as little as possible of it. So that's why you're not getting the mAh estimate after a discharge, only after a charge.
 
Just some perspective on this machine: at full power, presumably it's drawing about 40W from the wall to charge 2 2100mAh 1.2V (i.e. ~2.5Wh) cells up to 70%. In that time it consumes 10Wh to get a total of 3.5Wh into batteries. That's pretty shitty, no wonder it's go to vent all that excess energy!

[Zbig]

So that's why you're not getting the mAh estimate after a discharge, only after a charge.

You do, actually. That's how the "Test" function works.

[gorf]

You don't need to have independent current measurement during discharge. Use the same current measurement that you used during charge. Regulate the DC/DC so that the voltage on the shunt resistor drop is +ve during charge and -ve during discharge. ie the dc/dc will output 1.5ish V/cell out during charge and 0.7ish V/cell during discharge. Current flows both ways through a FET when it is on.  Probe the Fet gates to check which ones are on during charge and discharge.

[mikerj]

If you have four cells in series being charged, then you're going to need somewhat over 6v (w.r.t ground) to switch the top MOSFET on, so presumably there is some kind of gate driver circuit rather than the gates being driven direct from the micro?

Do the voltage sense connections go into diff-amps or just to potential dividers with the diff operation performed in software?

I also assume the series resistors for the AAAs only indirectly limit the current, e.g. the micro regulates the current to ensure some maximum cell voltage isn't exceeded, and the resistors increase the apparent cell voltage of AAAs during a charge cycle.

[AF6LJ]

I have a little different take on what is going on in the charger.
Only one cell is being charged at any given time.
They are multiplexed.

[hamdi.tn]

I have a little different take on what is going on in the charger.
Only one cell is being charged at any given time.
They are multiplexed.

+1

[mariush]

Bad user interface, those two buttons were just confusing to me. Lack of configuration for charge current is another fail for me. Could have added a third button and allow configuring current for each battery.

Would have loved to see the insides of the AC adapter.. is it a proper quality one that can actually do 45w or it's just something that overheats if used a lot (and they're relying on duty cycle,  people only using it for 15 minute periods and then idling for long times),.. how about temperatures inside that ac adapter when it outputs that current...

[IanB]

Exactly that. The "-dV" detection method implies that anyway: you send a current for a fixed period of time down the cell, then you let go and measure the open circuit voltage.

No! Not that. To do -dV detection reliably you must measure the voltage while the charging current is still being applied. The -dV signal is actually measuring the drop in internal resistance of the cell that happens when the cell starts to warm up at full charge. Internal resistance is only successfully measured by examining the voltage change caused by an applied current. It cannot be measured open circuit.

[andiz]

When I look at the wide-spread availability of the modern low self discharge cells, those crazy fast chargers are nothing else than pure marketing bullsh*t.
You pay a premium for those super-duper fast-chargers and all they are doing is ruining your cells by toasting them. It is a way better practice (and cheaper) to have a spare number of Eneloops or equivalent good quality cells to exchange the discharged cells in whatever device, and use a low-speed charger to recharge those discharged cells time controlled at a rate of 1/10C for about 12-14h.

What NiMHs don't like at all ist heat and overcharging (combined it's even worse). They will build up higher inner resistance because of separator and electrolyte damage, which leads to even higher heat losses (very practical for high-speed charging, isn't it? ). There are cells on the market, which can't even cope with 1C charge rate for long until they are building up additional inner resistance (after about only 20-50 cylces at 0,5-1C charge rate). This higher inner resistance makes those -dV controlled chargers pretty useless, because they only work (more or less reliable) with new cells (but not for the first 1-3 cycles, where they are likely to get hopelessly overcharged, because the chargers won't find the cut-off point). Ironically most -dV chargers claim to not overcharge, but without overcharging they wouldn't even work.

Actively cooling the cells during charging (like Varta) does not make anything better. First, because the inner heat of the cells will be way higher than on the outside and second, the active cooling will distort the detection of temperature- and voltage-signals.

[dr.diesel]

Not to mention that 15 minutes isn't fast enough to sit/stand and wait for it either, so why boil them? 

[Zbig]

Exactly that. The "-dV" detection method implies that anyway: you send a current for a fixed period of time down the cell, then you let go and measure the open circuit voltage.

No! Not that. To do -dV detection reliably you must measure the voltage while the charging current is still being applied. The -dV signal is actually measuring the drop in internal resistance of the cell that happens when the cell starts to warm up at full charge. Internal resistance is only successfully measured by examining the voltage change caused by an applied current. It cannot be measured open circuit.

You're right, I was wrong there. Anyway, I'm still pretty sure some multiplexing action is happening there. I have different (but I doubt radically different in terms of internal architecture) charger and this switching has apparent audible effects.

[andiz]

No! Not that. To do -dV detection reliably you must measure the voltage while the charging current is still being applied. The -dV signal is actually measuring the drop in internal resistance of the cell that happens when the cell starts to warm up at full charge. Internal resistance is only successfully measured by examining the voltage change caused by an applied current. It cannot be measured open circuit.

Sorry to say that, but this is not correct.
Measuring the voltage under load is a really bad technique (unless the cells are being soldered into a circiut), because of varying contact resistance between the battery terminals that can lead to false voltage readings of more than 10 mV and this is more than the detection threshold of the -dV cut-off. This is the reason, why commercially available charging controllers are measuring the open circuit voltage for -dV detection.
NiMH or NiCd cells have a negative temperature coefficient (because of the dropping of differential resistance) driven by heat an pressure, that reduces the open circuit voltage of the cells as well.

If you want to measure the inner resistance of a cell in order to calculate the charging termination point, you have to measure the difference between open circuit and load voltage. But also this technique calls for a proper connection eg. cells being soldered into a circuit.

[mikerj]

I have a little different take on what is going on in the charger.
Only one cell is being charged at any given time.
They are multiplexed.

That makes some sense I guess, the gate and cell sense voltages would always be referenced to ground then (albeit via a variable number of MOSFET RDSon drops).

However, that means the peak charging current would need to be 16 amps to get an 8 amp average with two cells (and a 4 amp average with 4 cells).  The switcher doesn't really look capable of delivering that kind of current, even at a low voltage.  What are the MOSFETs rated at?

[IanB]

Sorry to say that, but this is not correct.
Measuring the voltage under load is a really bad technique (unless the cells are being soldered into a circiut), because of varying contact resistance between the battery terminals that can lead to false voltage readings of more than 10 mV and this is more than the detection threshold of the -dV cut-off. This is the reason, why commercially available charging controllers are measuring the open circuit voltage for -dV detection.
NiMH or NiCd cells have a negative temperature coefficient (because of the dropping of differential resistance) driven by heat an pressure, that reduces the open circuit voltage of the cells as well.

If you want to measure the inner resistance of a cell in order to calculate the charging termination point, you have to measure the difference between open circuit and load voltage. But also this technique calls for a proper connection eg. cells being soldered into a circuit.

I agree the contact resistance at the cell terminals can be a source of problems, but since the charger is left unattended while operating whatever contact resistance exists is likely to remain the same unless anyone touches it. (I always spin the cells in the slots before charging to remove any oxide layer and create a good contact.)

The -dV under charging current is unfortunately how the electro-chemistry works. We are stuck with it if we want to detect -dV reliably.

As soon as the charging current is disconnected the cell terminal voltage decays in an asymptotic fashion towards some lower stable voltage, but it takes a few seconds to stabilize. So if you are doing pulsed charging and you try to measure the voltage in the gaps between pulses you are aiming at a moving target.

Maha had this problem in their design of the C9000 charger. The first version to market tried to do -dV detection in the gaps between charging pulses and it became heavily criticized for missed terminations and overcharging. In later revisions Maha modified the firmware with an additional termination condition upon reaching an open circuit voltage of approximately 1.47 V. This terminates modern cells like Eneloops reliably, but leaves them at slightly less than a full charge.

Of course, if you try to measure the voltage while the charge current is being applied, you not only have contact resistance to deal with, but also noise signals from the switching converter that supplies the charging current.

Good modern chargers such as the ones sold by Panasonic for Eneloops seem to have very sensitive and sophisticated end of charge detection algorithms. They appear highly reliable. I have seen any number of other brand chargers that are were desperately unreliable and could not be trusted at all.

[AF6LJ]

I have a little different take on what is going on in the charger.
Only one cell is being charged at any given time.
They are multiplexed.

That makes some sense I guess, the gate and cell sense voltages would always be referenced to ground then (albeit via a variable number of MOSFET RDSon drops).

Yes it would be variable, however I argue it would be small. +/- 20% is no big deal in this equation.

However, that means the peak charging current would need to be 16 amps to get an 8 amp average with two cells (and a 4 amp average with 4 cells).  The switcher doesn't really look capable of delivering that kind of current, even at a low voltage.  What are the MOSFETs rated at?

Not if we are charging one battery at a time with eight amp pulses.
And one other thing; nobody said the eight amps was an average, continuous or peak.
As I said I see the charger multiplexing the charging, only charging one battery at a time by doing some switching. There certainly is enough MOSFETs to do the switching.
I would love to see Dave get in there with a four trace scope and and see what sequence those MOSFETs are being switched in.

[drussell]

And one other thing; nobody said the eight amps was an average, continuous or peak.
As I said I see the charger multiplexing the charging, only charging one battery at a time by doing some switching. There certainly is enough MOSFETs to do the switching.
I would love to see Dave get in there with a four trace scope and and see what sequence those MOSFETs are being switched in.

It is quite possibly charging up to two cells at a time, hence the reason it switches down to 4A max average with more than two cells inserted.  It must be averaging right around that 8A per cell to be able to get anywhere close to full charge on a 2Ah cell in 1/4 hour, though, regardless of whether it is a higher peak current being pulsed or a continuous 8A.

[AF6LJ]

And one other thing; nobody said the eight amps was an average, continuous or peak.
As I said I see the charger multiplexing the charging, only charging one battery at a time by doing some switching. There certainly is enough MOSFETs to do the switching.
I would love to see Dave get in there with a four trace scope and and see what sequence those MOSFETs are being switched in.

It is quite possibly charging up to two cells at a time, hence the reason it switches down to 4A max average with more than two cells inserted.  It must be averaging right around that 8A per cell to be able to get anywhere close to full charge on a 2Ah cell in 1/4 hour, though, regardless of whether it is a higher peak current being pulsed or a continuous 8A.

It could very well be doing two at a time, that would explain why the current is halved when the third and fourth cells are installed.
 
If I had that on my bench I would be so all over it with my scope to see what is actually going on.

[EEVblog]

No! Not that. To do -dV detection reliably you must measure the voltage while the charging current is still being applied. The -dV signal is actually measuring the drop in internal resistance of the cell that happens when the cell starts to warm up at full charge. Internal resistance is only successfully measured by examining the voltage change caused by an applied current. It cannot be measured open circuit.

Sorry to say that, but this is not correct.

IanB is in fact correct.
http://na.industrial.panasonic.com/sites/default/pidsa/files/panasonic_nimh_chargemethods.pdf
The voltage drops 5-10mV per cell when full charge is reached.

[mikerj]

However, that means the peak charging current would need to be 16 amps to get an 8 amp average with two cells (and a 4 amp average with 4 cells).  The switcher doesn't really look capable of delivering that kind of current, even at a low voltage.  What are the MOSFETs rated at?

Not if we are charging one battery at a time with eight amp pulses.
And one other thing; nobody said the eight amps was an average, continuous or peak.

8 amps multiplexed evenly between two cells is a maximum average of 4 amps each.  4 amps * 15 minutes = 1 ampere/hour, so you couldn't possibly charge a 2100mAh cell to 75% in 15 minutes.

[mikerj]

IanB is in fact correct.
http://na.industrial.panasonic.com/sites/default/pidsa/files/panasonic_nimh_chargemethods.pdf
The voltage drops 5-10mV per cell when full charge is reached.

I always understood the dv/dt was performed under load, but I noticed that my cheap Imax B6 hobby charger periodically stops charging on the NiMh/NiCd profile, presumably to sample the open circuit battery voltage.

[uwezi]

They are multiplexed.

That makes some sense I guess, the gate and cell sense voltages would always be referenced to ground then (albeit via a variable number of MOSFET RDSon drops).

However, that means the peak charging current would need to be 16 amps to get an 8 amp average with two cells (and a 4 amp average with 4 cells).  The switcher doesn't really look capable of delivering that kind of current, even at a low voltage.  What are the MOSFETs rated at?

If you look at the coil you will see that it has two parallel windings (at 17:32 into the video) and also given the quite big core I would assume that it could possibly be rated for 16+ amps. And the MOSFETs would be up to the job with up to 18A continuous and 50A pulsed drain current.

It would also possibly explain the weird use of the upper n-MOSFETs: instead of switching off these (which is not really possible, because the body diode would conduct) you turn on the lower n-MOSFET and bypass the battery in that slot. After all a constant current source would not like to be disconnected anyway...

And the discharge goes the same way, pulsed through the shunt resistor at the bottom of the chain, with all the measurement possibilities you need.

However, this also means that the batteries are exposed to pulsed currents of 8C instead of dc at 4C. But didn't I recently read something about a quick-charge method which used pulsed currents in order to optimize charge speed and battery health?

[Groucho2005]

Maha had this problem in their design of the C9000 charger. The first version to market tried to do -dV detection in the gaps between charging pulses and it became heavily criticized for missed terminations and overcharging. In later revisions Maha modified the firmware with an additional termination condition upon reaching an open circuit voltage of approximately 1.47 V. This terminates modern cells like Eneloops reliably, but leaves them at slightly less than a full charge.

I have the Maha (Powerex) C9000 and use exclusively Eneloops. It's the best charger I have used. The voltage termination and following trickle charge do charge the cells to almost 100% of their capacity. Besides, -dV algorithms don't work reliably at low charge currents and/or older cells.
Check out this excellent review of the C9000:
http://lygte-info.dk/review/Review%20Charger%20Powerex%20MH-C9000%20UK.html

[IanB]

I always understood the dv/dt was performed under load, but I noticed that my cheap Imax B6 hobby charger periodically stops charging on the NiMh/NiCd profile, presumably to sample the open circuit battery voltage.

If you measure the open circuit voltage immediately after stopping the charging current you get a residual "echo" of the charging voltage until it decays away. Although not as strong an indicator as the actual voltage under load it can still be used as a means to detect the -dV signal. Some chargers evidently are designed this way.

[nuno]

1/2 cells @ 8A or 3/4 cells @ 4A may have something to do with power, the power the DC-DC can provide. Although, if it were the case I guess they could do better, but there can be a limit in some other place like PCB current capacity and so on.

[BravoV]

I have the Maha (Powerex) C9000 and use exclusively Eneloops. It's the best charger I have used. The voltage termination and following trickle charge do charge the cells to almost 100% of their capacity. Besides, -dV algorithms don't work reliably at low charge currents and/or older cells.
Check out this excellent review of the C9000:
http://lygte-info.dk/review/Review%20Charger%20Powerex%20MH-C9000%20UK.html

Same here for C9000, have been using it for years, and my 1st gen eneloops are still performing very well, I'm guessing it has to do with they're not fully charged all this time. No complain here with C9000 + Eneloop combination. 

Just curious if there is better charger out there especially for Eneloop cells ? C9000 is quite old now.

[Skimask]

Could the DAVE CAD circuit show in the video be modified to charge a bank of 4 batteries, albeit one at a time, using an input source which would normally only be able to charge one battery, and at the same time, be able to draw a load from the entire pack?
For instance, 4 NiMH cells, Vout about 4.8v on average, an input V of only 3.3v, battery 1 gets a bit of a charge, switch, battery 2 gets a bit of a charge, switch, battery 3 gets a bit of a charge...and so on.  Of course there would have to be a boost circuit in there somewhere to get Vgs high enough to kick the FET gates.

[ez24]

Check out this excellent review of the C9000:
http://lygte-info.dk/review/Review%20Charger%20Powerex%20MH-C9000%20UK.html

lygte-info is a member here - but I forgot his forum name.

Lygte  are you in on this topic?

[HKJ]

I am reading here.

All the chargers I have tested looks to be doing -dv/dt detection with current off or use voltage termination. Besides connection resistance another reason to do it with current off is noise from the switching circuit, the filtering requirements are much less when measuring with current off..

My guess is that the charger is fixed 8A and it will timeshare the charging current with 3 or 4 batteries in the charger.

[apis]

Whenever reading about quick charging NiMHs they recommend dT/dt termination and using max-voltage, max-temperature and max-duration for backup since detecting dV/dt is going to be difficult.

Charging them in series shouldn't be a problem as long as you can detect when individual cells are fully charged and disconnect them, but I suppose you could pulse charge them one by one as well.

For detecting discharge current, could they somehow be grounding the positive battery terminals one by one and thereby measure current via the shunt resistor?

Don't really understand how it detects if it's an AA or AAA battery though, there must be more to it and all the 1 ohm resistors must have some purpose, but the only reason I can think of is current sensing?

This thing needs more probing.

[drussell]

I would love to see Dave get in there with a four trace scope and and see what sequence those MOSFETs are being switched in.

Oh, I forgot to mention....

I'd love to see a four trace scope of the four + battery terminals during a charge cycle.  Even just looking at the voltages there relative to each other would tell us pretty much exactly what it's doing with the charge profiles.  Current to each cell and investigating how the mosfets are being switched would be interesting also but even just seeing those voltage curves would be very revealing.

[AF6LJ]

I would love to see Dave get in there with a four trace scope and and see what sequence those MOSFETs are being switched in.

Oh, I forgot to mention....

I'd love to see a four trace scope of the four + battery terminals during a charge cycle.  Even just looking at the voltages there relative to each other would tell us pretty much exactly what it's doing with the charge profiles.  Current to each cell and investigating how the mosfets are being switched would be interesting also but even just seeing those voltage curves would be very revealing.

Agreed. 

[HKJ]

Whenever reading about quick charging NiMHs they recommend dT/dt termination and using max-voltage, max-temperature and max-duration for backup since detecting dV/dt is going to be difficult.

That charger probably uses all the methods. -dv/dt is easy at 8 amps.

Don't really understand how it detects if it's an AA or AAA battery though, there must be more to it and all the 1 ohm resistors must have some purpose, but the only reason I can think of is current sensing?

My guess is that they measures the voltage on both sides of the resistors (Maybe measuring the switcher output voltage). To do that they may use the mosfets to switch connections around, but with only a few mOhm in each they will not affect the result significantly.

[tszaboo]

Actually it would be fairly simple to test any battery lifetime with 4C charging. All you need is a good battery tester, set up 4C charging, dv/dt or even cc/cv to fixed voltage which would take about 20-30 minutes and discharge with 1C/2C rate. Measure the coulumbic efficiency, and in an hour you get the expected lifetime.
Or even 200 cycles would be finished in a week and a half. I would do it myself and post results if there wouldnt be that bloody NDA.

[IanB]

Actually it would be fairly simple to test any battery lifetime with 4C charging. All you need is a good battery tester, set up 4C charging, dv/dt or even cc/cv to fixed voltage which would take about 20-30 minutes and discharge with 1C/2C rate. Measure the coulumbic efficiency, and in an hour you get the expected lifetime.
Or even 200 cycles would be finished in a week and a half. I would do it myself and post results if there wouldnt be that bloody NDA.

I recall that user SilverFox on CandlePowerForums tested some Eneloops on a 15 minute charger. He put them through continuous charge discharge cycles. They were still holding up quite well after 100 cycles, but they lost some capacity and also lost some of their low self discharge properties.

I can't seem to find the thread right now to link to it.

[drussell]

They were still holding up quite well after 100 cycles, but they lost some capacity and also lost some of their low self discharge properties.

The loss of low-self-discharge (increase in internal leakage current) is by far the worst result of any kind of fast charging of most types of rechargeables.  Even 1h chargers will severely increase the self-discharge compared to being trickle charged over the cycle lifetime.  I have lots of batteries here that still hold quite a good charge after 10+ years and hundreds and hundreds of cycles but they're totally dead after just a few days off the charger.  That might be a useable situation for a power tool but not so good if you have cells you want to use in a remote control or something. 

Best to keep some cells marked as trickle-charge only for low-drain, long term, low-cycle use and blast the crap out of your other ones, your older ones, etc., whatever, for your heavy repeated charge/discharge use and beat 'em until they just won't charge anymore...  lol

[apis]

Whenever reading about quick charging NiMHs they recommend dT/dt termination and using max-voltage, max-temperature and max-duration for backup since detecting dV/dt is going to be difficult.

That charger probably uses all the methods. -dv/dt is easy at 8 amps.

Yes you're right, the reson charging in the .1C to .5C region isn't recomended is because even dT/dt detection becomes unreliable, but at 4C it should be much easier.

[tombi]

This come up on Dangerous Prototypes recently - here is an appnote from Vishay on dT/dt sensing for fast charging.
http://www.vishay.com/docs/29089/fastappl.pdf

Not clear how much current they are pushing.

[Stonent]

I would think at some point, all this heat is breaking down the chemicals inside.  The external temperature may be quite different than what's going on in the center.

[VK3DRB]

To measure each battery discharge current, the micro might be multiplexing the FETs to measure the current from each battery bay one at a time periodically, and using the single sense resistor. Same for charging. Most the duty cycle is not measuring but actual charging and discharging.

Comments about the board...

1. In general, 45 degree micro is also a good idea for hot air and vapour phase reflow. The reason is the paste screening is generally a lot more uniform. You get less chance of shadowing and therefore less chance of dry joints.

2. No pin one on the micro and the SOIC. Crap.

3. Could have done better in the component designators. Not what I would call artwork. Also some white ink labels going over vias. Sloppy work.

4. Breaks in the high current tracks were a good idea. I have not tried this myself but I assume this tends to prevent solder voids at one end of long tracks due to surface tension during a wave soldering.

5. They used a crappy sleeve fan (probably bronze bushes) that dries out and then wears out, creating a very annoying whirring sound and eventually seizing. So it is clearly not a quality product.

[EEVblog]

Another video uploading now.

[VK3DRB]

As I mentioned, the fan in the charger is crappy sleeve type.

Good products use sealed ball bearing fans. Expensive graphics cards for gamers and CAD gurus and cheapo power supplies often also have fans with cheap bushes. First thing that fails is the fan. You can oil the bearings with sewing machine oil, but that only delays the inevitable... finding a replacement fan.

I own an expensive Pioneer 7.1 channel audio amplifier. Pioneer used a cheapo fan rather than a ball bearing fan. I have replaced the fan twice in 5 years and it is a unique form factor in a tight spot so I have to get the same rubbishy fan each time at never-to-be-repeated prices. How was I supposed to know Pioneers would put a crappy fan in? The product was not old enough when I bought it so I guess there were no failures in the field yet. Sales people won't tell you the product has a crap fan in it. In some stores you have to buy the device sight unseen (cannot take it out of the box.) The only solution is "once bitten, twice shy". That is, never buy Pioneer again. Stores will not allow you to tear down a product to look at its fan. I would not have bought the amplifier if I have known the trouble I would have with its fan. You'd think Pioneer being a Japanese company would know better.

Manufacturers should use quality fans like Panasonic or NMB DC brushless ball bearing fans, and avoid Hu Flung Dung brand fans if they want to build trust in the market place.

It is false economy using cheap and nasty fans - you lose customers. 

[bktemp]

As I mentioned, the fan in the charger is crappy sleeve type.

But does it matter? The typical lifetime of a sleeve bearing type fan depends on its temperature. You can find values in the range of several 10000 hours.
Lets assume the fan fails after 10000 hours. With one charge cycle every 30 minutes you can charge 80000 batteries. Most likely something else will fail before the bearing of the fan. If you charge one set of batteries per day the fan will last 50 years. I doubt many people charge that many batteries in their life.

A pc or some other device running 24/7 is a different story, but in rarely used charger even the cheapest bearing will last long enough.

[Groucho2005]

With one charge cycle every 30 minutes you can charge 80000 batteries. Most likely something else will fail before the bearing of the fan.

Yes, at 8 amps charge current probably all 80000 batteries. 

[AF6LJ]

Another video uploading now.

Good Something to watch while the test equipment warms up this afternoon...

[VK3DRB]

As I mentioned, the fan in the charger is crappy sleeve type.

But does it matter? The typical lifetime of a sleeve bearing type fan depends on its temperature. You can find values in the range of several 10000 hours.
Lets assume the fan fails after 10000 hours. With one charge cycle every 30 minutes you can charge 80000 batteries. Most likely something else will fail before the bearing of the fan. If you charge one set of batteries per day the fan will last 50 years. I doubt many people charge that many batteries in their life.

A pc or some other device running 24/7 is a different story, but in rarely used charger even the cheapest bearing will last long enough.

Your 50 years life from a cheap phosphor bronze bushing is far too optimistic. The lubricant would dry out way before then and start vibrating or seize. Maybe try 5 years, irrespective of the manufacturers' claims. I mostly use my Pioneer amp with Sennheiser wireless headphones, so the internal temperature is low, but the fans failed after only a few hundred hours of use over 2 or 3 years.

There are re-chargers too that get a heck of a lot of use, like in concert halls, churches etc where band mics have their batteries recharged and swapped out regularly. Whilst the 15 minute recharge time is impressive, the recharge life of the cells will be adversely affected compared to the recommended charge rates by reputable manufacturers.

I use a La Crosse BC-900 for recharging my genuine Sanyo Eneloops, all which I imported from a reputable dealer in the USA. The charger is excellent and no fan is required (slow recharging time). After 6 years and hundreds of recharges, the Eneloops are still going strong. Best batteries around in my opinion.

There are fake Sanyo Eneloops made where else but in "copywatch" China and sold on eBay. https://thecounterfeitreport.com/product/524/Eneloop-AA-Batteries.html

[rr100]

Good Something to watch while the test equipment warms up this afternoon...

Not so fast, still traveling through the intertubes I guess...

I wonder how well it does different size (mix of AA and AAA). I ordered one myself for the heck of it, the old BC-900 was starting to have trouble with the LCD (tried to clean it up but seems to be a losing battle). Plus I've a feeling that it fails to terminate charging in time many times.

[AF6LJ]

Good Something to watch while the test equipment warms up this afternoon...

Not so fast, still traveling through the intertubes I guess...

I wonder how well it does different size (mix of AA and AAA). I ordered one myself for the heck of it, the old BC-900 was starting to have trouble with the LCD (tried to clean it up but seems to be a losing battle). Plus I've a feeling that it fails to terminate charging in time many times.

This will be interesting to find out how well it works with mixed cells.

A search online reveals a hand full of 15minute wonders.
I'm not in a big hurry to charge batteries so something like this wouldn't appeal to me.
What does appeal to me is how they go about doing it.

[felixd]

Dave I was wondering while watching 2nd part: Why not to use FLIR to track current flow for discharge and charge, test modes ?
Wouldn't that help to reverse engineer the process? BTW that would also look freaking GOOD to watch how this thing heats up!

I hold my fingers for that shot. Please make 3rd part with thermal images !

[EEVblog]

Dave I was wondering while watching 2nd part: Why not to use FLIR to track current flow for discharge and charge, test modes ?
Wouldn't that help to reverse engineer the process? BTW that would also look freaking GOOD to watch how this thing heats up!

I would have done that, but the iProber is a more interesting tool I haven't shown much.

[oaliey]

When can we expect a teardown of that multimeter?

[oaliey]

If you meant the current probe, Mike had done that.

I am referring to the Brymen BM235. The interwebs is suspiciously lacking any information on this meter.

[SeanB]

Coming soon to an online store familiar to you........

[Svuppe]

That charger probably uses all the methods. -dv/dt is easy at 8 amps.

I found the datasheet of the charger:
http://www.produktinfo.conrad.com/datenblaetter/1100000-1199999/001172036-da-01-en-VARTA_LADER_LCD_ULTRA_FAST_4X_2400MAH_AA.pdf
According to that, the temperature is only used as a backup safety cutoff, when the temperature exceeds 75 degrees C. It doesn't mention dT/dt at all.

I am also curious as to how AAA batteries are handled. If the switchmode is a constant current source, then all those resistors in series with the AAA cells won't make a difference. I may have a look at that myself, as I have the same charger coming in the mail one of these days. I have had the old 15 minute charger, but it is about to give up now (screaming fan, that sometimes won't start until I smack it a couple of times). I didn't know Varta had made a new version until I saw the video.

[apis]

That charger probably uses all the methods. -dv/dt is easy at 8 amps.

I found the datasheet of the charger:
http://www.produktinfo.conrad.com/datenblaetter/1100000-1199999/001172036-da-01-en-VARTA_LADER_LCD_ULTRA_FAST_4X_2400MAH_AA.pdf
According to that, the temperature is only used as a backup safety cutoff, when the temperature exceeds 75 degrees C. It doesn't mention dT/dt at all.

I am also curious as to how AAA batteries are handled. If the switchmode is a constant current source, then all those resistors in series with the AAA cells won't make a difference. I may have a look at that myself, as I have the same charger coming in the mail one of these days. I have had the old 15 minute charger, but it is about to give up now (screaming fan, that sometimes won't start until I smack it a couple of times). I didn't know Varta had made a new version until I saw the video.

Hmm, yeah, maybe dT/dt doesn't work when they use a fan (or maybe it would have a cost a cent extra) and, as HKJ pointed out, at 8 ampere detecting dV/dt should be easy. The battery manufacturers datasheets that Iv'e read usually recommend dT/dt though (and all the backups) as a more reliable method when quick-charging (at 0.8C). But I don't have practical experience, it's just what I've read in datasheets and guides.

I also don't get the purpose of the 1 omh resistors, I assume they are used when detecting if it's an AA or AAA battery inserted but I don't really see how. Maybe they measure voltage drop across to see if there is current flowing through the AAA slot or not? Limiting current to the specified 3 Amps for AAA would proabably be done by limiting the duty cycle like it does when charging more than 2 AAs.

[SeanB]

The resistors on the AAA connectors are likely there for the discharge only, extra resistance so the AAA cell has a lower discharge current. On charge they add an extra voltage and power loss, so the unit likely reduces the constant current for the AAA cell charge, unless they allow you to mix cell sizes in charging. they have to reduce current for AAA as otherwise those resistors will have to dissipate 21W and will cook even with fan cooling.

[AF6LJ]

Thanks Dave; the video answers some of the questions I had.

[apis]

The resistors on the AAA connectors are likely there for the discharge only, extra resistance so the AAA cell has a lower discharge current. On charge they add an extra voltage and power loss, so the unit likely reduces the constant current for the AAA cell charge, unless they allow you to mix cell sizes in charging. they have to reduce current for AAA as otherwise those resistors will have to dissipate 21W and will cook even with fan cooling.

But when charging it's a constant current source so if you add resistance it will just increase the voltage and force the same current through the battery anyway, extra resistance will only waste energy/generate heat. On discharge the normal resistance looks like 3 ohm, so the extra 1/6 ohm (if I remember the number of resistors correctly) wouldn't make much of a difference: 0.38 A instead of 0.40 A. AAAs should be able to handle .4 A discharge anyway. Or am I missing something?

[bktemp]

Maybe the 1/6 ohm resistor is only used to detect AAA batteries and then the multiplex ratio or the current is reduced?

[twice11]

In the new video (you don't seem to have a thread for EEVblog #812 yet), you complain about the discrepancy for the battery voltage at 8A (or so) charge. I guess the voltage displayed by the charger is the no-load voltage in the short charging break, which is (of course) lower than the average voltage measured with the multimeter.

[HKJ]

It looks like Dave hit a trap for young players with the voltage.

My guess is that the charger will select lower current with AAA batteries and not only do it with pwm (This is the most common behaviour).

With the temperature I obvious gave to much credit to the charger, the maximum temperature is useful for finding old batteries and detecting low airflow.

Maybe it could be fun testing the charger to see how well it does.

[rr100]

I have the charger.

Mixing AA and AAA is handled like having AAA across the board, 3000 or 1500 mA depinding on the number of cells.

Funny thing is that I bought it thinking to go through my huge pile of AAs throwing out what's really bad and keeping what's still decent. Now both the good and bad is that it shows Err on ALL the cells I knew are bad. You know, those that you take fresh off the charger to use with a remote control and they die next day or week. Incidentally the ones that are left are ALL low-self-discharge (and I think except for the Varta's some kind of rebranded Eneloops, be it from Panasonic or Fujitsu). And are much newer because at some point I switched from high capacity cells (2500-2700 mAh) to LSDs because the higher capacity ones were dying on me all the time.

Frankly I don't use so much AAs anymore and I don't like that you can't set the current but for the rest of the people in the house is just what's needed: just drop the batteries in and take them out when the fan cuts off. In most cases even a two minutes charge would get you through the day.

[rr100]

One more thing: about the life of the fan - I think it is more or less a race to the bottom, to see what gives out first - the fan or the battery contacts. I wouldn't say they're particularly flimsy but unless I get much better at this (and quickly) I think we'll have a problem here.

[K6TR]

Dave I was wondering while watching 2nd part: Why not to use FLIR to track current flow for discharge and charge, test modes ?
Wouldn't that help to reverse engineer the process? BTW that would also look freaking GOOD to watch how this thing heats up!

I would have done that, but the iProber is a more interesting tool I haven't shown much.

Thanks Dave....I hate it when I have to cut tracks to do troubleshooting. But sometimes its the only way. The IProber provides a very useful alternative.

[Ampere]

Great video, Dave. I would like to see more reverse engineering videos in the future. It's always interesting to see how something was designed.

With regards to the Iprober... is there a way to do something similar with hobbyist-level equipment?

[oldway]

For me, the biggest flaw of this charger is that it can only charge the batteries with a single current value.
8A for an AA battery looks a little insane for me !

It should allow to choose the charging current between 3 values, high, medium or low rate.

For 2 batteries:
High: 8A
Medium: 2A
Low: 0.4A

For 4 batteries, half of these values.

[HKJ]

Low: 0.4A

Bad idea, -dv/dt termination will not work reliable at this low charge current.

[oldway]

Poor answer: at 0.4A, only max V termination is enough...Is this difficult to implement ?
Fan is also not needed.

[HKJ]

Poor answer: at 0.4A, only max V termination is enough...Is this difficult to implement ?

If you want a full cell without over charge, it is impossible to do. Chargers with voltage termination either under charges the cells, misses termination or uses a secret* top-off charge. Top-off is over charging, but NiMH batteries can usual take it.

*With secret I mean that the charger is saying finished before it applies the top-off charge, i.e. the user may remove a under charged cell, believing it is full.

[alxpo]

What do you think regarding using of zero (±opamp offset) burden voltage current shunts for measurements like 2nd part?
Like this, but sure with powerful opamps and lower resistors.

From EDN DI

[oldway]

All the low cost, low current AA and AAA battery chargers work on the principle of max V termination.
Very little over charging give a great over voltage, for this reason, ever if there is an over charging, it is very reduced and not harmfull for the battery.

Wat harmfull is for the battery, that's to charge it with current as high as 4C...
It's out of specifications of (most or all ?) the battery manufacturers.

It is also an insane loss of energy (It even need a fan to cool the batteries  )

@alxpo: There already exist an implemented battery voltage monitoring, as individual voltage of each cell can be displayed.
It is not necessary to add nothing as hardware.

But Dave has discovered that the displayed voltage is lower than the measured voltage...we should investigate why this happen...?

[HKJ]

All the low cost, low current AA and AAA battery chargers work on the principle of max V termination.
Very little over charging give a great over voltage, for this reason, ever if there is an over charging, it is very reduced and not harmfull for the battery.

No (Many low cost do never termination or only uses a timer) and it is as I wrote before.
I will suggest you go to my website and check how different chargers works: http://www.lygte-info.dk/info/indexBatteriesAndChargers%20UK.html

[KD0CAC John]

HKJ ,

I see you have looked into a lot of these battery chargers etc.
After watching Dave's video , I looked / Googled around looking for one of these and only found the Varda on eBay from Australia & 220v .
With your testing etc. , with similar functions as the charger in the video [ and batteries ] what would you recommend for US .   

[rr100]

It is also an insane loss of energy (It even need a fan to cool the batteries  )

The batteries need to be cooled anyway due to the way chemistry works, by probably roughly the same amount (energy-wise), except that because things happen much quicker we can't rely on the breeze to take out the excess heat. Yea, sure running the fan is extra energy expense but it is really next to nothing. And in any case a compromise we're willing to take, even at much larger scale with our servers, desktops, laptops, even Surface Pro tablets and Intel's compute sticks have fans, cars, A/C units and much more.

As total energy goes probably the regular slow overnight charger that is just an old school transformer, a diode or a bridge plus some resistor(s) as a crude constant current source would eat more energy to deliver the same charge to the same batteries.

[HKJ]

HKJ ,

I see you have looked into a lot of these battery chargers etc.
After watching Dave's video , I looked / Googled around looking for one of these and only found the Varda on eBay from Australia & 220v .
With your testing etc. , with similar functions as the charger in the video [ and batteries ] what would you recommend for US .

For charging only the Panasonic BQ-CC16 and BQ-CC17 works very well.
For analyzing chargers Opus works good (They are very good at terminating correctly).

The old classic analyzing charger is Powerex MH-C9000, it uses voltage termination and terminates before the batteries are full, then it supplements with two hours top-off charge after it has reported ready.

The project called Ultrasmartcharger is probably also a very good charger, but I have not looked at it (It is open source).

[oldway]

It is also an insane loss of energy (It even need a fan to cool the batteries  )

The batteries need to be cooled anyway due to the way chemistry works, by probably roughly the same amount (energy-wise), except that because things happen much quicker we can't rely on the breeze to take out the excess heat. Yea, sure running the fan is extra energy expense but it is really next to nothing. And in any case a compromise we're willing to take, even at much larger scale with our servers, desktops, laptops, even Surface Pro tablets and Intel's compute sticks have fans, cars, A/C units and much more.

As total energy goes probably the regular slow overnight charger that is just an old school transformer, a diode or a bridge plus some resistor(s) as a crude constant current source would eat more energy to deliver the same charge to the same batteries.

The energy losses are coming from the Joule effect in the internal resistance of the battery.
These losses are proportional to the square of the charging current.
8A is 20 x 0.4A, so the joule losses/s (W) are 400 x greater with a charging current of 8A instead of 0.4A.

For the same charge (quantity of Coulombs), time is only divided by 20 so it is still a huge waste of energy 
(twenty times more energy lost in joule effect in the internal resistance of the battery)

[rr100]

You are thinking about the internal resistance of a battery like a normal resistor just in series with the battery. This is not a correct model and you can easily test it just by injecting currents and plotting the voltage across the battery versus current (at the very same charge level, preferably take all the measurements within a few seconds on the same battery), it won't be a linear dependency.

[oldway]

Nobody said that internal resistance of a battery is a fixed value...It varies with temperature, concentration of electrolyte (and thus stade of charge), and so on...

[rr100]

Well, you did. If you say the wasted energy when charging a battery is proportional with I squared we can call that proportionality constant R.

If you also say the "real energy" that goes into the battery is proportional with I we can call that constant U.

Therefore the equivalent circuit would be a constant voltage sink that takes somehow the energy with 100% efficiency in series with a resistor. I don't think this model is correct.

[ez24]

HKJ ,
what would you recommend for US .

For charging only the Panasonic BQ-CC16 and BQ-CC17 works very well.
For analyzing chargers Opus works good (They are very good at terminating correctly).

The old classic analyzing charger is Powerex MH-C9000, it uses voltage termination and terminates before the batteries are full, then it supplements with two hours top-off charge after it has reported ready.

The project called Ultrasmartcharger is probably also a very good charger, but I have not looked at it (It is open source).

Thanks HKJ
I was thinking of asking the same question 

[oldway]

Well, you did. If you say the wasted energy when charging a battery is proportional with I squared we can call that proportionality constant R.

If you also say the "real energy" that goes into the battery is proportional with I we can call that constant U.

Therefore the equivalent circuit would be a constant voltage sink that takes somehow the energy with 100% efficiency in series with a resistor. I don't think this model is correct.

You are the kind of people who like to cut hair in four !!! 

The internal resistance of the battery increases when the charging current is higher because the formation of bubbles of H² on the electrodes is greater and reduced the active area.

http://theses.insa-lyon.fr/publication/2013ISAL0108/these.pdf

What interest has your post ? The internal resistance increases with the high charging current  and further increases the energy loss...Thus strenghtens what I wrote.

[Don Hills]

... But Dave has discovered that the displayed voltage is lower than the measured voltage...we should investigate why this happen...?

Maybe it's measuring (and displaying) the voltage during the momentary "off" period?

Way back in the day of NiCD cells, I built a charger that charged at 1C for 55 seonds, then discharged at C/10 for 5 seconds. It measured the voltage at the end of each cycle and used the difference to determine charge state. I found that the difference between charge and discharge voltages was fairly constant until the cells reached full charge (confirmed by their temperature starting to rise), at which point the difference voltage as much as doubled.

[NiHaoMike]

Since resistive losses increase with the square of the current, I'm surprised the "4A" with 3 or 4 cells is implemented as a 50% duty cycle of 8A switched between the two banks. If they just did a constant 4A, it would theoretically cut the resistance losses in half.

[drussell]

Way back in the day of NiCD cells, I built a charger that charged at 1C for 55 seonds, then discharged at C/10 for 5 seconds. It measured the voltage at the end of each cycle and used the difference to determine charge state. I found that the difference between charge and discharge voltages was fairly constant until the cells reached full charge (confirmed by their temperature starting to rise), at which point the difference voltage as much as doubled.

That is a valid way of measuring and monitoring the true cell voltage during a charge cycle and should be quite accurate. 

I'd be willing to bet that your charger worked quite well on the vast majority of cells.

[coppice]

... But Dave has discovered that the displayed voltage is lower than the measured voltage...we should investigate why this happen...?

Maybe it's measuring (and displaying) the voltage during the momentary "off" period?

Way back in the day of NiCD cells, I built a charger that charged at 1C for 55 seonds, then discharged at C/10 for 5 seconds. It measured the voltage at the end of each cycle and used the difference to determine charge state. I found that the difference between charge and discharge voltages was fairly constant until the cells reached full charge (confirmed by their temperature starting to rise), at which point the difference voltage as much as doubled.

I think the 15 minute recharge NiCd battery packs and chargers Vivitar made for some of their flash guns in the 80s worked in this way. They used special batteries with heavier conductors, and, of course, they were charging at about 4 times your rate. They didn't need any cooling fans, and the batteries didn't get very warm.

[rr100]

You are the kind of people who like to cut hair in four !!! 

The internal resistance of the battery increases when the charging current is higher because the formation of bubbles of H² on the electrodes is greater and reduced the active area.

It isn't about cutting hairs, it is about not making unfounded assumptions. The internal resistance is not from a given resistor inside the battery, it is just an imaginary concept that help us to model how the battery behaves. It is nothing more and nothing less than the proportionality constant between the delta-v under charge and the charge current. Once you say "oh, but this constant isn't constant" there is no point in continuing with any argument that assumes this proportionality (including specifically the one about the loss being proportional powere-wise with the square of the current while the "charge" is proportional with the current).

[f4eru]

Hello,
The 15V gate drive of the mosfet is because with that arrangement they don't care that the 6V or so battery voltage is substracted from the gate, i.e. they don't need level translators.

[oldway]

It isn't about cutting hairs, it is about not making unfounded assumptions. The internal resistance is not from a given resistor inside the battery, it is just an imaginary concept that help us to model how the battery behaves. It is nothing more and nothing less than the proportionality constant between the delta-v under charge and the charge current. Once you say "oh, but this constant isn't constant" there is no point in continuing with any argument that assumes this proportionality (including specifically the one about the loss being proportional powere-wise with the square of the current while the "charge" is proportional with the current).

Your argument is nonsense. 
The internal resistance is not an imaginary concept.
It is real and due to internal connections of the battery and the ohmic resistance of the electrolyte.
This is not because the resistance can vary by a few percent that the fundamental principles of electricity are no longer aplicables.

[rr100]

If you are talking about the ohmic resistance of the electolyte itself then (even if this would be reasonably constant) it is incorrect to say the waste is proportional with the square of the current because this resistance isn't in series with the circuit, but in parallel. Actually, given the fact that the voltage doesn't increase that much (let's say at most 1.5 times) the "waste" on this "resistor" would increase just 2.25 times. That is even if you increase the current 100 times for example from 80 mA to 8000 mA.

The resistance of the internal (and not only) wires yes, that's correct - the wasted power does behave like you said, roughly.

In any case while the discussion is interesting I hope you agree that from the practical standpoint for people with access to "line power" it just doesn't matter. You'd be using much more electricity if you turn on the lights to take the batteries from the dead device and put them into the charger or even to RTFM :-)

[oldway]

If you are talking about the ohmic resistance of the electolyte itself then (even if this would be reasonably constant) it is incorrect to say the waste is proportional with the square of the current because this resistance isn't in series with the circuit, but in parallel.

The charging current passes through the electrolyte and his resistance is in series with the circuit and not in parallel.
NB: everybody knows that the internal resistance of a battery varies with the concentration of the electrolyte 

Actually, given the fact that the voltage doesn't increase that much (let's say at most 1.5 times) the "waste" on this "resistor" would increase just 2.25 times. That is even if you increase the current 100 times for example from 80 mA to 8000 mA.

Your reasoning is wrong.
Basic equation of a battery voltage with charging current I is:
U = E + RI
Joule losses/s (W) are RI²
E is far greater than RI.
For this reason, you can't base your reasoning on the increase of U to calculate the % increase of joule losses.

[f4eru]

Since resistive losses increase with the square of the current, I'm surprised the "4A" with 3 or 4 cells is implemented as a 50% duty cycle of 8A switched between the two banks. If they just did a constant 4A, it would theoretically cut the resistance losses in half.

Yes, but it would cost much more in the inductor of the DC/DC converter to allow for the higher dynamic in voltage
The additional loss per mosfet is in the order of 72mW, which is negligible, especially considering they have fan cooling

[VK3DRB]

NB: everybody knows that the internal resistance of a battery varies with the concentration of the electrolyte 

Everybody? Absolute rubbish. Most one year old babies would not even know what a battery is, let alone anything about the internal resistance varying with the concentration of the electrolyte. Even Justin Beiber who is "worth" $200 million might not know either. In fact, it would not surprise me that less than 1% of the world's population know.

The tone of some of the comments in this thread reminded me of when I walked into an electronic components store in 1979 to buy a replacement 6X5 vacuum tube (valve) for my Kikusui oscilloscope for $2.15. The old valve's filament had gone open circuit. Another customer, whom I had never met before, was standing next to me. He big noted himself and belittled me by raising his voice in public something like, "A 6X5 valve is a bridge rectifier valve! Why don't you just use a couple of diodes! You're hopeless. Ha Ha Ha. I guess we all had to start somewhere." Embarrassed as I was, I was tempted to give this know-all a black eye. Instead I tried to explain I did not have a schematic for the oscilloscope and it was not worth the time mucking around, but this bombastic clown just laughed in contempt.

Assuming someone else hasn't already "taken him out" in a drive-by shooting or laced one of his junior burgers with cyanide, it would be poetic to find he is has been long term unemployable due to his inability to get along with people. I went on to become an electronics engineer as a life long career, and whilst I don't know it all and am learning all the time, I certainly know a lot more than I did in 1979. I have come across similar dick heads to Mr. 6X5 in business, but fortunately they are rare. I fired one once.

By the way, analysing Mr. 6X5's argument of using the rectifier diodes...

- I would need a schematic to the oscilloscope or a valve datasheet to work out where the diodes go
- I would need two diodes that are rated for the PIV, and current drain for B+ line in the oscilloscope
- I would probably need some resistors as well
- Back then diodes were not that cheap, around 60 cents each
- Spending an afternoon for the sake of saving $1, hoping I got it right is not time put to good use. Instead with the 6X5 valve, I just plugged it in and the Kikusui came back to life.
- The new 6X5 would have outlasted the old oscilloscope anyway

So even if it happened today, I would have done the same thing in buying the 6X5 valve. The only thing different is I would have wiped that smile off Mr 6X5's face 

(I am not saying you guys are like Mr. 6X5, but I do come across them electronics and programming weblogs and on Youtube. I think it is called trolling.)

[Don Hills]

... That is a valid way of measuring and monitoring the true cell voltage during a charge cycle and should be quite accurate. 

I'd be willing to bet that your charger worked quite well on the vast majority of cells.

Yes, it was quite reliable. It was "microprocessor driven" - an XT class PC. I used pins on the parallel port to switch the charge/discharge relays, and a lamp / LDR connected to the joystick port to measure the voltage. The lamp conveniently applied the C/10 discharge load. It was all written in BASIC.

[oldway]

Everybody? Absolute rubbish. Most one year old babies would not even know what a battery is, let alone anything about the internal resistance varying with the concentration of the electrolyte. Even Justin Beiber who is "worth" $200 million might not know either. In fact, it would not surprise me that less than 1% of the world's population know.

You should understood that this mean "everybody on this forum" with the exception of those involved in the "beginners" section of this forum.
So far I know, one year babies, nor even Justin Bieber are not posting in this topic. 

When you have bought a Varta 15 minutes charger and you want to proove that charging batteries with high current rate does not causes higher losses, only because you don't like to read that, and for this, you use arguments totally absurd and against the fundamental laws of electricity, that's what I call to troll.

NB: speaking of trolling, what your post has to do with "How The Varta 15 minute Battery charger Works" ?

[rr100]

you want to proove that charging batteries with high current rate does not causes higher losses

It is you who came wanting to prove something and in fact provided just hot air and a verbose and strong worded answer to everybody doubting you.

As I said it is debatable. If you charge for example with a slow charger that has inside a regular transformer and you burn some watts for 16 hours you'll be about the same as going full tilt some tens of watts for 15 minutes. That is in practical terms.

If you want to prove your point the stage is yours. But please come either with experimental data or a proven model that can be applied here. No, I do not accept U = E + IR as a good enough model for a battery charging, and certainly not across orders of magnitude for current. If you have something better than "you are wrong" to back it up we can discuss it.

[oldway]

No, I do not accept U = E + IR as a good enough model for a battery charging

U = E - RI (discharge)
U = E + RI (charge)
Are basic equations for batteries, as U = RI is for Ohm's law.
I don't have to prove this !
If you don't accept them, whe don't have to loose our time discuting here.

[mikerj]

As I said it is debatable.

It absolutely is not debatable, this is very basic stuff.  If you increase the charging current, you increase I2R losses.

[rr100]

That is for the wires and anything resistive between the battery and power supply, yes - I agreed to that.

By the way many of the slow chargers have a linear regulator and the really slow ones I was referring to earlier have just a resistor as "regulator" - they just supply some voltage high enough that the voltage change of the battery and the variation of the internal resistance just won't have much effect. Frankly I find it totally ridiculous that we're fighting over whatever power loss might be over those tens of miliohms from the batteries and cables when nobody complained over 200% of the energy wasted on any dumb slow charger.

[IanB]

That is for the wires and anything resistive between the battery and power supply, yes - I agreed to that.

No, it is for any resistive losses inside the battery. As mikerj says, this is completely fundamental and not open to debate. When current flows through a device and there is power dissipated as heat, then the I²R law applies for the dissipated quantity, where R is a proportionality parameter. Of course R may vary with temperature, time and other factors, but to a first order approximation we may assume R is a constant.

[rr100]

What if the voltage doesn't increase proportionally with the current (like is the case here)?

[IanB]

What if the voltage doesn't increase proportionally with the current (like is the case here)?

But the voltage associated with resistance losses does increase proportionally with the current. What makes you think it doesn't?

[rr100]

As I said, I already agreed to the fact that the RESISTIVE loses, practically by definition, behave as we'd be expecting from a resistor.

BUT what I try to say is that those are just A PART of the loses when charging a battery. An eneloop has a specified 25 mOhm internal resistance [edit: AA "regular" ~2Ah ones]. If you charge at 0.2A that's a theoretical 5mV drop. To make things simple let's take the nominal voltage of the battery at 1V (we'll be exaggerating all relative losses by about 20%) so we're looking at 0.5% loss compared with the energy that goes into the "battery". Does this mean the charging is 99.5% efficient? Of course not, and we didn't even start to think about the charger. At 0.2A and 5 mV that's a 1 mW loss. Are we looking in the right place? 1mW is down in the noise compared to what a charger eats just by sitting in the wall.

YES, if we want to take THIS 0.5% loss and say that we waste 20x more if charging at 4A and 40x at 8A yea, sure, that's correct. But a slow charger (or for that matter ANY charger lest in the wall) can easily and plausibly waste 3000x.

Back to this one for example eats 80mA (@15V) just sitting there. That's 1.2W, you'd need theoretically more than 10xAA eneloops to feed it ever day if you just leave it sitting there. And that is at the DC output, we need to add the wall-wart too but I can't measure that power with any reasonable degree of accuracy.

[oldway]

You are an incredible bad faith.

To justify your mistakes regarding batteries losses during charging, you now mix it with losses of the batteries charger.

But again your explanations are wrong.

We can now make low power switching power supplies (eg with STRA6069H SANKEN) that are as efficient (efficiency, no-load current) as SMPS 20 times more powerful (for the same output voltage).

Your reasoning is wrong again.

The 0.4A charger losses will therefore be 20x lower than the 8A charger but last 20 times longer, which means that there is no difference in energy loss regarding battery chargers.

In fact, there is a small difference in favor of the 0.4A charger as the charger 8A will have to provide more energy to compensate the additional losses from the battery, which will cause some additional losses in the 8A charger.

[rr100]

I do not try to justify anything, I'm just trying to clarify that your oversimplification ("huge waste of energy") based on the fact that you lose 20x or 40x more over the RESISTIVE LOSES IN THE BATTERY is an exaggeration. Yea, sure if you're using a 1/10C charger for a 2Ah battery you're pissing away only 1mW "resistively", sure. And energy-wise you'll be using 40x more with a 4C charger. But it won't be 40 times the TOTAL losses, it will be 40x those tiny resistive loses. There are more loses in the battery itself (no, you don't get a charging efficiency of 99.5% with NiMh, even at 1/10C), losses in the charger, etc. You need to be looking at the whole package.

[mikerj]

YES, if we want to take THIS 0.5% loss and say that we waste 20x more if charging at 4A and 40x at 8A yea, sure, that's correct. But a slow charger (or for that matter ANY charger lest in the wall) can easily and plausibly waste 3000x.

Only if it's left switched on continuously after the battery is charged, which is completely irrelevant to the charging efficiency of the battery.  If you apply the same conditions to a slow and fast charger (i.e. charge battery to the same level) the slow charger is always the more efficient one. 

You might want to read this PowerStream article:

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

[rr100]

Only if it's left switched on continuously after the battery is charged

DURING the charge, of course. Do you think it is implausible for a charger to lose 3W (that is 3000x the 1mW waste) without doing anything? This one eats 1.2W without taking into account the wall-wart.

If you apply the same conditions to a slow and fast charger (i.e. charge battery to the same level) the slow charger is always the more efficient one. 

First of all there are no "same conditions" in practical terms between a 4C charger and a (let's say) 0.1C charger. They'll differ wildly and those differences would matter much more than the internal resistance of the battery. But even if we have two chargers as close as we can build them, let's say that this Varta charger has a 0.1C mode. It'll waste only (see previous approximation) 0.5% energy compared to the battery capacity when charging at 0.1C while it'll waste 40x more at 4C. That's 20%. FINE. Now please estimate how much it'll waste by eating even one extra watt for the rest of the 10-16 hours extra it takes to charge at 0.1C! How much is that, x00%+ ?

[oldway]

I do not try to justify anything, I'm just trying to clarify that your oversimplification ("huge waste of energy") based on the fact that you lose 20x or 40x more over the RESISTIVE LOSES IN THE BATTERY is an exaggeration. Yea, sure if you're using a 1/10C charger for a 2Ah battery you're pissing away only 1mW "resistively", sure. And energy-wise you'll be using 40x more with a 4C charger. But it won't be 40 times the TOTAL losses, it will be 40x those tiny resistive loses. There are more loses in the battery itself (no, you don't get a charging efficiency of 99.5% with NiMh, even at 1/10C), losses in the charger, etc. You need to be looking at the whole package.

If you are looking at the whole package, you must also look for eficiency of the battery and you will conclude simply that it is better not to use batteries.
That's again a wrong way to analyse the situation.
If you are using batteries, it is because you need to do it.
But need you really charge your batteries in 15' at a tremendous high current rate with all the negative consequences this has ? (shortend live expectation, higher losses, very limited brands and types of batteries, even risk of explosion of the battery !)
Is it not better and more clever to buy several batteries more and to charge them at a lower rate ?

When you try to improve eficiency of a whole package, you have to improve eficiency of each part of this package.

1) battery charger: must be switch mode technology and be used in 25 - 100% output current range.
In this range, SMPS have a high eficiency.
You will say that my proposition of 3 charging currents (for 2 batteries, 8A, 2A and 0.4A) does not comply with this.
0.4A is only 5% of 8A.
But I never said that schematic of Varta is the good one.
I would have a different approach: the two converters principle, as used in computer SMPS and most of TV's sets and monitors: a low power auxiliary/stand by converter which would feed the µc and electronics control and also assume the 0.4A charging current of the batteries, and a power converter only switch on for charging with 2A or 8A rates.

2) Batteries: We can do nothing about battery technology and his efficiency, we can only reduce the losses during charging.
For this reason, never use the 8A current charging rate if it is not absolutely necessary.
I explained why but you don't seem to understand (or to accept !) what I wrote.

3) eficiency of what is powered by batteries: You seems to have forgotten this !

[oldway]

Another shortcoming of this Varta battery charger is the multiplexing when charging 4 batteries.

Why is this a shortcoming?

With unilateral pulsed current (here square wave), the charging current is the average current, but the current which cause Joule losses is the RMS current.
For the square wave, we have an average value of 1/2 the peak value and a RMS value of peak value/1.41 = 0.707 x peak current value.

Power losses in the batteries during charging are then 2x greater with a multiplexing that with a dc charging current.

[HKJ]

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

It is more like 90% to 95%, except the last part.

[splin]

Since resistive losses increase with the square of the current, I'm surprised the "4A" with 3 or 4 cells is implemented as a 50% duty cycle of 8A switched between the two banks. If they just did a constant 4A, it would theoretically cut the resistance losses in half.

Yes, but it would cost much more in the inductor of the DC/DC converter to allow for the higher dynamic in voltage
The additional loss per mosfet is in the order of 72mW, which is negligible, especially considering they have fan cooling

Actually if you assume 4.3momhs for the the mosfets (they'll be hotter than 25C) the extra loss is actually 206mW; add in the battery IR (25mohms), battery contacts (approx 1mohm each) and PCB traces (say 2mohms per cell) and the total extra loss for charging 4 batteries is 3.8W - not brilliant.

About those DTM4410 FETs - another bullshit datasheet. The datasheet says 3.8mohms typical, 4.5 max at 10V Vgs, 10A Ids - but the Rds(on) v Ids chart shows 7.8mohms typical at 10A, 10V - more than double! Same story for 4.5V Vgs . The Rds/Vgs chart also shows 7.8mohms at 10V, 10A at 10V, 10A so at least the two charts are consistent.

I had a look at several other Din-Tek mosfet specs and in almost all of them the Rds(on) charts bore no resemblance to the specs; e.g. the DTC2058 - spec is 26mohms, chart shows 15.7 (Vgs = 4.5V, Ids=8.3A). Many have Rds(on) scales which didn't make sense such as the attached chart for the DTL60N03 (specs are 3.1mohms @ 10V and 7.1millohms at 4.5V, both at 20A). You expect the odd error in datasheets but these seem to be mostly wrong - as a Japanese company I would have expected better.

So what do you trust? Presumably the DTM4410 3.8mohm typ, 6.5mohm max at 10A spec are the 'legal' figures but does that mean all the charts are worthless? Do they just make up the numbers or perhaps they have converted the graphs from pen and paper form using some dodgy OCR type software (surely not)?

[mux]

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

It is more like 90% to 95%, except the last part.

No, NiMH is inherently much less efficient. This has to do with the difference between electrode potential and neutral charging voltage, which annoyingly both change throughout charging/discharging. They even change at the same charging setting and %DoD because the electrolyte depletes fairly easily. If you have a perfectly tracking charger at about 0.05-0.1C (i.e. one that stays on the exact right voltage to force the right current through the electrolyte and internal ohmic resistance), the best that can be attained is about 85%. Charging slower won't improve overall efficiency as you start running into significant self-discharging effects (due to the effect that higher electrolyte mobility during charging also induces higher self-discharge rates). In practice, chargers usually are constant-voltage with some dissipative element forcing constant-current; as well as charging at 0.5C, which is why they quote the 66% figure.

It's very important to note that NiMH is fundamentally different from li-ion in this respect. NiMH has very good ohmic internal resistance (basically on par with li-ion), but (relatively) bad ionic resistance. Li-ion has incredibly low ionic resistance, which means that charging and discharging is basically only limited by the resistance of the electrodes. This is why efficiency is not an issue with li-ion, and li-ion can sustain much higher currents without nearly as much impact on capacity.

[HKJ]

The coulometric charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out. The faster you charge the worse this gets.

It is more like 90% to 95%, except the last part.

No, NiMH is inherently much less efficient. This has to do with the difference between electrode potential and neutral charging voltage, which annoyingly both change throughout charging/discharging.

Instead of writing theories, try measuring it, I did.

[mikerj]

Instead of writing theories, try measuring it, I did.

Maybe you should take that up with PowerStream, since you apparently know far more about batteries than they do.  It's not like it's their core business or anything.

[HKJ]

Instead of writing theories, try measuring it, I did.

Maybe you should take that up with PowerStream, since you apparently know far more about batteries than they do.  It's not like it's their core business or anything.

They and you can just read on my website, where this will be published, together with all my other battery stuff. You could also do you own testing and learn something, instead of just believing a random article.

[apis]

Why is there even an argument about the energy efficiency of the charger, no-one is going to buy a 15 min NiMh charger based on the efficiency!

What matters is if it works and the economy of it, i.e. how long the batteries are going to last (the electricity cost is going to be negligible unless there is some large standby current and it's always plugged in, but that would not be because it's a 15-minute charger). I think most realize there will be a compromise between battery life and charge current. I agree its not a good solution technically if you can plan ahead and manage your batteries properly, but most people know nothing about batteries and they don't have enough foresight to charge the batteries in advance. I remember being a kid and never having fresh batteries. Charging them would take a day so it effectively killed whatever activity you needed them for (and my parents were too cheap (or devious) to have enough spares that could be charged in advance). So in short: a 15 minute charger would have been awesome. The same must be true today, if someone picks up a game controller (or whatever) and realize the batteries are flat, waiting 15 minutes is not so bad, 2 hours (or 15 hours for trickle charging) is going to be a show stopper.

[mux]

Instead of writing theories, try measuring it, I did.

Maybe you should take that up with PowerStream, since you apparently know far more about batteries than they do.  It's not like it's their core business or anything.

They and you can just read on my website, where this will be published, together with all my other battery stuff. You could also do you own testing and learn something, instead of just believing a random article.

Do you have a link?

[HKJ]

Do you have a link?

Here: http://lygte-info.dk/

Mostly it is technical reviews, but once in a while I publish some articles. You can find them by careful looking through the battery & charger section.
The one about NiMH charging is not published yet.

[oldway]

@apis:

Why is there even an argument about the energy efficiency of the charger, no-one is going to buy a 15 min NiMh charger based on the efficiency!

Eficiency is a major concern in the modern world.
Perhaps electric energy is very cheap in Sweden, that's not true in other lands.

(and my parents were too cheap (or devious) to have enough spares that could be charged in advance)

For sure, your parents would also buy low cost batteries who would never accept to be charged with 4C.
Charging at 4C is above specifications of every AA batteries manufacturer and, if I remember right, Dave has showed that even Varta don't allow such a high rate with his batteries.
Using such a tremendous high rate has a high cost:
1) only high quality batteries can be used.
2) life expectancy is reduced.
3) losses are higher.

[oldway]

Instead of writing theories, try measuring it, I did.

Maybe you should take that up with PowerStream, since you apparently know far more about batteries than they do.  It's not like it's their core business or anything.

They and you can just read on my website, where this will be published, together with all my other battery stuff. You could also do you own testing and learn something, instead of just believing a random article.

I did not seen any efficiency value of batteries...
What efficiency are you speaking about ?
Faraday or energetic efficiency ?
Energetic efficiency is lower than Faraday one.
How will you measure and calculate energetic efficiency of a AA battery ?
Easiest way would be to charge and discharge the battery at constant power and measure the times of charge and discharge.
You can discharge the battery at constant power with your electronic load, but how are you charging the battery with constant power ?
Your power supply does not have this option.

[HKJ]

I did not seen any efficiency value of batteries...
What efficiency are you speaking about ?
Faraday or energetic efficiency ?
Energetic efficiency is lower than Faraday one.
How will you measure and calculate energetic efficiency of a AA battery ?
Easiest way would be to charge and discharge the battery at constant power and measure the times of charge and discharge.
You can discharge the battery at constant power with your electronic load, but how are you charging the battery with constant power ?
Your power supply does not have this option.

I have not published the article yet. The test is for mAh in and mAh out, where I uses a 2000mAh cell and test it with values from 1500mAh to 2300mAh.

[oldway]

Ok, that's the Faraday efficiency, that's the reason why your values are so high.
The true efficiency is the energetic efficiency, that's how much energy in and how much energy out. (energy = power x time. If power varies during the time of charge or discharge, you must integrate the power during this time)
I suggest you to measure this efficiency and not the Faraday efficiency who does not mean nothing !

[HKJ]

Ok, that's the Faraday efficiency, that's the reason why your values are so high.
The true efficiency is the energetic efficiency, that's how much energy in and how much energy out.
I suggest you to measure this efficiency and not the Faraday efficiency who does not mean nothing !

The statement I answered was this "meaning that you must put 150 amp hours into the battery for every 100 amp hours you get out" and that is not correct.
I do have the voltages during charge and discharge and can calculate the energy efficiency, if I can find the time I may do it.

[mikerj]

Instead of writing theories, try measuring it, I did.

Maybe you should take that up with PowerStream, since you apparently know far more about batteries than they do.  It's not like it's their core business or anything.

They and you can just read on my website, where this will be published, together with all my other battery stuff. You could also do you own testing and learn something, instead of just believing a random article.

I suspect your test methodology was incorrect then, though as we can't see it we can't tell.  Coulombic efficiency can be high if you only partly charge the cells, but rapidly drops as you approach full capacity and 66% is a typical figure for a fully charged NiMh cell.

[HKJ]

I suspect your test methodology was incorrect then, though as we can't see it we can't tell. 

As I have already explained that I charged different mAh into the cell and the discharged it again to measure the mAh

Coulombic efficiency can be high if you only partly charge the cells, but rapidly drops as you approach full capacity and 66% is a typical figure for a fully charged NiMh cell.

That is correct mostly correct, with the 2000mAh cell the efficiency was high up to about 1900mAh into the cell (>1850mAh out). If you insist on filling 2500mAh into it you will get a bad efficiency.

[oldway]

@HKJ:

I do have the voltages during charge and discharge and can calculate the energy efficiency, if I can find the time I may do it.

Calculate ? I wonder how you could do this as voltages are varying during charge and discharge in fonction of the time at a non linear rate ?
Could you explain how should you calculate the energetic efficiency ? What are the formulas you intend to use ?

[HKJ]

@HKJ:

I do have the voltages during charge and discharge and can calculate the energy efficiency, if I can find the time I may do it.

Calculate ? I wonder how you could do this as voltages are varying during charge and discharge in fonction of the time at a non linear rate ?
Could you explain how should you calculate the energetic efficiency ? What are the formulas you intend to use ?

I have log points for each second, it is easy to calculate power for each log point and then sum power*time. This would give my total charged and discharged energy.
My logfile looks like this (i.e. I have already done the calculations, I just need to collect them from a couple of files):
time   SMMeasure   SMSource   SMOhm   SMW   SMAh   SMWh
0   1,028179   0,99999   1,028189   1,028169   0,000257   0,000264
1   1,044294   0,999991   1,044303   1,044285   0,000535   0,000555
2   1,061145   0,999992   1,061154   1,061136   0,000813   0,000849
3   1,078734   0,999991   1,078744   1,078724   0,001091   0,001149
4   1,096882   0,999992   1,096891   1,096873   0,001368   0,001454
5   1,115237   0,999992   1,115246   1,115228   0,001646   0,001763

[oldway]

That's an approximation, but it is ok.
Have you already make this calculation for one kind of battery ? (as eneloop for example).
I am curious to see wat you find as energetic efficiency. I have in mind something like 80 to 90% but I did not measure this.

[HKJ]

That's an approximation, but it is ok.

It will be difficult to do it any better. With a real cell you have to measure in discrete time intervals. Converting the data to a smooth curve may give slightly better precision.

Have you already make this calculation for one kind of battery ? (as eneloop for example).
I am curious to see wat you find as energetic efficiency. I have in mind something like 80 to 90% but I did not measure this.

I have the data for a single NiMH cell at different charge capacities, but have not picked them out of the log files.

[oldway]

I hope you will share the results of your compiled data, it will be very interesting, you have done a very fine job 

[HKJ]

I hope you will share the results of your compiled data, it will be very interesting, you have done a very fine job 

Here you have the curve with energy:

Delta voltage is voltage above 1.50 volt

At best you get 80% of the energy back.

For the rest of the article see here: http://lygte-info.dk/info/batteryChargingNiMH%20UK.html
The mAh efficiency is just a small part of the article.

[Svuppe]

Has anyone figured out what processor is used in this charger yet?
I had a quick look at the voltages on the pins, and most are either 0 or 5V, indicating a 5V processor. But one pin puzzled me: Pin 18 is at 5.65V

They could be feeding it higher than Vcc through a resistor, and relying on the processors internal esd protection diode to clamp the voltage.

[Mr Smiley]

Hi All,

I had a few VANSON 1 hour chargers. The first one failed by cooking the battery in the first slot. Then the second one started to do the same, again in the first slot. I tried drawing out the circuit and was as confused as Dave was at the beginning.

I did eventually find a schematic for a VARSON V6000, which was a re-branded version of the one's I had. It was multiplexed in operation.

Not quite the same as this VARTA, the fets are transistor driven and there's no discharge function but I've added the schematic for you to see.

Might put some insight on how the VARTA works