Switching Charging 12 and 24 volt batteries either/on

Running a split 24V/12V setup in my motorhome and honestly the DC-DC charger rabbit hole nearly broke me — anyone else been down here?

My drive system runs 24V (Fogstar Drift LiFePO4s, lovely things) and I've got a separate 12V bank in the living space for lighting, USB points, and the fridge. Currently using a Victron Orion-Tr Smart 24/12-30A isolated unit to charge the 12V side from the 24V alternator circuit, which works a treat most of the time.

The bit that's doing my head in is managing when each bank charges and in what priority. Right now it's basically just… always on when the engine runs, which means if the 24V bank is low after a wild weekend on a Welsh hillside, it's still bleeding amperage across to the 12V side like a generous but slightly dim relative.

A few things I'm mulling over:

• Does anyone run a BMS-to-Orion comms link to properly pause DC-DC charging when the 24V side needs protecting?
• Is there a clever relay/VSR setup that only enables the Orion once the 24V bank hits a certain voltage?
• Or am I overcomplicating this and should I just whack a voltage-sensing relay and call it a day?

Curious whether anyone's cracked a tidy solution for priority-based charging across mixed voltage systems — feels like there should be an elegant answer but forums keep going in circles on it (no offence, us included).

What's your setup doing?

@WattSue oh mate, the DC-DC rabbit hole is REAL. I went down it for my static van and emerged three weeks later, slightly unhinged, clutching a Victron Orion-Tr Smart.

The 24-to-12 isolated version is the one you want for that split setup — isolation means your battery banks aren't doing weird electrical handshakes with each other. Fogstar Drifts are lovely but they're chatty via BMS and you don't want cross-contamination of charging signals between different voltage banks.

Key thing nobody tells you: size the Orion for your actual 12V loads, not just "biggest one fits". Running a 30A unit at 5A constantly is a bit sad for everyone involved.

What's sitting on your 12V side? That'll dictate the amperage you actually need.

@WattSue exactly the same boat here — I've got a 24V main bank and need to keep a 12V auxiliary topped up for my emergency backup loads (comms gear mainly).

What I've been trying to figure out is whether a Victron Orion-Tr Smart DC-DC isolates properly between the two banks? I've read conflicting things about whether the galvanic isolation actually matters in a LiFePO4 setup or if that's more of a lead-acid concern.

Also curious — are you using a BMS on each bank independently? I'm worried about what happens if one BMS disconnects under load while the DC-DC charger is mid-cycle. Does that cause any problems on the source side?

@WattSue the Orion-Tr Smart is almost certainly your friend here — the non-isolated version is fine if your 24V and 12V banks share a common negative (most motorhome setups do), and it'll save you a fair bit of money over the isolated variant.

Key thing people miss: size the DC-DC charger to your actual 12V consumption, not your battery capacity. If you're only running 12V lighting and a water pump, a 30A unit is overkill — the 17A Orion handles most auxiliary setups comfortably.

Mine runs 24V Fogstar bank → Orion-Tr Smart 24/12-17 → separate 12V AGM for engine-bay accessories. Configured it through VictronConnect in about 20 minutes. The absorption/float settings are worth adjusting properly for LiFePO4 if your 12V side is also lithium.

What's your approximate 12V daily load? That'll help narrow things down considerably.

@ExFirefighter42 worth flagging though — if your 24V and 12V negatives aren't properly bonded, the non-isolated version can cause headaches. Learned that the hard way in my cabin build before I switched to the isolated Orion-Tr.

Quick question for anyone who knows — can you run two Orion-Tr Smart units in parallel on the 12V output side if you need more current? My 12V loads are creeping up and I'm wondering whether I'd be better off doing that or just upsizing to a single larger unit. The 30A isolated version seems to be the sweet spot pricewise but I can't find clear guidance on whether Victron supports paralleling these specifically.