Strange BMS cell voltage readings

squowse

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I have put together a 16S 272Ah Lishen cell pack for testing. I neglected these cells a little so not sure what to expect. Some of them were stored for quite a few months empty.

Doing the capacity test, I've rigged up the computer to the UART interface of Jiabaida (JBD) 6S-21S 150A BMS. Model AP21S001.

Early in the test at approx 18.6A I noticed that cell 3 was close to 3.0V already. When checked with voltmeter it measured about 3.17, lower than the others at approx 3.28, but not as drastically low.

Then - even weirder the computer display showed it up at 3.28ish. But the voltmeter still read it close to 3.19!

It's continuing to fluctuate up and down, approximately every minute or so. Actually mostly reading 3.27 now. I will be able to get the plot once the test has concluded. Possibly early, as the BMS may well trip out for low cell voltage.

I've checked the other cells and all read near enough the same on voltmeter as on computer. All 3.28ish with minimal variation so far.

Has anyone experienced something like this? If it was a bad connection (ive checked tightness) how would the voltage end up reading higher?

If the BMS voltage ADC was bad onthat channel - it's a coincidence it happened on the "bad" cell?

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squowse

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So I decided to switch cells 2 and 3 and now it's behaving itself. So far. Unfortunately it's done the thing where it resets the SoC to its bad guess based in voltage. So I really need to charge back up again so I can do the full cycle to initialise the BMS SoC measuring. Very annoying you can't manually input the SoC and force it to use the shunt only.
 

squowse

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Charging back up to 100% and cell2 is now having a fluctuating reading - up to about 100mA either side of the multimeter reading, about every 4-5 seconds now. I wonder if this could be linked to the BMS balance lead wiring? I've checked for loose connections.

When I first switch it on, it auto adjusts itself to 17 cells. I have followed the instructions and also this diagram -
My model is slightly different to others in that the first connector has 15 pins, and the second one 7 pins.
 

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squowse

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I think that may have been the balancing, I could see how you might get a variation each end of the balance cable if it's doing something with it at the BMS end?

Anyway Im back on the discharge test, and now the cell 12 reading is fluctuating. It's the lowest cell but the reading drops 200mV for no apparent reason.
 

RCinFLA

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If cell voltage readings go significantly different with load current, there are a several possible reasons.

First are some poor terminal connections. A cell terminal connection with higher resistance will add to voltage reading when current is applied. Make sure the sense wiring is done consistent so there is not more than one bus bar between adjacent BMS sense wires.

Cells with low state of charge will have greater internal impedance so will drop more in cell voltage with load. Same applies to charge current.

A poor condition cell, even at same state of charge, will have greater cell internal impedance causing greater cell voltage slump with load current.
 

squowse

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Jan 4, 2021
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392
If cell voltage readings go significantly different with load current, there are a several possible reasons.

First are some poor terminal connections. A cell terminal connection with higher resistance will add to voltage reading when current is applied. Make sure the sense wiring is done consistent so there is not more than one bus bar between adjacent BMS sense wires.

Cells with low state of charge will have greater internal impedance so will drop more in cell voltage with load. Same applies to charge current.

A poor condition cell, even at same state of charge, will have greater cell internal impedance causing greater cell voltage slump with load current.
How does that fit with the voltmeter readings being normal? I even put the probes on the balancing lead lugs.
 
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