12 volt from 24 volt battery bank

tangle
tangle Registered Users Posts: 1
in Off Grid Solar & Battery Systems #1
I am new to solar power and have a question. I have a battery bank consisting of 2 banks of 4 6 volt batteries in series for a 24 volt system. I was wondering if it would be safe to wire the batteries in series for a 12 volt option while keeping the 24 volt series connection? I was wanting to run some 12 volt lights and charge the batteries with a 12 volt charger I have. Thanks
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: 12 volt from 24 volt battery bank

    Welcome to the forum.

    First of all you've got your terms a bit mixed up. Four 6 Volt batteries in series gives you 24 Volts. You can't have them wired in series/parallel for 12 Volts at the same time. What you are wondering about is if you can 'tap' one half of that battery bank to provide 12 Volts for some items.

    The answer is yes, but with some problems.
    If you draw off one half of a battery bank it will unbalance the bank, so it will not recharge properly (the two halves will always be at a different SOC). As such the battery life will be shortened. The greater the imbalance, the bigger the problem.

    The best solution is to use a DC to DC converter (something like this: http://www.solar-electric.com/12to24or24to.html ) to draw the 12 Volts from the whole 24 Volt bank.

    Another solution is to build a separate 12 Volt system, which can be as simple (but inefficient) as a 12 Volt battery kept charged from the 24 Volt system (direct DC using MPPT controller - expensive - or plug-in battery charger - inefficient).

    Any idea how much power your 12 Volt devices will demand?
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  • earlvillestu
    earlvillestu Registered Users Posts: 5 ✭
    #3
    Re: 12 volt from 24 volt battery bank
    Cariboocoot wrote: »
    Welcome to the forum.

    First of all you've got your terms a bit mixed up. Four 6 Volt batteries in series gives you 24 Volts. You can't have them wired in series/parallel for 12 Volts at the same time. What you are wondering about is if you can 'tap' one half of that battery bank to provide 12 Volts for some items.

    The answer is yes, but with some problems.
    If you draw off one half of a battery bank it will unbalance the bank, so it will not recharge properly (the two halves will always be at a different SOC). As such the battery life will be shortened. The greater the imbalance, the bigger the problem.

    The best solution is to use a DC to DC converter (something like this: http://www.solar-electric.com/12to24or24to.html ) to draw the 12 Volts from the whole 24 Volt bank.

    Another solution is to build a separate 12 Volt system, which can be as simple (but inefficient) as a 12 Volt battery kept charged from the 24 Volt system (direct DC using MPPT controller - expensive - or plug-in battery charger - inefficient).

    Any idea how much power your 12 Volt devices will demand?

    So if you were to get 12V from a 24V system using a 12V MPPT, how would the MPPT be connected? Would you tie the PV inputs of the two MPPTs together? Or feed the 12V MPPT from the 24V battery bank?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #4
    Re: 12 volt from 24 volt battery bank
    earlvillestu wrote: »
    So if you were to get 12V from a 24V system using a 12V MPPT, how would the MPPT be connected? Would you tie the PV inputs of the two MPPTs together? Or feed the 12V MPPT from the 24V battery bank?

    When you use an MPPT controller to do this you need a 12 Volt battery on its output and the 24 Volt battery bank feeds its input. Correct wiring size and circuit protection is absolutely necessary as the source battery can dump huge amounts of current.
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  • earlvillestu
    earlvillestu Registered Users Posts: 5 ✭
    #5
    Re: 12 volt from 24 volt battery bank

    How would you size the 12 volt controller. If, for example, you had a battery bank with ~400 AH at 24V and another at ~100 AH at 12V, what current rating would you need for the 12V MPPT?


    And am I correct that assuming you kept the 24V bank at 50% SOC or better, the 12V battery would almost always be in float? (Assuming the 12V battery was appropriately sized for its load.)
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #6
    Re: 12 volt from 24 volt battery bank
    earlvillestu wrote: »
    How would you size the 12 volt controller. If, for example, you had a battery bank with ~400 AH at 24V and another at ~100 AH at 12V, what current rating would you need for the 12V MPPT?


    And am I correct that assuming you kept the 24V bank at 50% SOC or better, the 12V battery would almost always be in float? (Assuming the 12V battery was appropriately sized for its load.)

    It depends largely on how heavy the 12 Volt loads are. You need to do some figuring to determine the battery size for that, how it is used, and how much power is needed to recharge it just as you would with any other system.

    Then you can get creative, such as having the connection between the 24 Volt bank and the MPPT controller active only when the 24 Volt system is fully charged. That way the 12 Volt bank may cycle as normal and recharge as normal. Or you can leave it connected and yes, keep the 12 Volt in Float all the time (which has to be taken into account when choosing the battery as some don't like to be 'floated' all the time).
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #7
    Re: 12 volt from 24 volt battery bank
    Cariboocoot wrote: »
    The best solution is to use a DC to DC converter (something like this: http://www.solar-electric.com/12to24or24to.html ) to draw the 12 Volts from the whole 24 Volt bank.

    Unless your 12 volt loads have some high start-up surges (and therefore need the buffering of a battery), Cariboocoot is quite correct.
    earlvillestu wrote: »
    So if you were to get 12V from a 24V system using a 12V MPPT, how would the MPPT be connected? Would you tie the PV inputs of the two MPPTs together? Or feed the 12V MPPT from the 24V battery bank?

    A DC to DC converter is probably the best solution. What loads do you have at 12 volts? Why are you pursuing the MPPT solution?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • Skyko
    Skyko Solar Expert Posts: 121 ✭✭✭✭✭
    #8
    Re: 12 volt from 24 volt battery bank

    Another option for 12V from 24V is a battery equalizer like the Vanner Voltmaster 66-60W 60 amp model. Evidently this type of device alternates the batteries it draws power from, keeping all of the series connected batteries in balance.

    They seem to be very high efficiency (>97%) but very expensive ($500 for the 60 amp model). Not sure if they are worth it.

    http://www.vanner.com/manuals/EQUALIZER-60-SERIES.pdf
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: 12 volt from 24 volt battery bank
    Skyko wrote: »
    Another option for 12V from 24V is a battery equalizer like the Vanner Voltmaster 66-60W 60 amp model. Evidently this type of device alternates the batteries it draws power from, keeping all of the series connected batteries in balance.

    They seem to be very high efficiency (>97%) but very expensive ($500 for the 60 amp model). Not sure if they are worth it.

    http://www.vanner.com/manuals/EQUALIZER-60-SERIES.pdf

    Yeah, that price ... You could buy a small MPPT controller and a 12 Volt battery and the wire and have money leftover.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #10
    Re: 12 volt from 24 volt battery bank

    Thats all too complicated. If you only a couple amps 12VDC here and there get a couple of these and call it good. No muss no fuss.

    http://dx.com/p/mini-dc-dc-voltage-stabilizer-regulator-module-red-126106
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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  • Skyko
    Skyko Solar Expert Posts: 121 ✭✭✭✭✭
    #11
    Re: 12 volt from 24 volt battery bank

    I actually don't understand what could be in the battery equalizer that is so expensive.

    With a 8 bit Atmel micro and a few power mosfets I could make a 30 or 40 amp device for $5 that kept the batteries within a tenth of a volt or less while self consuming milliwatts.

    I guess making it rugged and extra safe is the other $495.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #12
    Re: 12 volt from 24 volt battery bank

    Would guess you need to include some inductors and possible a transformer too (you need to level shift from 0-12 volts to 12-24 volts. And the typical equalizer is bi-directional--It can move power from the low battery to the high battery and back--Depending on loads and charging sources... It is surprising, but it all does add up.

    You compare it to a $500 MPPT solar charge controller--And it sounds pretty comparable.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #13
    Re: 12 volt from 24 volt battery bank
    Skyko wrote: »
    I actually don't understand what could be in the battery equalizer that is so expensive.

    Floating gate drives for the topside FETs, current monitoring for fault protection, heat sinks, the lugs and whatnot needed to handle the currents in the device and of course the big #1 reason - they don't make a lot of them so development cost is amortized over fewer units.

    I mean, you could do a non-isolated MPPT that would convert 200 volts in to a 50 amp output with the same micro, the same few FETs and a big inductor. But the devil is in the details (and in the cost of production for small-run systems.)
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  • Skyko
    Skyko Solar Expert Posts: 121 ✭✭✭✭✭
    #14
    Re: 12 volt from 24 volt battery bank

    I do not see the need to move energy from one battery to the other if both are kept in balance based on drawdown. Then the circuit becomes simple.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #15
    Re: 12 volt from 24 volt battery bank
    BB. wrote: »
    Would guess you need to include some inductors and possible a transformer too

    Well you could do it with a big DPDT relay, a capacitor and a timer. (It would just switch back and forth all day.) But again you have all that additional stuff that you don't think about at first - protection, wiring devices and the like.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #16
    Re: 12 volt from 24 volt battery bank
    Skyko wrote: »
    I do not see the need to move energy from one battery to the other if both are kept in balance based on drawdown. Then the circuit becomes simple.

    Yes - indeed you need no circuit at all if you do that! But most people would not be amenable to having to shut their lights off at sunset because they ran their fan too much during the day. (or other "balancing" actions taken to prevent an imbalance.) Thus the more complex designs that perform the balancing task automatically.
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  • Skyko
    Skyko Solar Expert Posts: 121 ✭✭✭✭✭
    #17
    Re: 12 volt from 24 volt battery bank
    bill von novak wrote: »
    Yes - indeed you need no circuit at all if you do that! But most people would not be amenable to having to shut their lights off at sunset because they ran their fan too much during the day. (or other "balancing" actions taken to prevent an imbalance.) Thus the more complex designs that perform the balancing task automatically.

    ???

    At 1000 times a second or more, the micro would check the balance of each battery and decide on the solid state switching to determine which battery should deliver the 12V, thus keeping them both at a constant discharge and somewhat equal.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #18
    Re: 12 volt from 24 volt battery bank

    If you are are making a "generic" battery balancer, you need to manage both charging and discharging on the "low 12 volt" bank as well as overall charging/discharging on the 24 volt bank.

    If all of your charging is on the 24 volt bank, you could (possibly) arrange for an upper 12 to lower 12 undirectional isolated down converter or even a 24:12 down converter on the same bank--But then you have to "protect" against "stupid stuff" (like disharging the 24 volt bank or upper 12 volt to keep the lower 12 volt battery section charged).

    The "balancer" (typically, as I understand) just aims to keep the upper 12 voltage equal to the lower 12 voltage section. That way, all the upper and lower banks maintain equal state of charge (for better or for worse). And you can mix 12/24 volt charging sources.

    I don't see the "high price" of the 60 amp balancer out of line for a "good quality" system. It sort of goes with the territory of off grid/solar/motor coach power systems and their costs for good/reliable systems.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #19
    Re: 12 volt from 24 volt battery bank
    Skyko wrote: »
    At 1000 times a second or more, the micro would check the balance of each battery and decide on the solid state switching to determine which battery should deliver the 12V, thus keeping them both at a constant discharge and somewhat equal.

    Yes, that would work. But as mentioned above, switching 30 amps 1000 times a second isn't super cheap.
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  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    #20
    Re: 12 volt from 24 volt battery bank

    And off the shelf 24v to 12v converters are much cheaper, produced in greater numbers and relatively efficient.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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