locked Re: Batteries starting to get weak - MMM - What to do -

Scott SV Tengah

Does anyone understand "continuous cell balancing", because I don't?

When I have had unbalanced batteries, the batteries are look perfectly balanced until you get to 90% + SOC. This is because of the flat voltage curve of lifepo4. Above that, the imbalances show up. How does an active balancer reliably detect an imbalance continuously when things look perfect below 90%? Through coulomb counting to determine amps out to see if the imbalance shows up during discharge? That wouldn't be reliable as cells age at different rates. 

Note that imbalanced batteries generally just reduce your total capacity until the imbalance is corrected. This is because if one of the 4 cells in the battery see an overvoltage condition, the BMS should stop charging altogether, even though the other cells could still accept more charge. Charging is restarted once the overvoltage cell drops below the high voltage threshold, either through a resistor to draw charge from it or a method to transfer that excess voltage to the other cells. Then the charging resumes. The charging will start and stop multiple times as during the cell balancing process. I have seem this before on my Victron setup and have been assured by Victron techs and my own research that it is totally normal.

The desire for active cell balancing seems a bit misplaced. Even if somehow they detect cell imbalances when there is no voltage difference, you still need to fully charge once in a while to reset the battery monitor as I believe all battery monitors use (amps in - amps out) to determine SOC and as batteries age, that needs to be recalibrated if you want any sort of accuracy. So solving the former problem still leaves the latter.

2007 A54 #69
SV Tengah

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