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Thread: 12v vs 24v battery bank amp hours

  1. #1

    Default 12v vs 24v battery bank amp hours

    When using say two 6v 200ah golf cart batteries in a 12v setup you would parallel them and get 200ah. If you had a 24v system then two of the same 6v 200ah batteris in series would make 400ah? If that's the case then you'd spend half the $ on a 24v system and you can use a charge controller that can interface 12v solar panels with a 24v battery bank right? Why go 12v then? (Besides the use of 12v DC electronics)

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    to use 6v batteries in a 12V setup, you SERIES them, battery_wire_diag.jpg (the top row of batteries in the sketch are in series)
    Batteries (pr PV panels) in SERIES, increases Volts
    Parallel, increases AMPS. Since Volts x Amps = Watts, either method still produces the exact same watts, but higher voltages have less losses, and a few % more efficiency.
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  3. #3

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    Quote Originally Posted by Mike90250 View Post
    to use 6v batteries in a 12V setup, you SERIES them, battery_wire_diag.jpg (the top row of batteries in the sketch are in series)
    Batteries (pr PV panels) in SERIES, increases Volts
    Parallel, increases AMPS. Since Volts x Amps = Watts, either method still produces the exact same watts, but higher voltages have less losses, and a few % more efficiency.
    I was laying in bed last night thinking about this and it occurred to me that I had it all wrong. One of these days I'll have this series/parallel thing down. So no matter how their wired you still get x volts/ah from a battery. Why is it that the higher the voltage the smaller the wire? Does the same principle apply to AC volts/amps? House wiring doesn't seem that thick and they're long runs through the walls too. Does that cause lots of losses on the ac end?

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    Quote Originally Posted by Alex9 View Post
    Why is it that the higher the voltage the smaller the wire?
    For the same amount of power (volts x amps if you leave out power factor which is an AC thing) the current is lower when you use a higher voltage. 12 volts time ten amps gives 120 watts, while 120 volts times 1 amp gives the same power.

    Since the power loss in a given size wire is lower when the current is lower, higher voltages translate to lower losses.

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    So if I decided to go with a 24V system over 12V system and using the same SIZE bank of say 440ah..

    Running a 2000 watt AC inverter (24v rated).. will I use less AH then the same setup at 12V rated (12v bank and 12v inverter and same 2000 watt inverter)??

    I know I can use lighter wire then the 3/000 I have for the 12V bank.. but if the DRAW is the same for the 2000 watt inverter (say 55amps) is there any advantage other then lighter gauge wiring??

    I also know I can use smaller breakers on the bank if I go with 24v..

    I haven't totally decided if I am going 12 or 24 yet (my controller can do both and I can get 1 more panel and get a total PV of 870 if I want to go 24).. which is better I think than 725 @ 12V that I currently have planned for my 5 panels..

    My controller limit (Morningstar TS45 (PWM)) is 5 panels @ 145 if I go 12V in parallel (725w).
    if I step to 24v I can go with 3 parallel strings of 2 (series) and be at the 870 watts of PV..

    I was leaning toward 12V for the less expensive 2000w inverters for 12v.. (the 24V ones of same 2K rating are like $600+)..

    I plan on running an Air Conditioner for 2 hours a day rated at about 55 amps / hour..

    So if the 24V system would give me a clear advantage (drawing less out of the same 24v bank) then I would consider heavily going with 24v and biting the bullet on the higher priced 2000w 24v inverter.

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    Quote Originally Posted by epsgunner View Post
    So if I decided to go with a 24V system over 12V system and using the same SIZE bank of say 440ah..

    Running a 2000 watt AC inverter (24v rated).. will I use less AH then the same setup at 12V rated (12v bank and 12v inverter and same 2000 watt inverter)?
    Absolutely yes because of Mr Peukert Law.

    First off a 12 volt 440 AH battery bank, and a 24 volt 440 AH bank are not equal in capacity, there is a 100% difference. 12 volts x 440 AH = 5280 watt hours. A 24 volt x 440 AH = 10560 watt hours.
    MSEE, PE

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    Quote Originally Posted by Sunking View Post
    Absolutely yes because of Mr Peukert Law.

    First off a 12 volt 440 AH battery bank, and a 24 volt 440 AH bank are not equal in capacity, there is a 100% difference. 12 volts x 440 AH = 5280 watt hours. A 24 volt x 440 AH = 10560 watt hours.
    So my Air Conditioner rated at 550 watts @ 115 AC (normal running and not surge) will use say 1/2 of the 55 amps it would use on a 12v setup with a 12v type inverter??

    So I would be pulling 22 1/2 amps from a 24v inverter and bank??

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    Quote Originally Posted by epsgunner View Post
    So my Air Conditioner rated at 550 watts @ 115 AC (normal running and not surge) will use say 1/2 of the 55 amps it would use on a 12v setup with a 12v type inverter??

    So I would be pulling 22 1/2 amps from a 24v inverter and bank??
    Assuming the same battery bank wiring you'd actually pull a little less. At 24V you will start with half the current, thus you get 1/4 of the losses that you would get at 12 volts. (Losses=I^2R in watts, so 2x current draw = 4x losses.) That means the inverter will not have to draw a little more current to counteract the additional losses in the wiring. Which is one of many reasons that higher voltages are in general better than lower voltages.

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    Quote Originally Posted by billvon View Post
    Assuming the same battery bank wiring you'd actually pull a little less. At 24V you will start with half the current, thus you get 1/4 of the losses that you would get at 12 volts. (Losses=I^2R in watts, so 2x current draw = 4x losses.) That means the inverter will not have to draw a little more current to counteract the additional losses in the wiring. Which is one of many reasons that higher voltages are in general better than lower voltages.
    Does that also mean with a 24v bank I could run my Air 4 hours vs the initial/originally planned 2??

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    Quote Originally Posted by epsgunner View Post
    Does that also mean with a 24v bank I could run my Air 4 hours vs the initial/originally planned 2??
    NO it will not. Battery capacity watt hours = Volts x Amp Hours.

    12 volts @ 440 AH = 24 volts @ 220 AH = 5280 Watt Hours.

    The gain is in efficiency.
    MSEE, PE

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