12 vs 24 question

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System
System Posts: 2,511 admin
in Solar Beginners Corner #1
Hi... I just abandoned a 12 volt system because of wire/distance problems. My question is this...what is the advantage of a 24 volt system over a 12 volt system? I know wire size is a factor and I know how to wire up series and parallel circuits to get different voltages and amps. I also know that I need to know what my load will be before I start getting parts. I have most of them now anyhow. Just want to know why some go with the higher voltage other than the wire/distance factors. It takes twice the batteries to get the amp hours of a 24 volt system over a 4 battery parallel wired system.

Thanks.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #2
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    Re: 12 vs 24 question

    Welcome to the Forum Andy.

    Regarding batteries--At least from a power point of view, series or parallel arrangement of batteries does not matter...

    Power = Voltage * Current

    If you put two batteries in series. Their voltage doubles, but the current/amp*hours remains the same:
    • Power = 2xVoltage * Current
    If you place the two batteries in parallel, the voltage remains the same but current/amp*hour capacity doubles:
    • Power = Voltage * 2xCurrent
    Same value of power.

    However, you are correct that going from 12 volts to 24 volts improves the ability for you to move power around...

    Doubling the voltage cuts the current by 1/2... So you can send power twice as far or on 1/2 the copper wire.

    Also, the voltage drop allowed also goes up...

    For example, a 12 volt system, you can run the battery down to 11.5 volts under load and 10.5 volt inverter cut-off. That gives you 1 volt head room...

    A 24 volt system will run down to 23 volts and the cut-off is 21 volts--That gives you 2 volt head room. Similar improvement for a 48 volt bank.

    Depending on your requirements--there are some things you can do to improve your chances for a successful system.

    First, put the battery bank and charge controller close to where the energy will be used.

    Second, look at using a 120 VAC inverter to send power any distance over a few handful of feet. Yes, the inverter add cost and is less efficient... But it is so much easier to send 120 (or even 240 VAC) long distances at 1/10th or 1/20th the current of a 12 volt system. Plus the voltage drop of 1 volt will kill a 12 volt system device--but who even worries much about a 5 volt drop in a 120 VAC system when under load.

    Next, if your Solar Array needs to be mounted away from the battery bank (better sun, clear area for array mounting)--Look at using a MPPT type solar charge controller (Maximum Power Point Tracking)...

    They are 3-5x the cost of a PWM controller--but allow you to run your solar Array Vmp to upwards of 100 VDC which you can now run 10's or even 100' to your MPPT Charge Controller / Battery Shed.

    An MPPT charge controller can efficiently (~95% or better) take the high voltage / low current solar array power and down convert it to the low voltage / high current required to charge the battery bank.

    So--depending on where your power / current issues were--there are several ways of solving the problem (higher voltage battery bank and/or higher voltage solar array + MPPT charge controller).

    The rough rules of thumb that we start with here--Roughly 1,200 watt maximum load on a 12 volt battery bank, 2,400 watt on a 24 volt battery bank, and over 2,400 watts, look at a 48 volt battery bank.

    Can you give us a bit more information on what you are trying to do (voltage, current, distance, AC or DC loads, etc.).

    Lastly, there are sometimes some limitations we have to live with... For example, many wind turbines which output a fair amount of power to a 12 volt battery bank will produce much less power into a 24 volt (or higher) battery bank.

    Or some people have a lot of 12 volt loads and all of the other conversions from 24/48/120 volts just become too complex, expensive, or have too many losses.

    -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 ✭✭✭
    #3
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    Re: 12 vs 24 question

    It comes down to that basic formula: Watts = Volts * Amps.
    Increase the Voltage and you can get the same Watts (total power) with less Amps. Less Amps means smaller wire size, less heat, and some simplified battery wiring if you need a lot of Amp hour capacity.

    True, you need 2X the batteries to get 24V instead of 12V, but you get twice the total power:

    100 Amp hours @ 12 Volts = 1200 Watt hours
    (2) 100 Amp hours @ 12 Volts for 24 Volt system = 2400 Watt hours.

    Part of the benefit can be simpler battery wiring: if you find yourself needing to put more than two 12V (or equivalent) in parallel then it might be better to go up to 24V. The over-all power is the same, but with the batteries in series there is less trouble with current sharing among all batteries (keeping charge/discharge equal across all cells).

    The bigger your needs, the more you benefit from increasing system Voltage. Sometimes it's a good idea to go up to 48.
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  • zeuspaul
    zeuspaul Solar Expert Posts: 59 ✭✭✭✭
    #4
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    Re: 12 vs 24 question

    I have been struggling with the same issue. I go back and forth between 12 and 24 volts. Today I am on 24 volts;)

    I am drawn to 24 volts because it is more efficient. Both of the charge controllers I am looking at- the Morningstar MPPT 45 and the Rogue are about two percent more efficient when converting to 24 volts.

    The Exeltech inverters I am looking at are about two percent more efficient when converting from 24 volts.

    There is less loss in the wiring with 24 volts.

    I prefer one string of series wired batteries.

    I don't have any 12 volt loads.

    I have been trying to figure out a way to connect my Harbor Freight 800 watt (900 surge) $90 generator to the solar system. The best solution I have found is a 24 volt 8 amp Soneil battery charger with a soft start http://soneil.com/24_volt.html. I like the Iotas but they have a huge inrush start up current which I believe would damage my cheapo generator.

    Currrent system design is 675 watts. 5 x 135 watt Kyocera, Morningstar MPPT 45, Exeltech 2000 watt inverter, four Crown golf cart batteries , HF 800 watt generator and a Soneil 2416SRF battery charger

    Subject to change without notice:D

    Zeuspaul
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
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    Re: 12 vs 24 question

    Adding a clarification here: sustained draws.

    An occasional "peak" to 2kW, as in starting a 'frige or having everybody turn on their stuff all at once is one thing. Continually or regularly using this sort of Wattage is another. If you just get the "odd moment" of 2kW use, you can probably stick to a 12 Volt system. Especially if you already have it and/or have need of 12VDC for other purposes.

    But if you start running washing machines and TV's and refrigerators and freezers and .... It adds up quickly, and 24 Volts becomes a smart choice.

    I have to say that I feel quite silly running a 24V 3.5kW inverter to supply a load that most of the time is less than 300 Watts. Unfortunately every day the demand is there for that big power, and this is a lazy old man's way of avoiding starting a generator. And of course there's no worry about future expansion. Eventually it will be "on demand" for everything, with no worries about repeating this Summer's "drain the batteries dead every day for a week" performance courtesy of the children. Never leave your children unattended. Even if they are over 30 years old! :p
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