Using an MPPT solar charger for DC-DC conversion

[Corvidae]

I'm getting ready to put new solar on my boat that will directly charge my 48v motor banks, instead of the 12v house banks. The 12v bank will still be charged by a 110 battery charger that runs off the inverter from the motor banks. What I'm trying to decide is a backup method for charging if there's a problem with the inverter.

Does anyone know if I take a 48v MPPT charger, hook the PV inputs to my 48v battery bank and the 12v battery output to my house bank, would it work? I know the 48v batteries will supply insanely higher amperage than solar panels would, but would the charger controller clamp that down without melting?

The main issue is that without the solar I have now, the only way I have to charge the house bank without the inverter is off the generator. I just got done rebuilding the generator, and my inverter is having problems. So I'm thinking I want another path for charging the house bank, just in case.

[Hartleyg]

I'm thinking you actually meant: "...take a 12V MPPT charger..." which would make sense - and yes, it should work a charm, as long as it's input voltage rating will handle the peak 48V pack voltage. You should size the 12V MPPT charger for the required amperage you need.
FWIW, it should be much more efficient than using the inverter


One caution - if your motor places high-level transients on the 48V battery when starting/stopping, you will want to filter the line to the 12V charge controller.



Hartley
S/V Atsa

[Pitchondesign]

There is no "clamping" capability on an MPPT controller. MPPT controllers are rated by watts. Exceed the rated watts and it will burn out. The correct method is to use a dc-dc charger. Victron makes them.

[Corvidae]

I know Victron has one, they're just 4x the cost of a similar rated ebay solar controller. There are even cheaper straight dc-dc converters, but they have no control at all. The solar mppt controllers just looked like a cheap way to get the job done in a pinch. Especially since I'm hoping to just have it sitting on a shelf never being used.

[secessus]

Originally Posted by Pitchondesign

There is no "clamping" capability on an MPPT controller.

MPPT controllers clamp power output from solar panels by moving Vpanel off Vmp, usually toward Voc. They do this constantly in Absorption and Float, and use the same approach to limit the controller to max power if needed. One might contest the wisdom of running a controller hard enough to limit power, but it does work. I'm overdriving one of my MPPT brutally right now (experiment / expedience) and it manipulates Vpanel to clamp output precisely at the controller's rated current.

To be fair, I have no idea what really happens when an MPPT is fed by a 48v bank and needs to clamp power. I suspect it might work, as I have fed an MPPT with a 24v power supply (DIY 120vac --> 12vdc converter/charger) and it worked fine for the months I used it. I think OP should try it, watch controller temps carefully, and report back what happens. For science. :-)

[nfbr]

We use Tristar MPPT 60 as 24v charge control from DC supply sources at work

Works a treat.