Internal Resistance Changes

[SailingInfidels]

I've had my DIY bank (400ah, 4P4S, Sinopoly cells) running for nearly four years now, and I am starting to see some weird things with the internal resistance of a couple of cells.
Since just before Christmas, I've noticed that a couple of cell (pairs) have had slightly elevated IR over the other two (0.6 vs 0.14 highest and lowest).
Twice now, while checking the battery via the BMS software (Orion Jr II), I've watched the IR of all the cells, for no apparent reason, suddenly change to .25 mohms for all cells.
Yesterday, the highest cell resistance, during discharging at 20-25a, over a few hours, went to 1.92 mohms.
Now I'm getting concerned.



First, how do I pinpoint what the resistance of each of the cells in the pairs is?


If I can do that, I will also know if it is the BMS or the cell that is the actual problem.


Thanks in advance for any advice.

[Nicholson58]

Talk to the vendor.

Make sure all battery terminals are clean and tight.

[SailingInfidels]

Originally Posted by Nicholson58

Talk to the vendor.

Make sure all battery terminals are clean and tight.

Terminals are clean and tight. I can't talk to the vendor, until I know whether it's the BMS or the cells.

[Jammer]

Originally Posted by SailingInfidels

First, how do I pinpoint what the resistance of each of the cells in the pairs is?

https://www.fluke.com/en-us/product/...zers/fluke-500


They're expensive. Find a contractor who has one, or purchase one of the various cheaper alternatives.

[wholybee]

The only way to measure the IR of cells in parallel is to break them apart and test them individually. There are much cheaper meters than the fluke. I wouldn't consider them accurate, but certainly good enough to find a problem cell from a group.

https://www.amazon.com/allsun-Batter...dp/B0795D9PDY/

It is also possible to calculate the IR by measuring the voltage drop while discharging a cell and applying ohms law. You know the voltage drop, and you can measure the current, so you can calculate the resistance. Note that the IR is not constant and varies on SOC. So, all the cells need to be at the same SOC when comparing them.

[SailingInfidels]

Originally Posted by wholybee

The only way to measure the IR of cells in parallel is to break them apart and test them individually. There are much cheaper meters than the fluke. I wouldn't consider them accurate, but certainly good enough to find a problem cell from a group.

https://www.amazon.com/allsun-Batter...dp/B0795D9PDY/

It is also possible to calculate the IR by measuring the voltage drop while discharging a cell and applying ohms law. You know the voltage drop, and you can measure the current, so you can calculate the resistance. Note that the IR is not constant and varies on SOC. So, all the cells need to be at the same SOC when comparing them.

Thanks very much, I will have to figure out a way to that into Mexico at a reasonable price.