Find bad Cell (wrong resistance?)

[henk_lifepo4]

Hi,
I am a newby to building batteries so excuse me if I ask a stupid question?
I have build a 4s2p battery off 8 220 Ah cells wih a JBD 150 A bms.Total capacity 440Ah. In rest the cells are within 10 mV. But now I see that one cell (so consisting off two "units" ) triggers the 2.5v low thresshold when I switch on the Nespresso which draws about 110 A peak.
1. Am I right that (at least) one off the cells probably is bad.
2. Can i identify the wrong cell by diasembling the original battery, top balancing all cells and reasembeling it into two seperate bateries each just 4s with one off the disbehaving cells Then when I start the Nespresso one should be ok and one should trigger the 2,5v low voltage setting?
3. Is there a better way to identify the wrong cell?

Kind regards
Henk.

 

[sunshine_eggo]

You have a 2P4S battery. 4S2P implies two separate 4S batteries in parallel.

Before you go nuts, triple check that all your battery connections are properly torqued, and your BMS sensing leads are high quality (not corroded, loose or have bad crimps).

If it's not a connection problem:

1. Sounds like it.
2. Mostly. After a top balance, you might not see any issues for awhile depending on how series the issue is. You might have to use one battery for awhile before the outlier behavior crops up.
3.Not better, but maybe a little faster with improved usability: Parallel the cells and charge them overnight. Don't mess with a full top balance unless you want to and then shuffle the cells. Pair the two 2.5V cells with other cells. The problem should move with the bad cell. Example:

A & B are parallel cells
1-4 are the positions, thus:

1A1B 2A2B 3A3B 4A4B

If 4A4B is the 2.5V "cell", reconfigure as follows:

Move 4B to the 1B position and move the 1B cell to the 4B position:

1A4B 2A2B 3A3B 4A1B

If the 4A cell is bad, cell 1 will become problematic. If the 4B cell is bad, cell 4 will become problematic.

Once you identify the bad cell, it's probably worth breaking them out into two separate batteries with their own BMS to prevent future issues and have improved redundancy.

 

[henk_lifepo4]

Hi, thanks for the advice. I will do as you suggested. I'll post the result in some days. I also just ordered a second bms to make two 4S 220Ah batteries.
These two I can then just connect the BMS in parallel to have 12v 440Ah again? So just like with two lead-acid batteries both negative to one negative terminal and both positive to one plus terminal.
Henk

 

[henk_lifepo4]

Hi,
I recived the second BMS and build two batteries now. One is working ok. The other has the bad cell. But it is not going up to to fast now I think. When the bad cell hits 3.650 the others are at 3.480 and more. So the Battery is 100%.
Aside from replacing the cell is an active balancer maybe a cheaper option? If yes which one?

 

[sunshine_eggo]

Hi,
I recived the second BMS and build two batteries now. One is working ok. The other has the bad cell. But it is not going up to to fast now I think. When the bad cell hits 3.650 the others are at 3.480 and more. So the Battery is 100%.

Is this 3.65V cell also the first to drop out at empty? I got the impression from your original post that the cell was dropping out due to loads. Is this still happening?

Aside from replacing the cell is an active balancer maybe a cheaper option? If yes which one?

Active BMS aren't as amazing as their specs imply. While active balancers advertise 2, 4, 6 or 10A, there's something hidden there. Most commonly, that's if there's a full 1V different between cells. At the typical 5mV difference in resting cells, one won't get any balancing at all. As cells start to diverge at the top of the charge, one might see 200mA, 3-6X better than your typical passive BMS but nothing compared to the charge and discharge currents involved.

Are they a bad idea? No. Should you keep your expectations very very low? Yes.

 

[henk_lifepo4]

Hi I am testing the behavior under load right now. As i have not build the cells in my rv yet I can now test only with a 150 W inverter. I'll build them in the Rv next Saturday so I can test wifh the 1500W inverter and loads up to 110 A. ( nespresso machine).

 

[henk_lifepo4]

First strange things I noticed that both batteries went down very fast. In 3 hours from 90% to 50 % with a load of just 11W.
I have one jbd 150A and one 200A in parallel. The cells kept in the same range with just a max 4 mV difference.

 

[sunshine_eggo]

First strange things I noticed that both batteries went down very fast. In 3 hours from 90% to 50 % with a load of just 11W.
I have one jbd 150A and one 200A in parallel. The cells kept in the same range with just a max 4 mV difference.

In the working range, this is always the case even in substantial imbalance situations.

90% to 50% reported by what? You have two batteries and two BMSs, so they're likely going to report different SoC.