battery bank watt question

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edward87
edward87 Registered Users Posts: 4
in Off Grid Solar & Battery Systems #1
Ok i have been looking into pv systems to try and figure out if it would be a viable power source for a feed lot operation. I still have a ton of questions but what I can not seem to figure out currently is when calculating battery bank watts

battery bank trying to build is 24 trogan l16re-2v 2v 1110ah 20hr rate wired in series

my calculations are

1110 Ah 48v total
555 ah at 50% discharge
555/20 hr =27.5 amps per hour
if my math is correct to this point here is my question. When calculate watts do i multiply by 2v or by the total system 48v
27.5*2v=55 watts or 27.5*48v=1320 watts

and is that watts over 20hr or per hr
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  • microage97
    microage97 Registered Users Posts: 12 ✭
    #2
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    I believe your math is wrong. 24 2V 1110ah batts in series would be a 48V 1110ah bank.
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  • Mountain Don
    Mountain Don Solar Expert Posts: 494 ✭✭✭
    #3
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    edward87 wrote: »
    1110 Ah 48v total

    1110 AH X 48 volts = 53280 watt hours, because amps x volts = watts and therefore amp hours x volts = watt hours.
    Northern NM, 624 watts PV, The Kid CC, GC-2 batteries @ 24 VDC, Outback VFX3524M
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #4
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    edward87 wrote: »
    battery bank trying to build is 24 Trojan l16re-2v 2v 1110ah 20hr rate wired in series

    my calculations are [corrections are in bold]
    • 1110 Ah 48v total
    • 555 ah at 50% discharge
    • 555/10 hr =55 amps [just amps, not amps per hour--Note, you are discharging for only 10 hours at the 20 Hour Rate]
    if my math is correct to this point here is my question. When calculate watts do i multiply by 2v or by the total system 48v
    • 55a*48v=2,640 watts [for 12x 2v batteries in series discharged @ 20 hour rate for 10 hours to 50% capacity/state of charge]
    and is that watts over 20hr or per hr [Watts and Amps are already rates]
    • 2,640 Watts * 10 hours = 26,400 WH total "useful" 48 VDC energy to 50% discharge over 10 hours

    Note that Watts and Amps are already "rates" (like miles per hour).

    Watt*Hours and Amp*Hours are an amount (like Miles driven).

    Watts per hour and Amps per hour do not really make any sense in our day to day usage.

    Does that make a bit more sense to you?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • edward87
    edward87 Registered Users Posts: 4
    #5
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    There we go. thanks a ton guys. That really helps. New to this and was getting lost.
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  • edward87
    edward87 Registered Users Posts: 4
    #6
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    How would I go about calculating what run time would be for 20kw generator to recharge same bank from 50% to 100% Gen burns 1 gal at half load. Just thinking about if my input power can't keep up looking into wind pv most likely and won't really know 100% tell its on the ground running looking at 2 4kw 48v turbine units average wind speed in my area is 16mph. So I would assume they could keep up
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #7
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    You should charge these batteries around 10-13% rate of charge (the wide spec. would be around 5% to 20-25% rate of charge) of the 20 Hour Rate (as always, check the battery manual for exact answer). There are various reasons to pick a particular rate of charge (10% or 5% or 20%, etc.)... We can discuss if you wish.
    • 1,110 AH battery bank * 10% rate of charge = 111 Amp nominal rate of charge
    • 111 Amps * ~58 volts charging * 1/0.80 charger eff = 8,047.5 Watts from genset...
    Call the battery bank ~85% state of charge when battery goes from "Bulk" (can accept 100% rate of charge) to "Absorb" where the charger holds ~58 volts and the current slowly drops to ~2% charge rate (when battery is >~95% full.
    • 85% - 50% = 35% charge
    • 35% charge * 0.10 Rate of charge = ~3.5 Hours of charging.
    Very roughly, to finish from 85% to 95%+ state of charge would require the generator/battery charger to hold 58 volts for ~4 hours (perhaps as long as 6 hours).
    • 3.5 hours of Bulk + ~4 hours of Absorb = ~7.5 hours of generator run time.
    Of course, there are other ways of running your battery bank (solar, cycling 50-80% state of charge and recharging >90% once a week (some say even once a month).

    Or, you can bulk with the genset and use solar for the last ~15% or so charging.

    etc...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • edward87
    edward87 Registered Users Posts: 4
    #8
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    Wow you have great knowledge sir. I sure do appreciate it. I like the idea of Gen bulk charge then 15% from solar. Again thank you for your time. I have a lot of homework to do now.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,448 admin
    #9
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    Edward, you are very welcome... Note that I gave you some very rough numbers/estimates above--Good enough for back of the envelope calculations, but for any crictical needs (like predicting fuel consumption, choosing battery chargers, etc.)--A lot more research is needed.

    Batteries are the "weak point" for any off grid system. Some light reading:

    http://www.windsun.com/Batteries/Battery_FAQ.htm
    http://www.batteryfaq.org/
    http://batteryuniversity.com/

    As questions, if something does not make sense to you--Ask more questions. Details do matter. Rough estimates/guidelines are just that. As you size your system (conservation is important) and pick your hardware, you can replace the estimates with more accurate numbers.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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