Finding a bad cell in a Series Battery pack of 12 NiMh batteries

So did you measure each cell voltage or not? It is possible to do this; just put your volt meter across a cell and it will measure its and only its voltage.

If you have just measured the pack voltage, 16.65V/12=1.388V seems like it could be a reasonable value for each cell. You might not actually have a bad cell.

I was hesitant to reply when you posted some time ago.

I'm pretty sure, the only way you can single out the bad cell is to take them out of the series.

De-solder and test individually is the only way.

You have a math problem not a battery problem. :wink: 1.2 x 12 = 14.4V, straight off the charger it can be ~1.4V per cell or 16.8V.

In series each cell voltage can be measured separately, it’s when they are in parallel that the pack needs to be disassembled.

How old is the pack and what sort of use has it seen? If all cells measure exactly the same voltage it’s a really good pack! :slight_smile: It may just be that the cells are a little worn, or possibly that the charging current could be higher. What’s the capacity of the pack and at what current do you charge? With high drain NiMH, you need to charge at least at 1C.

Yes. Several times. They all measure within .01v of each other.
The voltage has dropped to 16.45 since about 2:00 this afternoon.
So 1.371v per cell now.

Yeah, I was referring to the charged voltage. They are 1.2v cells but “should” hold a charge of up to about 1.45v per cell.

But yeah, I’ve always had a math problem tbh :wink:

This is what I keep hearing….and all the batteries have almost the exact same voltage. I just thought it was because the better cells with higher voltage were pulling up the voltage of the weaker batterie(s) while the weaker batteries were pulling down the better batteries….making the whole pack SEEM like all the batteries had the same voltage.

Guess I need to study series battery connection science a bit more huh?

I know! See, you nailed it!
My thinking was…this is suspicious if ALL the batteries in a 12 pack had the exact same voltage….and pretty much they do!
These are sub C cells. Like $2.00 per cell. The claimed capacity is 3000mAh. (Probably less than half that in reality)

The pack has been used about 10 times and I just received the batteries about a month ago.

I charge them at ~400mA and ~17 - 18v using a CC/CV DC power supply.

Should I be using different charge settings?

Shouldn’t this pack be settling on full charge at about 1.42v x 12 = 17.04v ?

And your avatar……is that your pet or just an internet photo?

$2 per cell is suspiciously cheap so i wouldn’t be surprised if internal resistance is relatively high. (Years ago, decent sub Cs were $5ish each.) Fresh off the charger, a fully charged cell will be between 1.40V-1.45V however, the voltage drops a little initially. Mine (AAs) would settle at ~1.39V in a couple hours. 400mA charge current is low. A 1C charge current would be 3A for that pack. I can’t remember the correct way but do know that they are not changed the same way as LiIon chemistry ie CC/CV. Weather this is a factor in underperformance or not, I’m not sure.

The cat is from the Internet.

If there is a bad cell it will have lower voltage.
The cells it is connected to are not going to do anything more to charge it than the charger would.

You think the good cells will bring up the voltage of the bad cell.
This is the flawed thought.

The bad cell will exhibit lower voltage when you test the cells individually,
regardless of them being connected in series.

Thanks. Just the explanation I needed.
And this would also tell me the batteries are remarkable balanced as far as voltage.

Even after additional charging, the pack always seem to eventually settle back down to 16.45v total pack voltage.
So it seems 1.371v is their max charge capacity.

I also re-checked each cell and found each cell to measure 1.37v

Thanks to everyone for their help and advice. Great forum!

I think one thing I’ve learned is always check the batteries individually when I receive them BEFORE making a pack out of them.
Also learned a lot about testing batteries in a series pack.

Zebretta my curiosity is piqued. What is this pack for?
Is it for a handheld vacuum?

sounds like the pack is ok to me.
does it give the expected runtime?

Very good guess.
Hoover Impulse Cordless Power Mop

Yes. My wife likes to use it. No complaints from her over this. Seems to run as long as she needs it to.

I was just expecting a bit higher voltage. 12 x 1.2v (fully charged to 1.42v per cell I thought would be around 17.04v or so fully charged)

:smiley: :+1:

I think these cleaning tools are the only things that take such a cell configuration and voltage.
Usually you will get the best battery longevity by unplugging the charger right after the light goes off. It seems if the battery lasts more than 18 months before starting to rapidly decline, you are lucky. I don’t know why Lithium cells are not standard in equipment like this. Maybe they don’t want the tool to last too long so they can sell you a new one every so often.

Did you build this pack yourself to replace one that failed? If so where did you get the cells?

Yes, I built the battery pack because we had a brand new one in storage but the battery was kaput.
The batteries were bought on ebay. (I know…bad place to get batteries)

I’ve never seen the green “full charge” light. I don’t think this battery pack reaches a high enough voltage to trip the green “full charge” light so I time it and then check the voltage.

Maybe they don’t use lithium cells due to the risk of fire?

ok thx. May try this soon.