upgrading my power pack

bmet
bmet Solar Expert Posts: 630 ✭✭
in Solar Beginners Corner #1
I am upgrading from my 'toy' power pack. I wanted to do it big, but since my g/f lost her job in time for the holiday my budget shrank. :cry:

I bought an 80 watt, laminate panel (cheap) and I want to match it with an appropriate deep-cycle battery. It's specs are,
80W
18.0 Voc
4.40 Isc
16.20 Vmp
3.96 Imp

On average throughout the year I have 4 good sun hours a day. Using the fast formula I've read elsewhere in these forums, I cut my wattage in half, then multiply it by 4, for 160 watts.

What is the next Math I need to match the 5-13% charging range for a proper battery?

Thank you.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,714 admin
    #2
    Re: upgrading my power pack

    Roughly for a 12 volt battery bank using the 5-13% rule of thumb:
    • 80 watts * 1/14.5 volts * 0.77 derating * 1/0.13 rate of charge = 33 AH
    • 80 watts * 1/14.5 volts * 0.77 derating * 1/0.05 rate of charge = 85 AH
    The Vmp is a bit low (and Voc is surprisingly low)--So in very hot weather, the panel may not produce enough voltage to quickly recharge the battery (by the time controller and wiring voltage drop is taken into account).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bmet
    bmet Solar Expert Posts: 630 ✭✭
    #3
    Re: upgrading my power pack

    Thank you BB

    I got what I paid for. It'll only get used a few hours a month, so there are long stretches for recharging. I was figuring a 65 AH rating based on the 85% efficiency of my controller(all my wire runs are less than 3 feet). I'd rather get an 85 AH battery for future upgrade.

    Thanks for writing out the math. I was searching for this through several threads last night, and didn't hit any upon any of them:confused:
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,714 admin
    #4
    Re: upgrading my power pack

    You are very welcome... Sometimes I try to search for a math equation that I have forgotten--and it seems almost impossible to find--usually I try to find posts that are in context using Google and the "site:wind-sun.com" search flag. Something like this Google Search
    • 5% to 13% rule of thumb site:wind-sun.com
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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