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hoeken avatar image
hoeken asked

MPPT as Battery to Battery Charger?

I recently got a new build catamaran with a pretty complex Victron system that has a large 48v house bank, with 24v and 12v buffer batteries. The installer used solar MPPT chargers to convert from 48v to 24v and from 48v to 12v. I was always a bit hesitant about this, but it does seem to work. I was going to let it go, until I was working on the 48v bus with it turned off, and I noticed the 48v to 12v charger was backfeeding 12v onto the bus that I had assumed was dead.

When doing research I found a few scattered pages here and there that advise not to use the MPPTs like this, but I haven't been able to find anything definitive. Its hard to go back to the installer who has been doing it like this for years with random forum posts.

Can anyone from Victron point me to an authoritative post or manual or something?

MPPT Controllersbattery charging
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2 Answers
Johannes Boonstra (Victron Energy Staff) avatar image
Johannes Boonstra (Victron Energy Staff) answered ·

Hello, The point is mainly that an MPPT has a feature to stop getting power from solar panels when something might go wrong. This feature will short circuit the input, which works well for solar panels of course as the short circuit current is limited. Having the MPPT between (large) batteries this feature can result in a damaged MPPT after its decides it needs to short the input. In short, its indeed not advised to have the MPPT used as DcDc converter between battery banks.

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thanar avatar image thanar commented ·
I’ve read about this “shorting out the input” numerous times, but never had any technical info on this. Under which circumstances would the controller “short” the input?
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djsmiley avatar image
djsmiley answered ·

As @Johannes Boonstra (Victron Energy Staff) mentions, this is not recommended.

If you have a CC limiting (current limiting) in place on the PV input side, it can work safely, but a bare battery bank has no such thing. A power supply or similar might have, but you have to somehow guarantee the short circuit current is never going to exceed the PV input current (See datasheet)

It will work fine, as long as the MPPT doesn't have to 'disable' the PV input. If it has to, and there is no limit (or too high limit), there is a risk of frying the MPPT (killing the mosfets).

Just get the equipment made for the task, instead of abusing products not intended for this application.



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