Recharging several NiMH AA batteries in series

[13hm13]

For an audio application, I've been using this 10-AA batt. holder. Disregard the "9v" battery plug -- that's just used for convenience (my audio amplifier can accept 5-30v.)



While I have several std. AA battery chargers, I must INCONVENIENTLY remove all cells from that  10-AA batt. holder, and go thru a whole cumbersome procedure.

Time for some convenience.

I want to simply connect voltage to that 9v plug, and charge all 10 AA NiMH cells in situ.

What do you suggest?

Maybe something like:



Also, I do have several 12v wall-wart adapters lying around. Could I re-purpose those somehow?

[Kim Christensen]

You could DIY your own design, but it would be easier to build one using one of the many dedicated chips out there.

https://www.analog.com/media/en/technical-documentation/data-sheets/MAX712-MAX713.pdf

Or just buy something like in the video you posted if you don't want to build it.

[13hm13]

You could DIY your own design, but it would be easier to build one using one of the many dedicated chips out there.

https://www.analog.com/media/en/technical-documentation/data-sheets/MAX712-MAX713.pdf

Like I noted, I have many 12v wall warts lying around. How safe or (in)effective is is it to JUST plug that into my 10-AA holder?

The MAX712 you linked is a fast charger. I'm in no hurry Trickle is just fine.

BTW: The charger I do use (Maha energy 4-cell) ... can make batts very warm (even hot) during "normal" charging/use. I don't feel comfortable with this type of commercial charger at all.

https://mahaenergy.com/mh-c204w/

[RoGeorge]

Unless you need fast charging, a 1 day slow and dumb charging is preferable.  A resistor series with a voltage source would do it.  If you think you might forget to unplug it next day, add a relay timer.

[13hm13]

Unless you need fast charging, a 1 day slow and dumb charging is preferable.  A resistor series with a voltage source would do it.  If you think you might forget to unplug it next day, add a relay timer.

One day is okay.
Suggest a value for the R (ohms, wattage) if I use a common 12 wall wart.

[Infraviolet]

NiMH are trivial to charge in series, none of the complexities of Lithium cells.

The important thing though is that when the NiMH cells are draining you need to have a cut-off which activates when the whole pack gets below the threshold of (whole pack voltage(1.2*n_cells))-(one cell's voltage(1.2)), because otherwise if they were badly unbalanced it could start backcharging the one cell that was already empty. And a backcharged cell could leak, and might if you're really unlucky even leak a little H2 gas or smoke a bit, but thankfully won't go off like a Li cell could.

I've used series stacked NiMH cells in many projects with no balancing or other fancy circuitry, just a charger like this
https://uk.rs-online.com/web/p/battery-chargers/7760800
plus a self-made adapter to go from their DC barrel jack to the format of connector I needed for my pack.

If your pack ever starts to underperform then give the cells an individual charging separately, but for the most part the stacked pack will be absolutely fine.

Regarding the warming of the cells on fast charge, I once measured this, rose by about 10 celsius over the course of the whole charging cycle, so 10C above room temperature when it finished and switched to trickle charging. And this was with 8 cells in a relatively enclosed location, heating would be even less with greater air flow around the cells.

[RoGeorge]

On a cell is written 1900mAh.  Thumb rule for slow chargers used to be 16 hours at 0.1C, in this case 16 hours at 190mA.  To push about the same energy in 24h would mean 190*16/24 = 126.67mA, anything around that would do it.  The nominal voltage is 1.2V, while during charging it might be about 1.4V or so.  For 10 of them in series you would need at least 15V.  Even more volts would be needed to use a resistor as a constant current source, maybe find a 20-24V voltage source.

Say 24V for a numerical example.  24V source - 14V batteries = 10V voltage drop on the resistor.  To limit the current to 125mA, R=10V/125mA, so about 80 ohms.  Resistor's power is U*I, so about 1.25W, make it 2-3W so it doesn't heat.  For example 2 parallel 50\$\Omega\$/0.5W would make a 25\$\Omega\$/1W.  Put 3 of these 25\$\Omega\$/1W in series to get a 75\$\Omega\$/3W (6x50\$\Omega\$/0.5W resistors in total).

Redo the calculations for whatever resistors and voltage source you may have around.

Doesn't has to be very precise +/-20% is acceptable, batteries won't get any damage at such small currents unless you forget them in the charger for many days non-stop. 

[13hm13]

Let's simplify things...

For my 10-AA NiMH pack (total output: 1.2v x 10 = 12 v), shown in OP, I will plug in a common 12v wall wart (adapter) but NO resistor. Just a direct mating.

Note: I never leave any charger unattended. They are always placed in common area where I can see them and the status LEDs.

[Infraviolet]

I'm not sure if 12V Dc will actually charge 10 cells properly. The main part of the discharge curve is around 1.2V/cell, but when fully charged the charge per cell is higher. 12V DC might not get the stack of cells up to full charge?

[Kim Christensen]

Let's simplify things...
For my 10-AA NiMH pack (total output: 1.2v x 10 = 12 v), shown in OP, I will plug in a common 12v wall wart (adapter) but NO resistor. Just a direct mating.
Note: I never leave any charger unattended. They are always placed in common area where I can see them and the status LEDs.

You could do that. I assume the common 12v wallwart is just an old school 50Hz transformer, diodes, & capacitor type. Those usually put out a higher voltage than their rating when lightly loaded so it should be able to fully top up your NiMH pack over time (Usually the terminal voltage on a NiMH cell is around 1.5-1.6V)
You'll have to experiment to see the volts vs current curve for the line voltage in your area. NiMH don't like a continuous trickle charge greater than 0.05C but are OK around this level forever. (In case you do forget to turn it off)

But if it's the switching type and puts out exactly 12V it's never going to fully charge those cells and it'll be tough on the adapter if it spends too much time in current limit mode early in the charge cycle.

[floobydust]

I see ongoing problems with keeping cell balance in NiMH (Eneloop, Ladda) AA packs. Despite being from the same batch and package, treated identically, charging them in series I will notice one or two lazy cells that fall behind and need more charging than the rest. If I manually rebalance by charging only the lazy cells to match the rest, it seems to be fine afterwards. So the cells are not identical enough.

I just had a Sanyo Eneloop 4-cell charger go bad, terminating charging early so the batteries are not actually fully charged. The voltage reference seems to have drifted.
Looking at the circuit, it's kind of junky because it charges in pairs so an imbalanced pair does not seem to get proper care.

NiMH really do not like being treated like NiCd being slightly overcharged or always on the charger for long periods. They have a short life if you do this a lot.

[13hm13]

I just plugged in a 12v Canon scanner adapter. It's SMPS based.  Adapter tag notes: 12.0 V, 1.25A.
I measured adapter output directly to DMM: 12.3v (no load).
I broke the ckt -- between Canon and 10-pack -- and measured the current draw with a Fluke 87.
The current immediately drops from ~ 20mA to about 8.2, and then very, very slowly drips down from there in uA. My patience runs out here, so I stop testing!
I repeated the above measurements several times to estimate behavior of the charging scheme. Same.

I'll  check again in a few hrs, leaving the contraption plugged in for the duration.

[13hm13]

I see ongoing problems with keeping cell balance in NiMH (Eneloop, Ladda) AA packs. Despite being from the same batch and package, treated identically, charging them in series I will notice one or two lazy cells that fall behind and need more charging than the rest. If I manually rebalance by charging only the lazy cells to match the rest, it seems to be fine afterwards. So the cells are not identical enough.

I just had a Sanyo Eneloop 4-cell charger go bad, terminating charging early so the batteries are not actually fully charged. The voltage reference seems to have drifted.
Looking at the circuit, it's kind of junky because it charges in pairs so an imbalanced pair does not seem to get proper care.

NiMH really do not like being treated like NiCd being slightly overcharged or always on the charger for long periods. They have a short life if you do this a lot.

I hate  all the high-rated Maha (Powerex) chargers I purch's new over a decade ago. Yes, I bought three models at once because of "high ratings".
They are all very sensitive. And the Powerex batteries all failed within a year.
Since new, they -- all models -- fault out on many batteries.
No such issues with other manuf. chargers I own, including ones much older.

FYI. the inside of one of my Maha chargers (disregard the other green PCB; that's another project!!)

[RoGeorge]

Let's simplify things...

For my 10-AA NiMH pack (total output: 1.2v x 10 = 12 v), shown in OP, I will plug in a common 12v wall wart (adapter) but NO resistor. Just a direct mating.

That would be too oversimplified, I'm afraid.  Won't charge them very good.  When charged the voltage is 1.4V ... 1.5V for a 1.2V cell, not 1.2V, so 12V will only partially charge the bank, probably at only 10-25% of their capacity.  Then, some cells will remain with almost no charge, which will make them become reversed polarized during powering a load.

Find a 24V supply. or make a voltage doubler from the 12V SMPS you have, to get 24V + add a resistor (or a series 5W/12V light bulb if you don't find the right resistor).


Alternatively, you can use a LED driver for LED strips, those are isolated constant current sources (the current is written on their back).  Even the driver inside a 220V LED light bulb might do the job, though it's not a good idea to mess with the mains AC voltage.  The LED driver inside mains light bulbs are not isolated, so that's an electrocution hazard for you or for anybody else that might come and touch the batteries.

[mariush]

I just have to ask ... do you really NEED to use AA batteries?

You could easily replace the 10 batteries with 2 or 3 lithium batteries ... and you can source such batteries from old laptop batteries or even new.

For testing an audio amplifier you could even use a single battery and a boost / step-up regulator to boost 3.6-4.2v to 10v or more.

[13hm13]

I just have to ask ... do you really NEED to use AA batteries?

You could easily replace the 10 batteries with 2 or 3 lithium batteries ... and you can source such batteries from old laptop batteries or even new.

For testing an audio amplifier you could even use a single battery and a boost / step-up regulator to boost 3.6-4.2v to 10v or more.

No real need for AA, other than I already have them and they are still in working condition. I have been looking into 18650 cells packs.
Not a fan of buck/boost for hi-fi audio. Not audiophile
Ideally, my little headphone amp works better with more LINEAR, clean voltage up to about 30v max. Many diyers that built the same amp liked its sound around 25v.
The little, portable amp is based on a larger "home" unit and is  quite a current hog, given its used of class-A bias and output current buffers. Powering it using 18v yields a total idle current draw of 106mA (that's w/o any earphones or audio signal plugged in). Idle current draw drops to about 94mA at 13v and 75mA at 9v.

[13hm13]

BTW:
I have used that same 10-AA battery pack for over 11 years. Same batteries, too. The pack doesn't get used more than 2-3x month (2 hrs for each use). I'm curious about the total use hours or "wear hours" on those AA Sanyo's. Yes, not much used but most of them sit sandwiched between other batteries (in-series cells) . So I'm curious about that constant in-series  "sandwich load" . Must be pretty low.

[IanB]

Let's simplify things...

For my 10-AA NiMH pack (total output: 1.2v x 10 = 12 v), shown in OP, I will plug in a common 12v wall wart (adapter) but NO resistor. Just a direct mating.

Do not do this. It is contrary to the advice given in this thread and will not work well, if at all.

When fully charged, an Eneloop reaches about 1.47 V at the terminals, so 10 in series will reach 14.7 V. You will need a charging voltage higher than 15 V. A supply of 12 V will not do it.

To charge the set of 10 cells safely in series, get a supply of, say 24 V and arrange that the current when fully charged is about 100 mA. Therefore we can choose a series resistor like this:

  R = (24 - 10 x 1.47) / 0.1 = 93 ohms.

You could choose 100 ohms to make it a round number.

Then when the batteries are empty (cell voltage 1.2 V), the charging current would be:

  Imax = (24 - 10 x 1.2) / 100 = 120 mA

For faster charging you could probably go to a 50 ohm resistor, but not lower than that.

[radiolistener]

Charging Ni-MH batteries connected in in series can lead to unbalance charge, some batteries will be overcharged and some will be not fully charged. It's okay if you're doing it rarely - one or two times and then using normal charger, but if you're doing it often it leads to a fast battery degradation.

If you're decided to charge it in series do not use fast charge with a high current. Use slow charge current, it minimize battery unbalance.

[IanB]

Charging Ni-MH batteries connected in in series can lead to unbalance charge, some batteries will be overcharged and some will be not fully charged. It's okay if you're doing it rarely - one or two times and then using normal charger, but if you're doing it often it leads to a fast battery degradation.

This is only true if you attempt a fast charge. Doing a long slow charge is the opposite of this, and a long slow charge is how you balance a pack.

If you're decided to charge it in series do not use fast charge with a high current. Use slow charge current, it minimize battery unbalance.

The posts above have all recommended a slow charge.

[13hm13]

Charging Ni-MH batteries connected in in series can lead to unbalance charge, some batteries will be overcharged and some will be not fully charged.

And what about DISCHARGING? I.e., normal use. in a series config as low-voltage NIMH batts are in devices?
 

It's okay if you're doing it rarely - one or two times and then using normal charger, but if you're doing it often it leads to a fast battery degradation.

Not even as EXPERIMENT?

If you're decided to charge it in series do not use fast charge with a high current. Use slow charge current, it minimize battery unbalance.

Yes, and the worst offenders, as I earlier noted, are the COMMERCIAL chargers, that have fancy packaging and software and features like "Rejuvenate". And when you pull out your batteries, they are very, very warm. But the charger owner's manual says that's "normal".

https://powerex.helpdocs.com/manuals/user-manual-mh-c204w-charger
https://www.philips.com.sg/c-f/XC000012108/why-do-my-philips-rechargeable-batteries-heat-up-when-charged#:~:text=It%20is%20normal%20for%20batteries,warmer%20the%20batteries%20will%20become.

My simple 12v DIY hack job runs cool as do the in-series AA  Eneloop's.