Charging Start Battery via Solar

Been thinking about this setup a fair bit lately on my static caravan build.

Running a Victron SmartSolar 100/20 into a 24V LiFePO4 house bank, and I want to keep a separate 12V start battery topped up without just wiring them together. The obvious solution everyone jumps to is an Orion-Tr Smart DC-DC charger, but I'm still weighing it up.

The isolated versions are pricier but seem worth it to keep the two battery systems properly separated — especially with lithium on the house side. Anyone actually running this combo day-to-day?

A few things I'm unsure about:

• Does the Orion need to see a proper "charge signal" or does it just work from voltage alone?
• Is 20A overkill for just maintaining a small start battery that barely gets used?
• Any issues with the Orion pulling power constantly even when the start battery is full?

I've seen some builds using a simple VSR (voltage sensing relay) instead, but that feels a bit blunt when there's lithium involved — don't really want the house bank trying to drag a flat starter up.

Fogstar and similar suppliers seem to have good deals on the non-isolated Orions at the moment, but I'm leaning isolated just for peace of mind.

Curious what others have done — particularly on boats or motorhomes where the start battery setup is more critical than mine. Is the Orion the right tool here or am I overcomplicating it?

@FellLover this is almost exactly what I did for the shepherd's hut build last spring.

The bit people often overlook — you can't just tap the 24V bank directly into a 12V starter without a proper DC-DC charger in between. I used a Victron Orion-Tr Smart 24/12-9 isolated unit, triggered by a virtual load output from the SmartSolar when it hits absorption stage.

Keeps the start battery properly maintained without any risk of backfeed, and the isolation matters enormously if your chassis grounds differ between the two systems.

Worth checking the Orion's input voltage range too — on a 24V LiFePO4 bank you can see 29V+ during bulk, and some cheaper DC-DC units don't handle that gracefully.

@IslandOffGrid snap on the shepherd's hut life 🛖 — mine's been running a Victron Orion-Tr Smart 24/12-5A isolated DC-DC for exactly this job and it's been flawless for two winters now.

Key thing nobody mentions: set the Orion to charger mode (not alternator assist) when it's solar-fed, otherwise it gets confused about input voltage thresholds and sulks.

Also worth noting the 100/20 will need a bit of headroom in your house bank before the Orion kicks in — set your Orion's input start voltage to something sensible like 26.5V so you're not robbing the house bank when it's barely half full.

@FellLover worth flagging that the Orion-Tr Smart @CamperCarl mentioned has a built-in "engine detection" feature — but on a static caravan build with no alternator, you'll want to configure it in "always on" mode rather than relying on voltage-sense for start/stop triggering.

Also, on a 24V-to-12V setup, even the 5A isolated unit pushes 60W into your start battery — more than enough to keep it healthy without ever cooking it. I ran a similar config in my motorhome for two seasons before going full lithium on the start side too.

One thing I'd genuinely check: make sure your SmartSolar has enough headroom on its load output or you're pulling from the bus properly — don't starve the house bank just to pamper a start battery that barely works for anything.

Good points from everyone here. One thing worth adding — on a static build like yours @FellLover, you don't actually need the engine detection feature that @LiFePO4Nerd flagged, since there's no alternator in the picture. That simplifies your config considerably.

What I'd watch is the absorption/float voltage mismatch between your 24V LiFePO4 bank and the start battery's requirements. The Orion-Tr Smart lets you set custom charge profiles, so dial that in properly rather than leaving it on defaults — I made that mistake on my motorhome build and had a very confused Gel battery for about three months before I twigged what was happening.

Also consider whether 5A is sufficient for your use case. If that start battery is powering anything meaningful, the 24/12-10A variant costs very little more and gives you proper headroom.