DC to DC charger to charge engine battery from house battery

Interesting one this — slightly different situation to the usual "charge leisure from alternator" setup most of us deal with.

Got me thinking about my shepherd's hut build actually, where I ended up overcomplicating the charging topology before simplifying it. The principle carries across though.

If your coach sits for extended periods and the engine batteries are only getting attention when you fire up the generator, a DC-DC charger running from house to starter makes a lot of sense. Something like the Victron Orion-Tr Smart (isolated version) would handle this cleanly — you can configure it to only run when house SOC is above a set threshold, so you're not robbing your own bank to top up the cranking battery.

Few things worth thinking through:

• Isolation matters — especially if the two banks have different charging histories or battery types
• What's charging the house bank? If it's purely the generator, you've got a decent window post-charge to let the DC-DC do its thing
• Is the engine battery actually going flat, or just sitting at a lower resting voltage? Sometimes a quality maintenance charger on a timer solves it more simply

The Orion-Tr Smart has Bluetooth monitoring which is handy for checking it's behaving without crawling into lockers.

Anyone else running a dedicated DC-DC purely for engine battery maintenance rather than the typical leisure-charging direction? Curious whether others have gone for a simpler solution like a Ctek D250SE bridging both purposes, or kept the two systems completely separate.

Hey @Gill1982, ooh yes that's an interesting reversal isn't it!

I had a similar thought during my barn conversion build — sometimes the solar bank ends up being the most reliable charging source on site, so it makes perfect sense to want it topping up the starter battery rather than the other way round.

A Victron Orion-Tr Smart would handle this brilliantly in either direction honestly. The isolated versions are worth the extra pennies too, keeps everything properly separated.

One thing worth flagging — check your alternator situation carefully before assuming the DC-DC route is your only option. But if the engine genuinely sits unused for extended periods, having your house bank babysitting the starter battery via a DC-DC is a really elegant solution.

Curious to hear how your shepherd's hut build ended up — did you go lithium on that one?

Bit of an odd one but — has anyone actually tried this with a Victron Orion-Tr Smart? I know they're usually wired the other way round but I'm fairly sure you can designate either battery as input/output depending on how you physically connect it?

Asking because on my narrowboat I've occasionally had the opposite problem — engine battery absolutely fine but house bank flattened — and wondered whether a single unit could handle both scenarios depending on which way you're running that day.

What's the actual use case @Gill1982? Is the engine battery getting discharged by something specific, or just sitting unused for long periods? Because if it's the latter, would a simple trickle feed off the solar do the job rather than a dedicated DC-DC unit?

Also curious what voltage the house bank is — 12V both sides I'm assuming?

@ThingamyBob yes, the Orion-Tr Smart will do this — input and output are just labelled for the typical use case but there's nothing stopping you reversing the roles electrically, provided your voltage differential makes sense.

Worth noting though: if your house bank is LiFePO4 and the engine battery is a standard lead-acid starter, you'll want to set the output profile accordingly. The Orion isn't "battery chemistry aware" on the output side without some manual config in VictronConnect.

One practical gotcha — the Orion-Tr Smart non-isolated version doesn't like certain shared negative setups, so double check your earthing arrangement first. The isolated variant gives you more flexibility there.

I've got something vaguely similar at my cabin (charging a small AGM from a LiFePO4 bank via a DC-DC unit) and the isolated version saved me a headache with ground loops.

@WattAndrea is correct but I'd add a word of caution here — just because the Orion-Tr Smart can be wired "backwards" doesn't mean you should do it without thinking carefully about the charging profile.

Your engine battery is almost certainly a standard flooded lead-acid or AGM, and the bulk/absorption/float voltages need to match accordingly. I've seen people bodge this on campervans and wonder why their starter battery is cooked within a season.

Also worth considering: why has your engine battery gone flat in the first place? If there's a parasitic drain somewhere, a DC-DC charger is just papering over the cracks.

In my static caravan setup I ran a Victron Orion non-smart for ancillary charging tasks — perfectly capable unit, but you need to set it up properly rather than just plug-and-pray like half the people on here seem to do.

@FormerTeacher that's a fair point about the caution, and it got me thinking about my shepherd's hut setup where I ran a Victron Orion-Tr Smart the conventional way from a vehicle alternator.

One thing worth considering before going down this route — what's actually draining your engine battery in the first place? If it's a parasitic draw issue, you'd potentially be running your house bank down just to compensate rather than fixing the root cause.

If it's more of a "I want to top up the starter battery as part of normal system management" scenario, that makes more sense to me. Does anyone know if the Orion-Tr Smart's load output detection behaves differently when wired in reverse? Wondering if the ignition-sense logic becomes irrelevant or causes issues in that configuration.