24v bank - Best way to get 12V

doctorZeus
doctorZeus Registered Users Posts: 24 ✭
in Solar Beginners Corner #1
I don't have a clear understanding, but I'm of the perception that there's no "best" way to reduce nominal 24 and 48 volt banks to 12V.

Right now my bank is looking to be eight 6V golf car batteries ran in series/parallel to get 24V @ 430ah.
Do I simply buy the best and most efficient inverter I can find and then convert the A/C current back to D/C with a wall wart or power supply? (Anyone ever do this with regular computer PSUs? I'm actually interested in testing out some computer power supplies with the killawatt.)

What about DC-DC converters? I see Samlex (which appears respectable enough) sells one that reduces 24V to 12V: http://www.amazon.com/Samlex-SDC-12-Switching-DC-DC-Converter/dp/B0054MXCWM/ref=sr_1_6?s=electronics&ie=UTF8&qid=1378929505&sr=1-6&keywords=samlex+24v Since I've seen these for literally $2 from hong kong direct stores that brag about how efficient it is, I'm wondering (hoping) the $70 Samlex device would be worth looking into - at least testing.

Also, anyone know if a Killawatt will even measure DC, and assuming it doesn't, are there similar devices out there that will? I'm thinking I should be able to manually get roughly accurate results with my multimeter but I would love for the little wall plug to track it for me.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    Re: 24v bank - Best way to get 12V

    There are smaller DC AH/WH meters.

    There are larger, and more expensive, AH Battery Monitors (plus Victron).

    And you can get DC Current Clamp meters to do real time measurements (not Amp*Hours over time).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • sunbunny
    sunbunny Solar Expert Posts: 59 ✭✭✭✭
    #3
    Re: 24v bank - Best way to get 12V

    I've wondered about this, too, since I will initially be powering a travel trailer. After lots of looking around at produts, I decided to replace the converter in the trailer with a more modern one. ... or maybe a 'power supply' ?
    It will be 20+ feet from my power shed so I won't be trying to send the 24 volt across.
    It's pretty hard to decide if I should spend $25 or $65 on this. None of the brand names are familiar and NAWS dosen't offer anything.
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #4
    Re: 24v bank - Best way to get 12V
    doctorZeus wrote: »
    I don't have a clear understanding, but I'm of the perception that there's no "best" way to reduce nominal 24 and 48 volt banks to 12V.

    Your options are:

    Battery balancers http://www.allbatterysalesandservice.com/browse.cfm/4,3147.html https://www.altestore.com/forums/index.php/topic,1985.msg7534.html#msg7534

    DC/DC converters http://www.amazon.com/Pyle-PSWNV720-Power-Converter-Technology/dp/B003P17X8I/ref=sr_1_1?ie=UTF8&qid=1378933496&sr=8-1&keywords=24v+to+12v+converter

    The balancer lets you charge the 12V or 24V battery and then draw from the 12V or 24V battery (i.e. it's bidirectional.) The DC/DC is one way.
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  • RandomJoe
    RandomJoe Solar Expert Posts: 472 ✭✭✭
    #5
    Re: 24v bank - Best way to get 12V

    Another option, though perhaps a bit overcomplicated, is the Morningstar SunSaver MPPT. You'd have to have a 12V battery as well, but nothing huge. The SunSaver can take up to 75V input so could be attached to a 24V or 48V battery bank. Only 15A, so might be a bit pricey for the purpose as well.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #6
    Re: 24v bank - Best way to get 12V

    Since you're talking about wall warts ...

    If the load is very low, you can simply put it on half of the bank. But the voltage will not be 12V all the time, but will vary 11 to 15V or so. Therefore, if you need regulation, that's not a good option.

    You can also connect your loads in series. For example, if your load is two leds, you can connect them in series and attach to your bank. For this option, it will be no regulation neither.

    Other than this, DC->AC->DC is very inefficient. DC->DC converter will do much better.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #7
    Re: 24v bank - Best way to get 12V

    There are good, 90% eff DC-Dc convertors around, Some you may have to buy the bare "Brick" and add some wires and heat sink.
    http://www.digikey.com/product-search/en/power-supplies-external-internal-off-board/dc-dc-converters/590047?k=dc-dc
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

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  • tons001
    tons001 Solar Expert Posts: 71 ✭✭
    #8
    Re: 24v bank - Best way to get 12V

    I'm using on of these. Pretty happy wit it. http://www.mastervolt.com/marine/products/dc-dc-converters/
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  • doctorZeus
    doctorZeus Registered Users Posts: 24 ✭
    #9
    Re: 24v bank - Best way to get 12V
    NorthGuy wrote: »
    You can also connect your loads in series. For example, if your load is two leds, you can connect them in series and attach to your bank. For this option, it will be no regulation neither.

    This is very interesting.. most of what I'm wanting to power isn't voltage sensitive (12V fans and lights). Do both devices (wired in series) have to be ON (drawing power) in order for them to draw 12V each? Does that mean if there's a short or disconnect or bulb blowing or whatever on 1 of the 2 devices, then the other one will get smoked by 24v? Would I be able to put some kind of fuse in front of the series string? Are there 12V regulators designed to take 10-15 volts and hold it steady at 12.x volts that I could put in front of the string? (I would imagine these would be much more efficient than a 24v to 12v reducer.)

    BTW I fully realize this is an FAQ thread and appreciate everyone taking the time to give feedback.
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #10
    Re: 24v bank - Best way to get 12V
    doctorZeus wrote: »
    This is very interesting.. most of what I'm wanting to power isn't voltage sensitive (12V fans and lights). Do both devices (wired in series) have to be ON (drawing power) in order for them to draw 12V each? Does that mean if there's a short or disconnect or bulb blowing or whatever on 1 of the 2 devices, then the other one will get smoked by 24v? Would I be able to put some kind of fuse in front of the series string? Are there 12V regulators designed to take 10-15 volts and hold it steady at 12.x volts that I could put in front of the string? (I would imagine these would be much more efficient than a 24v to 12v reducer.).

    If you connect something in series they're either both on or both off. They also should draw fairly similar amount of current. Otherwise, one of them will get more voltage than the other. So, if you have two lights, you can use the this way, but you cannot do it with things that you're going to switch on at a different times. It also may not work with dimmable lights, although you can easily fix these problems by connecting the point between loads to the middle of the battery bank. In this case, you need to make sure that both items are turned on at the same time, or you will be discharging your battery unevenly.

    Voltage regulators can do it, but they simply waste voltage. Say, you load needs 2A @ 12V. When put on regulator, it still will be 2A, but 24V(or whatever is the battery voltage). So, you waste half of your energy. DC to DC convertors will preserve energy.
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  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    #11
    Re: 24v bank - Best way to get 12V
    doctorZeus wrote: »
    most of what I'm wanting to power isn't voltage sensitive (12V fans and lights). Do both devices (wired in series) have to be ON (drawing power) in order for them to draw 12V each?

    Do NOT connect fans in series... it won't work. Generally, do not connect motors in series. When one motor is going slower it will drop its impedance and try to draw more current, but the current is limited by the other motor in series. Therefore the voltage will drop on the slower motor and the faster motor will get higher voltage and spin faster.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
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  • sunbunny
    sunbunny Solar Expert Posts: 59 ✭✭✭✭
    #12
    Re: 24v bank - Best way to get 12V

    Would it be more efficient to charge a 12V battery with a 120 volt charger than to use a DC>DC or DC>AC>DC converter ?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #13
    Re: 24v bank - Best way to get 12V
    sunbunny wrote: »
    Would it be more efficient to charge a 12V battery with a 120 volt charger than to use a DC>DC or DC>AC>DC converter ?

    He's wanting to run 12VDC device(s) from a 24 or 48 Volt battery, not charge a 12 Volt battery.

    As to that, though, 120 VAC battery chargers can be pretty inefficient. Think about it: step low Voltage DC up to high Voltage AC (invert), drop back down to different low Voltage DC, charge battery. Every step uses power. Less loss would be had in dropping the first low Voltage DC down to directly charge (or supply) the lower VDC.

    It is sometimes simpler and cheaper to do it with a battery charger (or power supply) if you've already got an inverter running anyway.
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