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Normal? Battery voltage drops quickly after charging stops.

apctjb

Solar Enthusiast
Joined
Jun 16, 2020
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I have a 48V-560AH LiFePO4) battery bank (2-EVE 280 cells in parallel , 16 series) solar only charging and float set to 54.5. I notice that within 30 mins of sun going down and no load (other than tare loss of inverters ~75W) that the voltage drops from 54.5 down to 53.8 and then levels out. See graph of battery voltage for yesterday below.

So is it normal for LiFePO4 or indicative of a problem with my battery bank?

Are the voltage/SOC curves and tables, like the one below , indicative of when the battery is at rest or under charge or shouldn't it matter.

1661308499415.png

1661308641389.png
 
That 53.8 sounds like the resting voltage of 3.33 volts per cell. The higher float voltage maintained them above resting voltage until the sun went down.
 
sounds like the resting voltage of 3.33 volts per cell

Thx, but not fully following. Are you saying it is normal for voltage to drop to 3.33 when at rest ?
 
Yes. What voltage were the when you got them?
Not sure, going on 2 years ago now. So if 3.33 is typical can I assume then that the SOC tables estimate SOC when charging and not at rest?
 
The voltages in that table are when under charge. The voltages at the lower part of the table look to be under discharge.. That is why voltage tables are not a reliable indicator of State of Charge.
 
The voltages in that table are when under charge. The voltages at the lower part of the table look to be under discharge.. That is why voltage tables are not a reliable indicator of State of Charge.
Thanks, very helpful. I went back through data logger and it appears that the resting cell voltages on the first day I installed the system was 53.3 so 3.33 V/cell average.
 
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