Top balanced new cells, multiple cells bottoming out, causes?

Shale MacGregor

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I did capacity test on 4 cells to test and found them all in the 308-312 range. I put together a 16s pack of 304a cells. First I top balanced them all to 3.6 together. At the start of testing they were all in the 3.431-3 range. After ~ 100ah discharge I had a low voltage shutoff by BMS. One cell was at 2.5, and two others were at 2.9 and 3.0. I checked and all of the lugs were tight and I verified the voltage using multimeter to make sure it wasn't an issue with a bad connection.

The discharge was at around .25C max during test.

What am I missing besides these 3 cells being lower capacity?
 
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sunshine_eggo

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I did capacity test on 4 batteries to test and found them all in the 308-312 range. I put together a 16s pack of 304a cells. First I top balanced them all to 3.6 together. At the start of testing they were all in the 3.431-3 range. After ~ 100ah discharge I had a low voltage shutoff by BMS. One cell was at 2.5, and two others were at 2.9 and 3.0. I checked and all of the lugs were tight and I verified the voltage using multimeter to make sure it wasn't an issue with a bad connection.

The discharge was at around .25C max during test.

What am I missing besides these 3 batteries being lower capacity?

Terminology: the individual 3.2V LFP elements are cells. "Battery" typically refers to the collection of cells forming the energy storage device.

Aside from that - The obvious:

You verified by individual test that all 4 cells had capacities ranging from 308-312Ah, and you're surprised when the 4S battery behaves as though the 4 cells have different capacity... 😁

Also, when you group dissimilar cells, the battery capacity will be no larger than the WEAKEST cell, so your 4S battery will only have 308Ah.

In a perfect world, you would note:
The 2.5V should be the 308Ah cell.
The 2.9 and 3.0V cells should have had progressive more than 308Ah but less than 312Ah.
The highest voltage cell that you didn't mention should have been the 312Ah cell.

Lastly, it's worth noting that you confirmed your cells vary about 1.2%, but this is all that it takes to cause wildly different voltage behavior at the extremes.
 

Shale MacGregor

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Terminology: the individual 3.2V LFP elements are cells. "Battery" typically refers to the collection of cells forming the energy storage device.

Aside from that - The obvious:

You verified by individual test that all 4 cells had capacities ranging from 308-312Ah, and you're surprised when the 4S battery behaves as though the 4 cells have different capacity... 😁

Also, when you group dissimilar cells, the battery capacity will be no larger than the WEAKEST cell, so your 4S battery will only have 308Ah.

In a perfect world, you would note:
The 2.5V should be the 308Ah cell.
The 2.9 and 3.0V cells should have had progressive more than 308Ah but less than 312Ah.
The highest voltage cell that you didn't mention should have been the 312Ah cell.

Lastly, it's worth noting that you confirmed your cells vary about 1.2%, but this is all that it takes to cause wildly different voltage behavior at the extremes.
Guess I left out some information.

The rest of the cells were in the 3.2-3.3V range and as stated only about 30% of capacity had been drained from the pack as a whole, so while a minor deviation is expected in terms of 1-3% capacity difference, this is a completely different range.
 

sunshine_eggo

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Sorry. Some reading fail on my part. You put enough in there, but I missed it. My apologies.

If you're saying that all 16 cells were 3.431-3.433 at the time the test started, and they had been off charge for at least an hour, I would individually test the 3 cells in the same manner as those tested yielding 308-312Ah. They sound like they are severely under-spec. Personally, I would be testing each one of them.
 

Shale MacGregor

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Yeah that's what I will end up doing. I had cut some corners because these were new cells and I figured I would save time... always end up learning that lesson the hard way.

Thanks!
 

time2roll

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I would start by simply charging the battery and verify the top balance. Then move to testing the low cells.
 

fafrd

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I would start by simply charging the battery and verify the top balance. Then move to testing the low cells.
I’ll second that and also suggest the internal resistance of each cell be measured in-situ (delta_V / delta_I from both applying load and applying charge current).

What was the discharge current used when running your capacity test and what was the discharge current used when the low cell hit LVD?
 
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