Charging 10 AA im parallel, bad idea?

[malow]

hi, i was wondering, its there any problem in charging 10 AA battery in parallel, that i may not be aware?

i will buy the 10 battery at same time, (no new/old mixing) and will perform the "conditioning" to make them as even as possible (discharge independently till 0.9v, charge all at 0.1C-16h).

as my hobby charger (turnigy accucel-6) can do 6A charging (about 3.6C) the -dV will be small, but as i have the the serial>USB cable, i pretend to do a "manual" end-of-charge, by analyzing the graph.

may also use a contact thermometer also.

is there any problems in this scenario? :duh2:

also, on side-question, i was aware that when the -dV happens, technically, the battery its already overcharging. my question about this, is, "where" in the voltage climbing, stabilize, and then drop, its the best point to end charge? keeping in mind my goal is longevity, not so much capacity. (will use them a lot, an would be nice if they can last about 400 cycles at least. they are 2200mah LSD cells - hobbyking turnigy)

tnks in advance.

 

[VegasF6]

No, you shouldn't parallel charge nimh batteries. Even if matched as well as possible, they still won't be that well matched. You would probably have to buy 100 of them to impedance match 10, and at some point they would get out of balance.

For proper termination you would need ~.5C charge rate for the entire pack. The cell with lower resistance would receive the brunt of that charge and heat up, possible even explode. Stick with series charging.

 

[VegasF6]

You can look up negative delta V termination to get a better idea what exactly that means, but my brief understanding of it is while charging the battery will hit a "knee" voltage and actually drop some. That's when termination takes place. I know it is different for nicad than nimh, I think something like 200mV, and certain hobby chargers will actually allow you to set the voltage drop detection level.

Monitoring battery voltage will give a pretty good idea when to terminate too, a sudden rise in temperature means the battery is no longer absorbing the charge. So your idea to monitor cell temp is a good one, but I am not sure how useful in this case.

 

[malow]

tnks VegasF6 for the answers.

 

[45/70]

Hi malow. Lescan says "meeeow!". How is your kitty, forgot his/her name, a full grown "cat" now I suppose and doing well I hope.

Um, anyway, the problem as I understand it is that when you have as many as 10 cells in series (~12 Volts), the -dV is hard enough to distinguish, but can be detected with a hobby charger, reasonably well. When you have 10 cells in parallel at ~1.2 Volts however, it's harder to distinguish. Also, Mr Happy explained it to me once, that there are certain properties of nickel based chemistry cells that create current eddys within the cells while charging, that also make parallel charging impractical.

Dave

 

[Mr Happy]

The answer is definitely don't try to charge NiMH cells in parallel. The reason is similar to the reason for not wiring LEDs in parallel.

What you would want to happen is that your charging current is evenly distributed between the cells, but what actually may happen is that one cell might get a little more current than another (no two cells are identical). The cell with more current will get a bit warmer, causing its resistance to drop, and it will get yet more current. This can quickly escalate to a run-away situation where one cell gets way too much current and is drastically overcharged.

In commercial battery packs, well matched cells are sometimes wired in parallel, but in these packs the cells are tightly joined together so the heat is distributed and they all remain at about the same temperature. These packs also have a temperature sensor and they use temperature to regulate charging as well as voltage. By this means, two or three cells might be put in parallel, but I think ten in parallel would be a step too far.

So to restate the advice: don't attempt to charge NiMH cells in parallel. It will not end well.

 

[malow]

tnks 45/70 and Mr Happy for the answers 😉

45/70, Bambi its better than ever... and his new friends, Tripé, Polaco, and Malhadinha sent a bunch of meows to Lescan too 😉

 

[SilverFox]

Hello Malow,

It is not advised to charge NiMh cells in parallel, but that doesn't mean that you can't do it...

There are some applications that involve NiMh cells in parallel, and the charging is done at 0.1C with a timer shut off.

With your set up, if you want to try it... charge at 2 amps for 16 hours. This is not ideal, and there are a lot of issues with it, but if you want to give it a try it is the best way to proceed.

Tom

 

[VegasF6]

Why bother though if you have an accucel 6? Spec sheet says it will charge 1-15 nickel cells. Just pop them all into a piece of PVC with some magnets or a clamp to ensure good contact and away you go.

 

[45/70]

45/70, Bambi its better than ever... and his new friends, Tripé, Polaco, and Malhadinha sent a bunch of meows to Lescan too 😉

Cool! Thanks for the pics!

I was going to send an updated pic of Lescan, but don't have any recent ones on this computer. She just turned 7 YO, sorta her birthday this month. I don't know the exact date, as I found her under a bush.🙂

Why bother though if you have an accucel 6? Spec sheet says it will charge 1-15 nickel cells. Just pop them all into a piece of PVC with some magnets or a clamp to ensure good contact and away you go.

Vegas brings up a good point. By charging the 10 cells in series at a 0.1C rate with your hobby charger, you will better "balance" the cells, as NiMh cells balance well using this method when series charged. In fact, for cells that are welded together in a "pack", this is the only way of balancing them.

Dave