SimpBMS
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Tom-evnut
Problems with balancing - or just not understanding it.
I have added 4 more Tesla module to 4 which I already had in a power wall.
Two modules in series.
Now from the new modules there is one with two cells being 100mV above the others.
When I was charging the whole pack the first time the lowest cell did not go above 3.77V. While the some cells went higher. The highest cell went up to 4.01. Then the BMS shut the charging off because of imbalance.
Balancing was set to 3900mV and soon stopped doing so. So I lowered it to 3500mV and experimented the whole day with short strong charges, strong discharges, slow discharges and right now with a 3A charge which I will leave over night.
What really confuses me is: no matter what I do, the lowest cell never passes 3.77V while charging. I have attached some diagram and readings from simpBMS the "new" modules are the bottom 4.
Sometimes after burst charges the difference between the lowest and the highest decreased, while still 3.77V was not passed. But after that it increased again on discharge. I'm talking 10mV +/-.
Does it balance on discharge? What are optimal circumstances for balancing? I'm a bit lost here...
Figured on thing out. Module 5 was responsible for hanging on 3.77 because one pole did not conduct to the aluminum bar used for connecting the modules. Neither did the original screw. Changed the screw, now it charges. Will later change the bars to copper... Module 7 has stil 90mV differences, but now I let the pack charge at 80 Amps and will see what happens.
Almost a day later the difference between highest and lowest cell, fluctuating dependent on battery current and direction, is still not much smaller than before.
It's definitely getting worse. Any suggestions or information on how to get this to balance are appreciated:
And here a BMS reading when the battery stopped charging
BMS Status : ESS Mode : Happy |Balancing Active 48 Out:1100 Cont:0110 In:0000 Charge Current Limit : 4 A DisCharge Current Limit : 280 A
Modules: 8 Cells: 48 Strings: 4 Voltage: 47.985V Avg Cell Voltage: 3.999V Low Cell Voltage: 3.959V High Cell Voltage: 4.070V Delta Voltage: 111mV Avg Temp: 18.249C
Module #1 23.99V Cell0: 4.00V Cell1: 4.00V Cell2: 4.00V Cell3: 4.00V Cell4: 4.00V Cell5: 4.00V Neg Term Temp: 18.55C Pos Term Temp: 18.54C Module #2 23.99V Cell6: 4.00V Cell7: 4.00V Cell8: 4.00V Cell9: 4.00V Cell10: 4.00V Cell11: 4.00V Neg Term Temp: 19.27C Pos Term Temp: 17.52C Module #3 23.99V Cell12: 4.00V Cell13: 4.00V Cell14: 4.00V Cell15: 4.00V Cell16: 4.00V Cell17: 4.00V Neg Term Temp: 19.21C Pos Term Temp: 17.54C Module #4 24.00V Cell18: 4.00V Cell19: 4.00V Cell20: 4.00V Cell21: 4.00V Cell22: 4.00V Cell23: 4.00V Neg Term Temp: 18.68C Pos Term Temp: 18.66C Module #5 24.01V Cell24: 4.00V Cell25: 4.00V Cell26: 4.00V Cell27: 4.00V Cell28: 4.00V Cell29: 4.00V Neg Term Temp: 19.06C Pos Term Temp: 17.46C Module #6 24.01V Cell30: 4.00V Cell31: 4.00V Cell32: 4.00V Cell33: 4.00V Cell34: 4.00V Cell35: 4.00V Neg Term Temp: 17.63C Pos Term Temp: 16.26C Module #7 23.98V Cell36: 4.07V Cell37: 3.96V Cell38: 3.96V Cell39: 3.97V Cell40: 3.96V Cell41: 4.05V Neg Term Temp: 17.91C Pos Term Temp: 17.73C Module #8 23.96V Cell42: 3.99V Cell43: 3.99V Cell44: 4.00V Cell45: 4.00V Cell46: 3.99V Cell47: 3.99V Neg Term Temp: 18.93C Pos Term Temp: 19.03C CANbus 843.44mA 90% SOC 764399.94mAh