Orion Tri DC-DC converter connected to a 12v starter battery

Been meaning to post about this for a while after wrestling with the same setup in my static van.

Running a 24V Victron lithium bank as my main leisure setup, but the van's original 12V lead acid starter battery just sits there doing sod all most of the time — until I need it to start the heating system or run the step motor. Classic problem.

I ended up looking hard at the Orion-Tr Smart 24/12-5A non-isolated for exactly this float/maintenance role. The logic being: leisure bank is healthy and charging via solar all day, so why not bleed a trickle across to keep the starter battery honest?

Few things I wasn't sure about though:

• Non-isolated is cheaper and simpler, but is it actually fine when both batteries share a common negative? On a van setup with chassis grounds involved it felt slightly sketchy to me
• Setting the output to 12.7V makes sense for float, but does the Orion-Tr Smart let you dial that in precisely through the app, or is it fixed at certain steps?
• Anyone found the 5A version sufficient just for float/maintenance duty, or is it so light that the Orion barely does anything meaningful?

I ended up holding off and just running a basic Renogy DC-DC temporarily, which honestly isn't ideal because it hammers the starter battery rather than floats it gently.

If anyone's actually got the Orion-Tr configured this way — especially in a 24V to 12V scenario — I'd genuinely love to hear how you've set it up. Feels like there's a right and a wrong way to approach this and I'm not fully confident I know which is which yet.

@FellKev had almost the exact same headache with my cabin setup — 24V Fogstar lithium bank, 12V lead acid starter.

The Orion Tri is brilliant for this but the isolation is what really matters here. Make sure you're running the isolated version, not non-isolated — mixing voltage systems without it is asking for trouble.

A few things worth checking:

• Input voltage range — the Orion handles 12V starter fine but verify your absorption/float levels aren't confusing it
• Set the engine detection properly or it'll try to charge from a dead starter
• Fuse both ends close to the batteries

The Victron app makes configuring the charge profile pretty straightforward once you're connected via Bluetooth. What firmware version are you running? Older versions had some quirks with the engine detection threshold that tripped people up.

@FellKev worth flagging something neither you nor @EmmaEdwards82 have mentioned — the Orion Tri's input voltage range bottoms out at 6V but the absorption behaviour matters more than people realise when your source is a starter battery.

On my narrowboat I run nearly identical topology. The critical bit: configure the Orion as a fixed voltage output rather than letting it chase the starter battery's voltage fluctuations. Lead acid starter batteries under engine load can sag embarrassingly, and if your Orion interprets that as a fault condition it'll cycle on/off like a confused toaster.

Also check your cable sizing for the 12V side specifically — the current draw at 12V input to push meaningful watts into a 24V bank is twice what you'd naively expect. Victron's own calculator is decent, but I've found real-world losses consistently worse than their estimates.

@SomersetNomad good shout on the input range — caught me out in my van conversion when the alternator was doing its warm-up dance and the Orion Tri just sat there sulking until voltage settled.

One thing worth adding: if you're running the Orion Tri in engine-off mode (i.e. not using the ignition sense wire properly), you'll quietly flatten that 12V starter battery without realising. Done it. Embarrassing. Fogstar bank was laughing at me while I was bump-starting in a Tesco car park.

Wire the ignition sense correctly or use VE.Direct to set charge conditions — don't just assume the unit knows what it's doing unsupervised.

Great points all round — and this is exactly the rabbit hole I fell down last winter on the boat.

One thing worth adding: don't overlook the Orion Tri's isolated vs non-isolated variants. On my setup I initially grabbed the non-isolated unit, not realising that with separate battery banks at different voltages you can end up with a ground loop nightmare. Switched to the isolated version and the interference on my 12V instruments disappeared overnight.

If you're running a 24V leisure bank feeding down to a 12V starter, the isolated Tri-Smart is almost always the right call — the galvanic separation just keeps things tidy across two voltage systems that were never designed to talk to each other.

Worth double-checking which variant you've actually got fitted, @FellKev — the packaging isn't always obvious at a glance.

@MasterSolar raises a good point about the boat context — the moisture considerations there are brutal.

Something nobody's touched on yet: make sure your Orion Tri is configured for the correct battery type on the output side. The default profile out of the box isn't always appropriate for lithium, and I've seen people happily "charging" their LiFePO4 bank with an absorption voltage that's either too high or cutting off too early.

In VictronConnect you can set custom charge curves — absolutely worth five minutes of your life to verify this matches your BMS specs. My Fogstar Drift cells wanted a specific float voltage that differed from the Victron lithium preset.

Also worth noting: if your 12V starter battery is AGM rather than flooded lead acid, the input behaviour under load can differ slightly. Check what you've actually got before assuming the defaults are safe. 🔋