Battery Inquiry

jmtheman28
jmtheman28 Registered Users Posts: 7 ✭
in Solar Beginners Corner #1
Hello,
I am trying to size batteries to power an 800 Watt pump for 4hrs/day. I plan to use 5-Kyocera KD210GX-LPU solar panels. The max amperage of the pump is 7 amps which means I need a lot of voltage to achieve 800 watts. Can someone help explain how to size for AH based on this 4 hour requirement. The system is entirely DC. Thanks
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #2
    Re: Battery Inquiry

    7a x 4hrs = 28ah/day. 28ah x 3days (possible no sun)= 84ah. 84ah x 2 = 168ah to disallow going below 50% soc.
    168ah is minimal unless you can go shorter times for no sun, but i would not go below 2 days worth of battery power. 2 days worth would equate to 56ah x 2 for 50% soc = 112ah.

    edit to add:
    now if you average about 4-5hrs of sun per day then 1 or 2 of those pvs would cover the needs as each pv is rated 7.63a. 2 would be better and the charge rate with 2 of them would be about 9% for the 168ah capacity and 13.6% for the 112ah capacity. all 5 pvs might be too much for the battery unless going with something like a concorde agm like their sunxtender. all 5 will fry most standard batteries and would give higher maintenance minimally. 5 x 7.63a = 38.15a and with a 168ah battery is a 22.7% rate of charge.
    does this help?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #3
    Re: Battery Inquiry

    If I am understanding you--:
    • Power=Voltage*Current
    • Voltage = Power/Current = 800 watts / 7 amps = 114 VDC
    For standard Solar RE systems, 48 volt battery banks (~60 volts maximum when charging) is the standard maximum voltage...

    You could make a 120 VDC battery bank, and place two Solar Charge Controllers across the 1/2 each of the series battery bank and make such a system.

    However it is not standard and will run into some "Ground Fault Protection" standards...

    I would like to hear more about the pump itself and why you would not want to use an AC pump behind an inverter for this setup (and a 24 or 48 volt battery bank).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jmtheman28
    jmtheman28 Registered Users Posts: 7 ✭
    #4
    Re: Battery Inquiry

    I plan to use a Grundfos SQF pump capable of pumping 40-50 GPM with a 25' head. The pump will have water level switches wired in to turn the pump on if the level is too high and turn the pump off when the level is low (inside the sump). This pump has an MPPT in it. I guess I am trying to find the cheaper system, AC vs. DC, to power the pump and still work. One last thing I do not want to use a generator as back up. I could use an AC pump that I already have capable of those requiements. Thanks
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Battery Inquiry

    good catch bill, now if it is not 800w and is 7a at 12v (84w) then what i said applies.
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  • jmtheman28
    jmtheman28 Registered Users Posts: 7 ✭
    #6
    Re: Battery Inquiry

    So, since I am using the solar panels to charge the batteries I do not need to size the panels for the 800Watts to be used by the pump? If so then how do I size the panels to meet the needs of my system? Thanks for the replies I need to step out of the office to work in the field. Problem just came up. I plan to try and get back to you guys early next week. Thanks again.:D
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,641 admin
    #7
    Re: Battery Inquiry

    The Grundfos SQF pumps are very cool (but expensive).... They take both AC and DC power and have a MPPT/Linear Current Booster built into the pump itself...

    So, ideal, with this type of pump, you would want to use just solar panels wired to the pump+controller and avoid any batteries at all...

    The price for exactly the same system, with batteries, will double to 4x the kWhour pricing of your electricity (more losses from batteries and charge controllers, battery replacement every X years, etc.). Plus you now have battery maintenance (keep them clean and watered, make sure that the batteries are not depleted below 20% state of charge, etc.).

    I would only add a battery bank if you need pressurized water (house plumbing) or unintended pumping during bad weather (and you may still need a backup genset anyway--the batteries would only hold a few days worth of useful power anyway).

    The Grundfos pumps can be directly connected to an AC genset (backup or primary power)--So for non-sunny weather, you might need to bring out a small genset every few days (during bad weather) to keep the water level up--Or setup an autostart genset to pump during cloudy weather.

    I would avoid a battery bank if at all possible.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jmtheman28
    jmtheman28 Registered Users Posts: 7 ✭
    #8
    Re: Battery Inquiry

    If my pump requires 450 watts should I use:
    3 - Kyocera KD210GX-LPU 210 Watt Solar Panel (210 watt)
    OR
    3 - Kyocera KD185GX-LPU 185 Watt Solar Panel

    Would more wattage account for less direct sunlight, partly cloudy, etc.?
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