Need to run 24 volt loads off a 48 volt bank

openplanet
openplanet Registered Users Posts: 34 ✭✭
in Off Grid Solar & Battery Systems #1
I'm upgrading my system to 48 volts but still need to run some 24 volt loads: led lights, ceiling fan, small (SunDanzer) fridge, pressure pump).  From what I can see, my options are

1: 48-to-24 volt converter
2: small mppt charge controller charging small 24 volt bank from the 48 volt bank

Interested in knowing people's thoughts.  Thanks.
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Comments

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #2
    My first choice would be option 1, but I've read where converters can fail in a way that exposes lower voltage loads to the higher voltage.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #3
    48VDC-to-24VDC converters are not common enough to be a commodity item, so you will probably pay more than you think you should. Have you attempted to total up what amps or watts all your loads add up to? I only did a quick Google and found a few, but the power output was surprisingly low for the price on most of them. I would guess that the items you list would add up to at least 15A / 360W, and that may be tough. 

    On top of that, I'm not sure how dependable these buck converters are. I'm already somewhat dependent on a 24VDC-to-12VDC converter in our cabin, but at least nothing critical is on the 12VDC circuit. 24VDC to 12VDC is pretty common, so there are lots more choices out there (at lower prices).

    In your situation, you may looking at a single point of failure in this converter.  If I had a fridge, I'd consider looking for a new one.
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • openplanet
    openplanet Registered Users Posts: 34 ✭✭
    #4
    Thanks for that input.  The 24 volt loads add up to surprisingly little, because the fridge is super efficient.  And the Meanwell converters, which have a good reputation, are relatively inexpensive, such that I might buy two to hedge my bets.  But using a small MPPT controller to charge a small 24 volt bank from the 48 volt bank is still an interesting option to me.  Hoping to get some thoughts about that approach.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    edited July 2017 #5
    I see new 48->24V 15A buck converters on ebay for < $25.

    I'd only explore a second battery bank if you need high surge amps.

    I am available for custom hardware/firmware development

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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #6
    I'd use a small MPPT controller from Morningstar, and have it set to FLOAT SERVICE for a smallish 24V battery. Use the right size fuses and a 28V crowbar circuit.
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • patel
    patel Registered Users Posts: 1 ✭
    #7
    Actually I am little bit confused... My total load of house is 9kw then if I install on grid system then what will be inverter ratting? And if I install off grid system then what will be inverter ratting as well as battery battery capacity . If I want 7 hrs battery backup. Please guide me ..thank you
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  • bill von novak
    bill von novak Solar Expert Posts: 891 ✭✭✭✭
    #8
    patel said:
    Actually I am little bit confused... My total load of house is 9kw then if I install on grid system then what will be inverter ratting? And if I install off grid system then what will be inverter ratting as well as battery battery capacity . If I want 7 hrs battery backup. Please guide me ..thank you
    If you want to do an on-grid system your maximum power doesn't matter.  If your grid connection supports it right now then it will support it with solar added.

    If you want to do an off-grid system and your maximum load is 9kW then you will need a inverter good for 9kW or better.

    Your battery capacity will depend on you ENERGY not your POWER usage.  (Energy is kwhr; power is kw.)  If, for example, your average use is 1kW (so 7kwhr at a time) and you want to operate down to 50% (pretty standard) then you will need 14kwhr worth of batteries.  This will cost you around $4000 for decent lead acid batteries.

    However saying "I want 7hrs battery backup" is fairly meaningless if you are off-grid. In that case if your average draw is 1kw then you will need a ~144kwhr battery bank costing around $50,000.  (3 days autonomy down to 50%.)
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  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,894 ✭✭✭✭✭✭
    #9
    Here is another meaningless statement, I have been battery backed up for 25 years.... It definitely did not cost me $50,000. The Utility told me long ago I would be begging for them to connect me and they wanted about $100,000. Other than the yearly fire season preparation, it is a joy to be remote from all the hustle and bustle! We love it!
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net

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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #10
    In the on-grid case, the inverter rating should ideally be a little less than the rating of the panels - loads don't matter.

    I am available for custom hardware/firmware development

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  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
    edited July 2017 #11
    I have good experience with the Victron DC/DC converters. They are more expensive than the cheap ones from eBay but hopefully more reliable. I have had a 24/12 on for two years now in temperatures from -20C (-4F) to +30C (86F) with no issues. Victron has 100W and 250W versions. Datasheets here:
    https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-DC-DC-converters-isolated-100W-EN.pdf
    https://www.victronenergy.com/upload/documents/Datasheet-Orion-Tr-DC-DC-converters-isolated-250W-EN.pdf


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