DC - DC Battery Charging Using a Charge Controller

I've found DC-DC battery chargers to be quite expensive (>$500 for a 24V-12V 5A charger) but I need one for an off-grid solar application. I had an idea to use a small 12 V solar charge controller (because they are cheap and readily available) between the 24 V source (in this case a 24 V battery bank) and a 12 V lead acid battery. Would that work? Or would its just burn out the charge controller? 

Comments

  • firerescue712firerescue712 Posts: 93Solar Expert ✭✭
    I used a Morningstar SunSaver MPPT15L for charging a 12v bank from a 24v bank for 2 years.  Worked flawlessly! 
  • JohannJohann Posts: 240Solar Expert ✭✭✭
    It may be cheaper just to use an  24 volt/ 120 volt inverter and then plug up a 120 volt/ 12 volt power supply or a battery charger depending on the application for what it is used/needed for.
  • BrockBrock Posts: 631Solar Expert ✭✭✭✭
    I also have been using a morningstar fed from a 48v bank to keep a 12v battery floating and running a bunch of 12v loads, it works great.
    3kw solar PV, 8 L16's, xw 5548, Honda eu2000i, iota DLS-54-13, Leaf EV, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • SCMSCM Posts: 3Registered Users
    Thanks for the feedback. 
    Brock and firerescue712, I'll definitely give it a try then. 
    Johann, that is the configuration I currently use, but I'm trying to find a way to stick to purely DC to minimize losses.  
  • SCMSCM Posts: 3Registered Users
    Any idea how the MPPT does efficiency wise when used in this way?
  • jonrjonr Posts: 1,115Solar Expert ✭✭✭✭
    edited October 2016 #7
    Why use MPPT (unless you have insufficient amps) - a PWM controller would also work with very high efficiency. [don't do this--See below. -Bill B. moderator]
  • mike95490mike95490 Posts: 8,002Solar Expert ✭✭✭✭
    edited October 2016 #8
    MPPT can throttle/limit the power from the panel or battery, to operate safely,  
     
    If you try this with a PWM controller, the internal fuse or FET switch will blow nearly instantly, as the 24V 200aH battery tries to dump it's power into the 12V battery, when the PWM circuit turns on.   Not at all efficient.  If it survived, it would only be about 50% efficient, PWM relies on a solar panel acting as a current source, not a voltage source. The further the voltage of the PV is from the battery voltage, the more loss.
    The MPPT controller will be about 95% efficient, as they nearly always (for a quality engineered one).   Not all MPPT controllers are rated for Battery - battery charging like this, but Morningstar is capable.
    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 ,

  • jonrjonr Posts: 1,115Solar Expert ✭✭✭✭
    Good point (don't do it).
  • Dave AngeliniDave Angelini Posts: 4,120Solar Expert ✭✭✭✭
    Adding to Mike, anyone who does this must take care to breaker or fuse the mppt input wiring to the battery correctly as well as the mppt output wiring the lower voltage battery.
    This application is alot diiferent than hooking solar panels to the mppt. An mppt internal failure could start a fire.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail [email protected]

  • mike95490mike95490 Posts: 8,002Solar Expert ✭✭✭✭
    for your batteries, the terminal mount fuse blocks by Blue Sea are hard to beat.

    MRBF terminal fuse by blue sea (MRBF - Marine Rated Battery Fuse)
    http://bluesea.com/productline/overview/378

    Get the fuse & holder.

    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 ,

  • BB.BB. Posts: 28,139Super Moderators, Administrators admin
    Efficiency wise, a good MPPT charge controller will be around 95% efficient at rated output current. About as good as you will get with any DC to DC converter, and more efficient than DC to AC (85% eff) to DC (~80% eff) setup (0.85*0.80=68% estimated efficiency with "standard" inverter and AC.

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