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

[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.