DC - DC Battery Charging Using a Charge Controller

SCM · October 2016

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?

firerescue712 · October 2016

I used a Morningstar SunSaver MPPT15L for charging a 12v bank from a 24v bank for 2 years. Worked flawlessly!

Johann · October 2016

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.

Brock · October 2016

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.

SCM · October 2016

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.

SCM · October 2016

Any idea how the MPPT does efficiency wise when used in this way?

jonr · October 2016

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]

mike95490 · October 2016

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.

jonr · October 2016

Good point (don't do it).

Dave Angelini · October 2016

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.

mike95490 · October 2016

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.

BB. · October 2016

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

DC - DC Battery Charging Using a Charge Controller

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