Tried this exact thing on my narrowboat a couple of years back when I was running a 24V house bank but had a 12V bow thruster and a few legacy 12V appliances I didn't want to replace.
Short answer — it works, but it's a bit of a fudge. I used a small Victron SmartSolar 75/15 with dummy solar input (a regulated 28V bench supply standing in for panels) and it did step the voltage down reliably enough. The Victron held 12V out on the load side fairly steadily.
The problems I hit:
- It's not really what MPPT controllers are designed for, so efficiency wasn't brilliant
- You lose the benefit of proper DC-DC isolation
- Getting the "solar" input voltage right to keep the controller happy takes a bit of fiddling
Honestly, for a permanent install I'd just get a proper Victron Orion-Tr DC-DC converter. The isolated versions especially. They're designed precisely for 24V → 12V and they just work without the head-scratching. Fogstar and others often bundle them competitively too.
The MPPT workaround is fine for a temporary lash-up or if you've genuinely got a spare controller sitting in a box doing nothing. Otherwise you're spending the same money on something less suitable.
Curious whether anyone's done this with a Renogy controller — I imagine the results would vary depending on how the charge algorithm behaves at the bottom end. Anyone tried it on a cabin or static setup where current draw is more predictable?
