SOC under light load vs no load

jfinet
jfinet Registered Users Posts: 5 ✭
in Solar Beginners Corner #1
My new solar system seems to be working pretty well ... but I am still trying to understand how it all should work ... and what my actual battery capacity is ... I have light loads normally when we try to conserve ... like 0.6 amps ... for fridge/propane detector/radio memory/and instrumentation (Trimetrics/MS CC/Remote ) ... so with solar off for 3 days (a test) ... the system used 35 ah ... and the voltage read 12.5 v ... how would you interpret the 12.5 v ... as a unloaded voltage ... or voltage under load ... most of the charts for voltages under load are assuming a significant load ... but my load is so modest ... it is almost unloaded ... using 220 ah for my two Trojans ... the Trimetrics calculated 84% or 16% down ... and I am trying to match the 12.5 v with this ... appreciate any input on this ... John
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: SOC under light load vs no load

    My first question--The light loads of 0.6 amps. Is that at 12 VDC or 120 VAC (note that 0.6 amps at 120 VAC is about 6 amps at 12 VDC).

    A very lightly loaded battery is probably close (enough) to no-load.

    Do you have a hydrometer? Good temperature corrected specific gravity is always the gold standard. And you should be running checks on all cells about once per month (SG and water levels--Log the temperature corrected SG per cell).

    The Battery Monitors are estimates... Sometimes very good (or at least good enough), sometimes not so good. It is a good idea to double check with a hydrometer anyway. How the Battery Monitor resets to 100% (after a few hours of absorb, etc.) can vary between installations/configurations. You don't want the BM to lead you astray.

    Measuring resting voltages--Always a rough guess. But you should be looking for something strange... If you never pull your batteries below 12.2 volts, and you see them at 11.5 volts one day--Check everything--Something may be wrong.

    Similarly if your 6 volt battery is balanced at 6.25 volts each, and one day there is a 0.5 or 1.0 volt spread--Again, something may be wrong.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jfinet
    jfinet Registered Users Posts: 5 ✭
    #3
    Re: SOC under light load vs no load

    The amps are 12 vdc ... I do not have an inverter ... and because it was cool and nothing in fridge ... it was probably closer to 0.5 amps ... so the 35 a-h number seems right ... I have used SG tester to check batteries ... with full charge ... and they looked good ... I should have put a multimeter on the batteries to see what the 12.5 v actually was ... could have been 12.54 v which with open circuit charts would be very close to the 84% ... might look for a better SG tester ... and try using that at the same time ... John
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