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

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

I have a battery pack that seems to have a bad cell in the pack. I say that because it doesn’t seem to charge to it’s full capacity but to the capacity I would expect if one cell were bad.

Of course, the battery pack is 12, 1.2v NiMh sub C cells so 1.2v x 12 = 14.4 and fully charges I would expect ~ 1.4 × 12 = 16.8v

Would charging the pack fully, then testing the voltage across each cell to find the weak cell work?

Thanks

Lexel
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To find the weak cell its better to have it discharged till it cant deliver much current
The good cells should show 1.2V the bad below 1V

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EasyB
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It might just be that the pack has a bit less capacity than it’s rated for.

If one cell has much less capacity it would become full before the other cells when charging. The charger most likely would not detect the delta-V from this one cell, so it would continue to charge until all the other cells are full. The charge input might not even register as lower. The full charge would be input even to the bad cell, and it would get hotter than the other cells during charge as a result.

If there is a bad cell, testing voltage under load or Lexel’s suggestion would probably find it.

Zebretta
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Ok,
And I can just place the test leads on the plus and negative sides of the batteries while they are still connected in series right?
In other words, I do not have to disconnect the batteries from each other….
Being still connected in series wont affect the readings correct?

EasyB
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Zebretta wrote:
Ok,
And I can just place the test leads on the plus and negative sides of the batteries while they are still connected in series right?
In other words, I do not have to disconnect the batteries from each other….
Being still connected in series wont affect the readings correct?

Correct.
Zebretta
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Thanks!
Your assistance is appreciated lexel and Easy

snakebite
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Measure voltage of each cell looking for one or more significantly lower than the others, at 0v, or worse, reversed.
If all ok repeat under a load like a taillight bulb.

Zebretta
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The problem I am having is that all of the cells seem to have to same voltage. Why? Because the higher voltage cells seem to lend their voltage to the weaker cells until they all balance out. All the cells show the exact same voltage regardless.

When I charge it then stop charging, it will keep dropping in voltage until it finally settles back at around 16.65v
But since it’s 12 × 1.2v cells, it should charge to and hold around 17.4v (which is actually about a 1.2v difference btw)

Taking the battery pack apart and resoldering it would be a major task.

Are you certain it’s possible to test individual cells that are in series?

EasyB
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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.

ARsee
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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.

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

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

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eebowler
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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! 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.

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Zebretta
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EasyB wrote:
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.

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.

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

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

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

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?

Zebretta
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eebowler wrote:
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! 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.

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 ?

eebowler
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\$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.

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Angler
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Zebretta wrote:
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.

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.

Zebretta
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Angler wrote:
Zebretta wrote:
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.

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.

Zebretta
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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.

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

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

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

Very good guess.
Hoover Impulse Cordless Power Mop

Zebretta
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snakebite wrote:
sounds like the pack is ok to me. does it give the expected runtime?

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 × 1.2v (fully charged to 1.42v per cell I thought would be around 17.04v or so fully charged)

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

Very good guess.
Hoover Impulse Cordless Power Mop

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?

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

Very good guess.
Hoover Impulse Cordless Power Mop

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?

Zebretta
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Lexel wrote:
To find the weak cell its better to have it discharged till it cant deliver much current The good cells should show 1.2V the bad below 1V

ok thx. May try this soon.