Burping the battery

Re: Burping the battery

Well--You can do some paper models for panel amp*hours... For example with a PWM controller, a 175 watt Vmp~17.5 volts and Imp~10 amps (round numbers for ease of typing). Assuming 4 hours of "full noon time sun" per day for 9 months of the year, the amount of Amp*Hours per day for a PWM controller on a 12 volt battery bank would be (note the 77% derating maybe too much--perhaps 90% would be closer--but don't know for sure):

• 10 amps Imp * 0.77 derating * 4 hours of sun per day = 30.8 Amp*Hours per day into a 12 volt battery bank
• 10 amps Imp * 0.90 derating * 4 hours of sun per day = 36.0 Amp*Hours per day into a 12 volt battery bank (lower derating losses)

The same thing running an MPPT controller:

• 175 watts * 0.77 derating * 4 hours of sun per day * 1/14.5 volts charging = 37.2 AH per day 12 volt bank.

So--The 0.77 derating is probably "wrong" for an Amp*Hour solution. Solar panels tend to derate by ~0.82 on warm days/actual Vmp depression (hot panels). Imp actually (very slightly) rises with hot panels.

And the MPPT controller output will vary by 20% or so between very cold and very hot days (and even based on your actual battery charging voltage--i.e., 13.5 volts charging vs 14.5 volts finish charging)... So AH prediction is a bit more nebulous.

-Bill