Correction, the AC IR's I reported earlier are wrong, due to not making good contacts to the copper tabs with my IR tester due to melted plastic insulators on the cells. After cleaning up the contacts, the AC IR's of the 2 bad cells now measures 59 and 152 milli ohms respectively. I either over heated these 2 cells during the build or during unsoldering. Most likely the latter because I inspected every cell after I finished soldering the pack and all insulators looked fine back then.
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Rebuilt battery and now its worse?
- Thread starter pishta
- Start date
If you poke around thru my posts on BMS balancers, there's at least one thread that has pictures to compare a few random BMSes that have or don't have balancers, to give an idea of how to tell with certain types.
Some don't have individual channel balancers as it's a charge pump system in the BMS chip itself, so you have to look up the ICs on the board itself...but even if the chip has the function, it doens't mean it's enabled in the firmware for those that are controllable that way.
So there is also some description in one or more of those posts of how to test whether balancing is going on or not.
BatteryMooch
Power poster
Pulse the cell at two different discharge current levels and measure the voltage change. Then use Ohm’s Law to calculate the equivalent resistance that would cause that voltage drop for that current change.
SOC and cell temp affects this (as it does for AC IR too).
The are three different DC IR’s though, just as there are a huge number of different AC IR’s depending on frequency, so you need to know which you want….ohmic, diffusion, and polarization…each occurring at a different time after the discharge starts.
BatteryMooch
Power poster
You can check for an array of larger resistors (and maybe tiny chips, MOSFETs) that equal the number of cells. These are what’s used to drain the cells down to balance them. The resistors are larger since they create more heat than any other resistors on the BMS except the ones for main current sensing.
Many BMS controller chips can do low current balancing using internal resistors though, perhaps up to 25mA average, so a lack of resistors doesn’t prove no balancing.
pishta
New-ish here
I have quite a few silverfish 36-48v batteries and I charged 4 to full capacity (per green light on stock charger). I can measure each P group once I get the case off the battery and I find 2-3 groups in each (bad per the vendor) battery are under 4v (3.6v) so Im assuming the BMS are not balancing the pack, but are just cheaping out and keeping the pack safe. The rest were consistant 4.19-4.20V. Ill segregate those low V groups and test each cell and find wtf is happening.
It may take quite a long time keeping the charger connected at full charge to balance the cells. Sometimes 12 hours, to days or even weeks in extreme cases.
What do you mean by
?
pishta
New-ish here
All these batteries were "in the back" and I asked if I could take them off his hands after seeing them all lined up in a corner. He said take them all! It saved him the hassle of disposing of them "properly".
