Charging lifep04 48v, individual cell max v while under charge

[David B.]

I'm running eve 304 cells @48v 16 cells top balanced. Charging using jkbms and sungold hybrid inverter set at 54v charging at 40 amps. The inverter has float which will not turn off so its set at 50v.
The question I have is, while charging the battery voltage is around 52v with cell voltage at 3.4v or about.
(I'm starting the charge manually and stopping manually until I get the parameters set) then when I start charging @40 amps going up 53.5v per the jkbms and the inverter, the jkbms shows individual cell voltage at 3.7v and rising. So I shut it off and per the jkbms the cells drop back to 3.4 or there about. So what is the issue here? The battery voltage is low at 52v to 53v with cells balanced at around 3.4, but the individual cells jump above 3.65 when charging. I this normal? Do I need to worry about individual cell voltage when charging or just the total battery voltage? If so how high can the individual cell voltage go before I get nervous?

 

[BentleyJ]

52.0V / 16 = 3.25V per cell not 3.4V. If one cell is "running" up to 3.7V then the cells are no longer balanced. Probably because the cells are not getting fully charged and/or the absorb cycle is too short for any meaningful balancing to occur.

There seems to be a discrepancy between the bulk voltage which should be equal to the sum of 16 cells. Has the BMS been calibrated? Its time to verify the readings with a meter, both bulk voltage and each cell voltage compared to the BMS reading.

 

[David B.]

Thanks for replying. When charger has been off for say 20 minutes the battery voltage and cell voltage are correct. bms has been calibrated. Just when I'm putting 40 amps in that things get dicy. Cell voltage is rising very quickly while battery voltage is rising slower. Cell voltage with a calibrated meter quickly exceeds 3.65 while charging. But as stated falls back to normal readings. I probably misled you when I stated 3.4v I'm turning the charger on and off while watching the voltage. I should have said 3.2 or there about. Still though should individual cell voltage go up faster under charge while total battery voltage lags behind?

 

[BentleyJ]

That's one of the big surprises with Li chemistry batteries. Once a cell reaches full charge the voltage will run higher very fast while the others are still absorbing the current with stable voltage.
The cells either need to be top balanced again or the battery needs to float at low current while the balancer is given time to work. If you have identified the charge voltage that caused the high cell(s) to run up. Set the absorb voltage just below this value and reduce current to 10A or whatever the battery can tolerate without pushing any of the cells over the cut out limit. Make sure the balancer is enabled and working. With 304Ah cells it could take a while with a 1 or 2A balancer. A while meaning 24hrs or more.

The goal is to get all 16 cells to a minimum of 3.45V (55.2V) with a delta of 10mV or less. At his point the tail current should be low, 6A give or take.

 

[David B.]

The solar wizard you are, thanks.