Using a MPPT as a DC to DC battery charger?

TucsonAZ
TucsonAZ Solar Expert Posts: 139 ✭✭
edited May 2018 in Solar Beginners Corner #1
I am at it with another build and the 12v needs from a 24v system are always sort of a pain to deal with.  I will have my main battery bank (24v and 5,700 watts) hooked to a Midnite Classic 150 and 1,300 watts of panels.  I have been thinking about using a step down converter for my 12v needs but if I add an additional battery it will give me another 1,200 watts in my bank and I will likely be wasting solar anyway with a topped off 24v bank by sunset.

I was thinking about a DC to DC 4 stage charger but they're costly.  Then I saw mention of using an MPPT hooked to the 24v bank instead of PVs and charging the 12v bank like that.  I have a spare EPever 20a and 30a MPPT sitting here and this would afford me a lot of versatility and redundancy if anything broke while traveling.

Anyway, I've seen mixed opinions and was wondering what you all thought, is this a safe and sane option or no? 

Comments

  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited May 2018 #2
    From what I understand it is possible to do as you described as long as the controller is a true MPPT, which the EPever apparently is, I would recommend a fuse rated at no greater than controllers maximum current handling capacity between the two. Personally I've not tried doing this, but going rather on logic and information gathered.
    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.
  • TucsonAZ
    TucsonAZ Solar Expert Posts: 139 ✭✭
    Can anybody else confirm this is a viable option?
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    How big (watts or amps) are your 12VDC needs?  We installed this on our 24VDC system last summer, and have been pretty pleased with it:
    https://www.amazon.com/Pyle-PSWNV720-Power-Converter-Technology/dp/B003P17X8I

    It powers a stereo for our cabin (a car stereo), some night lights on two floors, and charging for cell phones and the like. It's rated at at 360W continuous (that's 30A at 12V), and up to 720W for a short time.

    I'd be hesitant to use something like an MPPT charger, which is meant to charge a 12VDC battery. Besides being overkill for most possible applications, even if you can override the charging phases, I don't know that you can force it to regulate at a constant 12VDC.
    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.
  • TucsonAZ
    TucsonAZ Solar Expert Posts: 139 ✭✭
    Horsefly said:
    How big (watts or amps) are your 12VDC needs?  We installed this on our 24VDC system last summer, and have been pretty pleased with it:
    https://www.amazon.com/Pyle-PSWNV720-Power-Converter-Technology/dp/B003P17X8I

    It powers a stereo for our cabin (a car stereo), some night lights on two floors, and charging for cell phones and the like. It's rated at at 360W continuous (that's 30A at 12V), and up to 720W for a short time.

    I'd be hesitant to use something like an MPPT charger, which is meant to charge a 12VDC battery. Besides being overkill for most possible applications, even if you can override the charging phases, I don't know that you can force it to regulate at a constant 12VDC.
    That has been my thought but since I already have the MPPT and a spare 12V if I can get the same outcome without having to spend any additional money plus another 1,200 watts of storage and a backup charge controller for my main system should that one go out is seems like a no brainer to me that it's a far better choice.

    It will be runny my security camera system, the water pump, USB chargers, maybe a projector and a few other 12v native items. 
  • Horsefly
    Horsefly Registered Users Posts: 470 ✭✭✭✭
    I would think if you put your "spare" 12V battery in for this function you will be replacing it after some cycles just like any other battery, and that will cost more than the $39 for the converter, but what do I know.

    If everything you want to run is Ok with being powered by something that will vary between 14.5V and 12V, I guess your plan is fine.
    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.
  • TucsonAZ
    TucsonAZ Solar Expert Posts: 139 ✭✭
    Horsefly said:
    I would think if you put your "spare" 12V battery in for this function you will be replacing it after some cycles just like any other battery, and that will cost more than the $39 for the converter, but what do I know.

    If everything you want to run is Ok with being powered by something that will vary between 14.5V and 12V, I guess your plan is fine.
    Arrrrrg, this is the fundamental flaw with the interweb.  I do super appreciate your outlook on it and you make some valid points but I need to only know if the plan stands at face value and if there are any potential pitfalls I'm missing.  The pros on cons beyond that I can weigh out based on the variables unique to me and my lifestyle. 

    A $5 buck converter at anything needing stable voltage addresses the varied voltage concern and that's cheap and easy but I don't think I have much or anything actually that would require it.   
  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    TucsonAZ said:
    Horsefly said:
    I would think if you put your "spare" 12V battery in for this function you will be replacing it after some cycles just like any other battery, and that will cost more than the $39 for the converter, but what do I know.

    If everything you want to run is Ok with being powered by something that will vary between 14.5V and 12V, I guess your plan is fine.
    Arrrrrg, this is the fundamental flaw with the interweb.  I do super appreciate your outlook on it and you make some valid points but I need to only know if the plan stands at face value and if there are any potential pitfalls I'm missing.  The pros on cons beyond that I can weigh out based on the variables unique to me and my lifestyle. 

    A $5 buck converter at anything needing stable voltage addresses the varied voltage concern and that's cheap and easy but I don't think I have much or anything actually that would require it.   
    All innovations took a risk to prove or disprove a theroy, if I had a spare MPPT controller handy I would give it a try if the input voltage was within the specs of the controller and a fuse to limit the current were installed to prevent runaway, my suspension is it will work. FWIW  
    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.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    I know that 2 brands of MPPT controllers are robust enough to charge batteries from batteries.   Morningstar and Midnight Classic -others, you take your chances
    What I would do, is use an automotive headlamp (or 2), in SERIES with the 24V and the Controller.  They will act as a ballast resistor and limit the current into the MPPT.  Different  MPPT controllers behave differently, the lamp helps insure  the controller does not see unlimited amps from the battery bank.
    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 ,