Voltage...

plongsonplongson Solar Expert Posts: 115 ✭✭
If a flooded cell battery is fully charged according to a SG of between 1.255 and 1.275 what should the voltage be per cell (around 50*f)?

What I'm trying to do here is figure what the 100% voltage should be on my 48v system and where I should set my low volt cut-off at calculating for a 25% DOD. Where would you set the LVCO/gen start at???

Am I on the right track? I might go for a 50% DOD setting later but I'm a scared-e-cat right now!!!

Man, thanks for the patience!!

Paul
3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Voltage...

    Paul;

    To be accurate, individual battery makers set their own specific per cell Voltages depending on the particular model and its intended use. Otherwise the general rule for FLA's is 2.12 VPC indicates full charge.

    If you want to be certain of not going below 50% DOD using a Voltage indicator alone the best you can do is not let it go below the nominal system Voltage (48 ) while under load. 25% DOD would be roughly 49.5 Volts (2.06 per cell).

    Nothing like a battery monitor for on-the-go accuracy.
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Voltage...

    The problem is that measuring battery voltage that has had current flow (charging/discharging) and not "rested" for 3-24 hours or more--is not very accurate.

    A few folks here have recommended trying 46 volts (11.5 volts for a 12 volt bank) as a good cutoff-point. You might want to "filter" that for a few seconds to let well pump starting surges/etc. not hit the alarm/cutoff/start the genset early.

    Some charge controllers have a Low Voltage Cutoff (intended for protecting loads) that also takes current flow into account (higher current, lower set point).

    I am not sure--but I would start around -5 mV per degree C per cell @ 25C ref as a starting point for a battery temperature offset... for a 48 volt/24 cell battery bank at 50F
    • 25C * 1.8 + 32F = 77F
    • 50F - 77F = -27F difference from Ref
    • 27F * 1/1.8F per C * -0.005 V/C * 24 Cells = +1.8 volt offset for 48 volt bank at 50F
    So, at ~50F, your charging set point should be ~1.8 volts higher, and, I would guess, your reference should be 46V+1.8=47.8 volts


    Note that I have seen other numbers of -2.7mV, -3.67mV, and -4mV per degree C per cell offsets...


    You should measure the temperature compensated specific gravity of your cells for more accurate readings.


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • plongsonplongson Solar Expert Posts: 115 ✭✭
    Re: Voltage...

    Ya know, I also have the Magnum BMK that has a SOC parameter. Maybe I'm getting off on the wrong foot and should use the state of charge parameter to safeguard the system.
    It appears one can use all three, SOC, AMP and VOLT parameter for the genstart.

    Have you had the chance to use the BMK and these features? Problem is, I'm not there all the time so it will be on autopilot sometimes for a week at a time.

    I think once i get it set up, I'll be less anxious.
    3500w solar, 800AH with Rolls Surrette, Magnum inverter, Midnite charge controller, Kubota 21kW diesel genset...private well...and just recently connected to city power for additional options...nice to have options 
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