Charge 12v battery bank from 24v battery bank

nickdearing88
nickdearing88 Registered Users Posts: 100 ✭✭✭
in General Solar Power Topics #1
Hi everyone. I'm back after being gone for some time.

I have an existing 24v 200Ah system but I have several 12v DC devices for the new cabin: 12v DC water pump, 12v DC lighting, 12v DC fan for composting toilet, etc.

I remember reading somewhere about using a simple PWM 12v charge controller with input from a 24v battery bank to keep a 12v battery charged for 12v devices, but I'm having trouble finding anything to that effect in search. Has anyone else done this successfully?
Current system: 8-100w Renogy panels mono/poly, 2 strings of 4 panels in series - 24v 100Ah AGM Battleborn LiFePO4 batteries - Morningstar MPPT40 CC - 1500W Samlex PSW inverter
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    You can't simply connect a PWM controller, the first pulse it sends down to the battery, will have hundreds of amps from your battery bank behind it and the FET's will fry faster then your fuse can activate.   You will need a ballast resistor, calculated to the value needed for your system.

    The Morningstar MPPT controllers WILL handle this sort of condition, charging from a battery  but they are a bit more pricey
    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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  • nickdearing88
    nickdearing88 Registered Users Posts: 100 ✭✭✭
    #3
    I wondered about some way to limit current besides the breaker. So a MPPT controller would function in this way? I'm using a Morningstar MPPT on my 24v bank currently. I'm trying to avoid having a separate array for the 12v system. Long-term I'm adding a bigger 48v system and will use the current array for the 12v system.
    Current system: 8-100w Renogy panels mono/poly, 2 strings of 4 panels in series - 24v 100Ah AGM Battleborn LiFePO4 batteries - Morningstar MPPT40 CC - 1500W Samlex PSW inverter
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #4
    You would need another MPPT (Morningstar) to use your 24V bank to charge the 12V bank.  

    A breaker does not "limit" current.   It stops it at it's threshold trigger.  Stops all of it.  Just like a fuse, except you reset it instead of replace it.
    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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  • nickdearing88
    nickdearing88 Registered Users Posts: 100 ✭✭✭
    #5
    Right, I understand that. When the breaker trips -- that's a pretty hard limit! :wink:
    Current system: 8-100w Renogy panels mono/poly, 2 strings of 4 panels in series - 24v 100Ah AGM Battleborn LiFePO4 batteries - Morningstar MPPT40 CC - 1500W Samlex PSW inverter
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #6
    There's always the option to just use an AC battery charger.  A bit more loss, but may not matter much if there's "extra" solar most days. 

    If going the controller route because solar is tight, I'd probably add some pv rather than taking from the 24v.
    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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  • nickdearing88
    nickdearing88 Registered Users Posts: 100 ✭✭✭
    #7
    Estragon said:
    There's always the option to just use an AC battery charger.  A bit more loss, but may not matter much if there's "extra" solar most days. 

    If going the controller route because solar is tight, I'd probably add some pv rather than taking from the 24v.
    Right, I'm thinking that's probably the best option now. Thanks!
    Current system: 8-100w Renogy panels mono/poly, 2 strings of 4 panels in series - 24v 100Ah AGM Battleborn LiFePO4 batteries - Morningstar MPPT40 CC - 1500W Samlex PSW inverter
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #8
    Short of converting to 24 VDC or 120 VAC (inverter) power... You could use buck mode DC to DC switching power supplies. The are pretty cheap these days (Amazon, EBay, etc.):

    https://www.amazon.com/s?k=dc+to+DC+buck+power+supply&ref=nb_sb_noss

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