How to charge 48V LiFePo4 from 12v LiFePo4 batteries

Been down this exact rabbit hole with my shepherds hut build — spent three weeks convincing myself there was some elegant solution before accepting the universe just wanted me to buy more kit.

The cleanest approach I found is a DC-DC boost converter rated for the current you need. Something like a Victron Orion (though those are typically buck/step-down, so check the spec carefully) or a dedicated 12V→48V boost unit. The key thing is matching your charge profile — your 48V Fogstar bank still needs a proper LiFePo4 absorption/float curve, not just "shove watts at it and hope."

A few things worth thinking about:

• Efficiency losses — you're probably looking at 85-92% conversion, so factor that in if you're already tight on capacity
• BMS communication — if both banks have smart BMS units, make sure neither cuts out mid-charge and leaves the converter in a weird state
• Fusing both sides — because of course, always fuse both sides

The awkward truth is that 12V and 48V systems are a bit like mixing metric and imperial spanners in your toolkit — technically possible, mildly irritating, and you'll question your life choices regularly.

Has anyone actually done this long-term with a motorhome setup? Curious whether the boost converter approach holds up on the road or whether it's more of a static-install solution.

@CharlieCampbell66 curious what you landed on in the end.

For anyone else finding this thread — the options as I understand them:

1. DC-DC boost converter (12V → 48V) — possible but efficiency losses stack up, and finding one rated for meaningful current isn't straightforward
2. Victron Orion-Tr Smart — but these are 12V → 12V/24V, not 48V, so doesn't directly apply
3. Separate charging sources — just charge each bank independently from solar/shore power

For my static caravan setup I've been considering something similar for EV charging headroom. The maths on conversion losses made me question whether it's worth it over just correctly sizing the 48V bank from the start.

What's the actual use case here — is the 12V bank legacy kit you're trying to integrate, or are you genuinely running two voltages intentionally?

@CharlieCampbell66 been through something similar wiring up the static caravan — ended up running a Victron Orion-Tr Smart 12-48V DC-DC converter which does the job cleanly. Not cheap, but the isolation means your BMSs aren't fighting each other, which matters more than people realise until something goes wrong at 2am.

Worth checking your 12V bank's continuous discharge rating before sizing it — the Orion will pull a fair current from that side depending on what you're pushing into the 48V bank.

If you're also thinking about EV charging down the line on the 48V system, factor that into your converter sizing now rather than retrofitting later. Learnt that one the hard way.

Worth adding to what @VivaroDream mentioned — the Orion-Tr Smart is solid but do double-check your input voltage range if your 12V bank ever sags under heavy load. LiFePo4 sits fairly flat across its discharge curve which helps, but I've seen setups where the Orion browns out unexpectedly because someone sized the 12V bank too lean.

Also worth considering whether you actually need bidirectional transfer or just one-way charging. A lot of people overcomplicate this when a simple DC-DC charger does the job perfectly well without the added cost.

What's the actual use case here — are you trying to top up the 48V bank from the 12V as a backup, or is this your primary charging path? That'll change the answer quite a bit in terms of what size unit makes sense.

Good points from @BrianKnight86 on the voltage range — worth adding that if your 12V bank is on the smaller side, the Orion-Tr Smart's quiescent draw can be a slow drain when the 48V system is demanding charge and the 12V hasn't quite recovered. Seen a few people catch this out when they've got mismatched bank sizes.

Also worth considering whether you actually need the two banks talking to each other continuously, or just occasionally. If it's the latter, a manual switch or a simple automation trigger on a voltage threshold can save you a lot of complexity and potential failure points. Sometimes the boring solution is the right one!

@CharlieCampbell66 — what are the relative capacities of your two banks? That might shape which approach makes most sense for your setup.