can I use 2 24v battery banks,combine to 48v for inverter but charged seperate at 24V?

patdufo
patdufo Registered Users Posts: 9 ✭✭
in Off Grid Solar & Battery Systems #1
I have a 28V DC military generator and want to be able to use 2 24v banks tied together for the inverter but charged seperately from the 28v generator?This will be for cloudy days.The charge controllers would be 48v,do you think that it would work?thanks
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited January 2021 #2
    The problem would be there would be a dead short on the charger on the interconnect of the 48V bank, the only options are isolating the individual 24V groups or using a Schottky diode to prevent reverse current flow. Neither is ideal, however  there are DC-DC chargers available but they tend to be low current, high $$$, what current output is the generator? 
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • Photowhit
    Photowhit Solar Expert Posts: 6,006 ✭✭✭✭✭
    edited January 2021 #3
    If you aren't using the battery bank at the same time you are charging, you could devise a method to charge as a 24 volt battery bank and use as a 48 volt. That would not be an option for most systems.

    I've done this the other way around, charging 2 - 6 volt batteries in series, and using them (at night) as a 6 volt battery bank in parallel. A tiny system for bicycling touring.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
    Wired directly - NO
    If you change the wiring each time you wish to charge, then - yes, it's possible.   But I'd advise against it, because of the complexity of the continual re-wiring.   Can't do high amps with schottky diodes - even with many in parallel, you still have too much loss.
    Might be able to rig up some DC circuit breakers with a transfer switch interlock,but still messy
    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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  • BB.
    BB. Super Moderators, Administrators Posts: 33,631 admin
    #5
    Just to be clear--2x 24 volt battery bank tied together in series for a 48 VDC input AC inverter?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • patdufo
    patdufo Registered Users Posts: 9 ✭✭
    #6
    BB. said:
    Just to be clear--2x 24 volt battery bank tied together in series for a 48 VDC input AC inverter?

    -Bill
    yes

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  • patdufo
    patdufo Registered Users Posts: 9 ✭✭
    #7
    ok,a battery isolator couldnt help?Will likiely put in a quick disconect at the inverter and charge with inverter on,this is a 280 amp at 28v DC gen,should charge the 2 24v 1000 AH batteries pretty quick,might have to turn it down a bit(charge to quick)comercial DC gen that I have seen are verry expensive and most not rated for continuos use
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,631 admin
    #8
    There are devices that can support splitting a 24 volt battery bank into 12 volt loads... Basically, a DC to DC converter that holds the "low 12 volt bank" = (equal) to the voltage on the "high 12 volt bank". Battery equalizer or balancer... There are quite a few for 12/24 volts systems (used on Bus/Truck conversions such as a 24 volt truck chassis and a 12 volt RV package). They are also used on some military vehicles.

    https://www.waytekwire.com/datasheet/extend battery life.pdf

    I looked for 24/48 volt versions--But I did not find any with a quick search (some that are rated a few amps, but no >60 Amp units).

    They are not cheap... As much as a similar AC inverter (or more, since it is a smaller market?)...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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