Confused about battery power and voltage

sub3marathonmansub3marathonman Solar Expert Posts: 300 ✭✭✭
I thought I've done all the correct calculations, but they don't seem to be working out in reality.

The battery bank is 4 Rolls/Surrette 12CS11PS, for 48 volts. Each battery is rated at 357ah at the 20hr rate of discharge, which is 17.9 amps. Or about 17kwh for the entire fully charged battery bank.

Now, it was my understanding that a fully charged battery is at 12.7v, or 50.8v for the 48v system. So if you go down to 49v, that should be about 40% discharged. Which should still be OK for the battery. Also, the XW6048 settings are set at 1.2v higher (my understanding of Solar Guppy's recomendation), at 52v for "sell" and when I tested it I set "sell" down to 50.2v.

However, it seems that it drops to 49v much too quickly, without supplying much electricity at all, maybe 1kwh, or about 5% of the capacity.

Am I doing something wrong? Or have I made a miscalculation?


  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Re: Confused about battery power and voltage

    Are you measuring the voltage under load or after ~3 hours of rest (no charge, no loads)? The resting voltage would be the "accurate" measurement. Also, is the DVM calibrated--there have been a few times here when it turned out the DVM's where not accurate.

    Have you checked each cell/battery voltage individually?

    Do you have a temperature corrected hydrometer? That would be the "gold standard" for estimating your battery capacity (or a battery monitor).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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