Using a MPPT as a DC to DC battery charger?

TucsonAZTucsonAZ Posts: 136Solar Expert ✭✭
edited May 29 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

  • mcgivormcgivor Posts: 2,205Solar Expert ✭✭✭✭✭
    edited May 29 #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 bank 

  • TucsonAZTucsonAZ Posts: 136Solar Expert ✭✭
    Can anybody else confirm this is a viable option?
  • HorseflyHorsefly Posts: 310Registered Users ✭✭✭✭
    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, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
  • TucsonAZTucsonAZ Posts: 136Solar Expert ✭✭
    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. 
  • HorseflyHorsefly Posts: 310Registered Users ✭✭✭✭
    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, 4 x Vmax XTR12-155 12V, 155AH batteries in a 2x2 24V 310AH bank.
  • TucsonAZTucsonAZ Posts: 136Solar Expert ✭✭
    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.   
  • mcgivormcgivor Posts: 2,205Solar Expert ✭✭✭✭✭
    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 bank 

  • mike95490mike95490 Posts: 7,902Solar Expert ✭✭✭✭
    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 ,

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