6v or 12 v batteries?

kennedy759
kennedy759 Registered Users Posts: 10 ✭
in Off Grid Solar & Battery Systems #1
starting to think about my next off grid system, and want to set up a 24v system. some on here use 6v bat and some 12v, why are 6v used? is it cost? what about needing more bat cables to connect them?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: 6v or 12 v batteries?

    It's a matter of how much total capacity you need.
    Four 6 Volt 200 Amp hour batteries in series is preferable to two parallel strings of two 12 Volt 100 Amp hour batteries.

    A bit more detailed explanation here: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power
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  • Alaska Man
    Alaska Man Solar Expert Posts: 252 ✭✭
    #3
    Re: 6v or 12 v batteries?

    So is this right?

    copied from the link: More information:

    An additional note on choosing system Voltage.
    The largest available charge controllers at this time can handle 80 Amps output at a given Voltage. If you consider the rule-of-thumb 10% peak current and 25% DOD that would limit the battery bank for a single controller to 800 Amp hours. Thus we get stored power of:

    12 Volts @ 800 Amp hours = 2.4 kW hours DC @ 25% DOD We use about 220KW in a day right now..
    24 Volts @ 800 Amp hours = 4.8 kW hours DC @ 25% DOD
    48 Volts @ 800 Amp hours = 9.6 kW hours DC @ 25% DOD

    Above those numbers and you are needing more than one charge controller. The same size battery bank we have now at 48V would give us more than 3.5X the stored power?
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  • kennedy759
    kennedy759 Registered Users Posts: 10 ✭
    #4
    Re: 6v or 12 v batteries?

    I read thru the links and it still does not tell me in a short answer as to why some go with 6v and some with 12 v bat, I am looking at a 24v system
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #5
    Re: 6v or 12 v batteries?

    Is this true?
    We use about 220KW in a day right now.

    That is almost 1 month's worth of power for my home--Lots of conservation, I am a pretty cheap guy (moderate climate, not too much in the way of electricity hungry appliaces--Natural gas for cooking/heating).

    Anyway, yes that would be "nominal" for a single 80 amp charge controller. You can parallel two or more charge controllers for more current/larger solar array.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: 6v or 12 v batteries?
    kennedy759 wrote: »
    I read thru the links and it still does not tell me in a short answer as to why some go with 6v and some with 12 v bat, I am looking at a 24v system

    Look at the capacities of the batteries involved.
    If you want 200 Amp hours @ 12 Volts, for example, you could have two 200 Amp hour 6 Volt batteries in series which is usually a preferred arrangement over two 100 Amp hour 12 Volt batteries in parallel (fewer connections, less chance of current sharing problems).

    I think Alaska Man's 220 kW hours per day is a typo. At least I hope it is.
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  • Alaska Man
    Alaska Man Solar Expert Posts: 252 ✭✭
    #7
    Re: 6v or 12 v batteries?

    Doh'!! Yes, typo 220/wh


    Okay off to NAWS to check out 48V inverters. Then off to Craig's list to post pics of the Xentrex.

    This also means more PVs on my Midnight Classic 150 too, correct? Off to the use the M.C. string sizing tool as well.
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  • Alaska Man
    Alaska Man Solar Expert Posts: 252 ✭✭
    #8
    Re: 6v or 12 v batteries?

    6V or 12V??

    I went with 6V because you get more storage with smaller voltage batteries. You should see the kind of amp hours you can get in 2V batteries.



    Hang on I'm confused............If I have 8x 6V 220 amp hour batteries wired for 48V then wouldn't my total amp hours go down?

    Right now at 12Volts I have 880 amp hours (440 amp hours useable at 50% DOD). Wouldn't wiring it for 48V bring that down to 220Amp hour 110 useable?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #9
    Re: 6v or 12 v batteries?

    8x 6 volt @ 220 AH batteries as a 12 volt bank:
    • 12 volts * 880 AH (two batteries in series x 4 parallel strings)
    8x 6 volt @ 220 AH as a 48 volt bank:
    • 48 volt @ 220 AH (all batteries in series)
    Note that the stored energy is the same (based on P=V*I):
    • 12 volts * 880 AH = 10,560 Watt*Hours of energy
    • 48 volts * 220 AH = 10,560 Watt*Hours of energy
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #10
    Re: 6v or 12 v batteries?

    kennedy,

    Generally, the lower the voltage of the individual batteries in a system, the better. In a moderately sized system, 200-ish AH 6 Volt batteries work well. Golf Cart batteries -- 6 V -- are true Deep Cycle batteries. Most 12 V batteries are not.

    Running single strings of batteries is much better than parallel strings, IMO.

    If a battery would develop a problem with a single cell, replacing a 12 V Flooded battery would mean throwing out five good cells, to replace that single bad cell. But if that single bad cell was in a 6, 4 or 2 V battery, there would be fewer "good" cells tossed out to replace the single defective cell.

    If one needs fairly high capacity in a battery bank, lower voltage batteries with higher Capacity is better that trying to get capacity by running parallel strings of smaller capacity batteries.

    Generalizations, in general. Opinions, Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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  • kennedy759
    kennedy759 Registered Users Posts: 10 ✭
    #11
    Re: 6v or 12 v batteries?

    ok, thanks, now I get it, now leaning towards 4-6v batteries in series, instead of 12v in parellel
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