Uneven battery discharge

jdriller · Oct 5, 2003

When I use 6 AA Nimh or 3 D Nimh, one or two of the batteries discharge unevenly. With 6, 5 might read 1.22v after use and one 1.07. With 3, two 1.2 and one 1.05. Why does one battery discharge unevenly?

Thanks for the help. /ubbthreads/images/graemlins/help.gif

eluminator · Oct 5, 2003

It had less of a charge to begin with, I guess. This always happens to some degree. Maybe the crystal size was greater in the weakest one. I wish I knew exactly. And because the weakest one gets discharged farther than the rest, it seems to get worse. Maybe you've discharged them enough to "reverse charge" the weak one. I think the only way to tell that is to measure it's voltage while it's in the device while the device is turned on.

I find that a few charge/discharge cycles will usually get the capacity of all the cells about the same, but that's not a very good solution either. And it's not permanent.

I think the moral of the story is be wary of using rechargeables in a device that has many cells in series.
Two in series can sometimes be one too many.

jayflash · Oct 5, 2003

See my acid test post to determine, before using, how much current each cell will sink into a dead-short. Try to use batteries, together, that are close in output. Out of a group of NiMh's some may output, say, 12amps into a short & others 8 or 6 etc. Use the closest in amp readings in the same device & they will usually all poop out at nearly the same time.

Brock · Oct 5, 2003

Another thing to help this situation is making sure your charging each battery individually. Once the batteries got "off" from one another they will only get further and further apart as time goes on. If you charge them full individually each time they should stay closer over time.

radellaf · Oct 5, 2003

Look up battery matching here or on google if you want to get into a lot of reading. The above ideas are good, but personally, unless they're really far off from each other (e.g. you have a bum cell that's not performing up to spec) I just wouldn't worry about it. When the light dims the first time, charge 'em all up. Less of a concern, of course, if you have something like a Maha 401 that charges each cell individually. With the Ds I have I sometimes charge them one at a time in the CCrane.

jdriller · Oct 5, 2003

Great advice! Thanks for the help.

JSWrightOC · Oct 6, 2003

I'm running into the same situation with four AA NiMH cells I have...while all of them may have a no-load voltage of 1.110 or so (after being partially discharged), there is definitely a weak one that drops down to a couple hundred mV under load. I'm trying to condition and exercise it right now in a RS pulse charger (just like the CCrane--except RS doesn't sell this model anymore) and I'll see what happens. Worst case scenereo is I replace the bad cell with one that's better matched (these are all used anyway) or just replace the entire set.