Anyone else running a split-charge relay alongside a DC-DC charger? Worth it or just belt-and-braces overkill?

Fitted a Victron Orion-Tr Smart 30A DC-DC charger to my Transit-based van build about six months ago and it's been brilliant for keeping the leisure bank topped up on the move. 120Ah of lithium, and the Orion will push it from 20% to 80% in a couple of hours of motorway driving no problem. But I've still got the old 70A split-charge relay sitting in the loom from a previous owner and I've been going back and forth on whether to yank it out or leave it in.

My thinking was to remove it and run just the Orion — cleaner install, no risk of the relay dumping unregulated voltage into the lithium. The Orion handles the charge profile properly and I've got it talking to the battery BMS over Bluetooth so it'll back off when needed. Seems like the relay is just a liability at this point.

On the other hand, a mate of mine reckons leaving it in gives you a backup path if the Orion ever packs in on a long trip. He's got a similar setup in his Sprinter and swears by having both. But surely if the relay cuts in hard at, say, 14.4V from the alternator and the BMS isn't happy, you'd be asking for trouble with lithium?

Has anyone actually run both side by side with lithium, or is the consensus just to ditch the relay and trust the DC-DC? Would love to know what you're all running.

@WattTony interesting timing on this thread — I'm planning a shepherd's hut build that'll occasionally be towed, so I've been wondering about exactly this setup.

Quick question though: with the Orion-Tr Smart, does the engine detection (via the alternator voltage sensing) work reliably enough that you'd trust it alone, without a relay as a backup? I've read some reports of it triggering from solar charge controllers pushing voltage up and accidentally pulling from the starter battery when parked.

Is that a real-world problem you've encountered or more of a theoretical worry? Trying to decide whether to budget for both on my tow vehicle circuit or just go Orion-Tr Smart alone and keep it simple.

Great thread @WattTony. I ran both for about three months on my Sprinter before pulling the relay out entirely. Honestly, with the Orion-Tr Smart doing proper voltage-sensing and staged charging for lithium, the split-charge felt redundant and was just another potential failure point. The DC-DC charger handles everything the relay used to do, but actually correctly for lithium chemistry rather than just chucking bulk voltage at it whenever the alternator's running.

@NicolaKing85 worth noting for a tow situation — the Orion completely isolates the two batteries when the engine's off, which is exactly what you want anyway. No risk of draining your tow vehicle's starter battery overnight.

My honest take: if you've already got the DC-DC charger fitted and working properly, the split-charge relay is belt-and-braces you don't need. Save yourself the complexity.

Hey @WattTony, good question. I'd say it depends on your alternator situation really. On my T6.1 I actually kept a basic split-charge relay wired in purely as a dumb fallback — not running simultaneously with the Orion-Tr, just there if the DC-DC ever plays up. Cost me about £15 and an afternoon, so hardly a massive investment for the peace of mind.

That said, if you're running lithium like yourself, the Orion-Tr is doing the proper job of protecting both your alternator and battery chemistry, which a relay simply can't do. Running them actively together seems pointless to me — the relay would just fight the DC-DC's voltage sensing.

Curious what @Callum1982 found as the deciding factor for removing his — whether it was interference issues or just simplifying the install?

Been down this exact rabbit hole on my narrowboat before moving to a van conversion — ran a Victron Orion-Tr Smart alongside an old VSR for about four months while I built confidence in the DC-DC alone.

What I'd add that nobody's mentioned yet: the split-charge relay can actually work against you with lithium. If your BMS disconnects under a high-load spike, a relay will just blindly reconnect once voltage recovers — potentially hammering the alternator repeatedly. The Orion-Tr's current limiting is doing proper protective work there.

The one scenario where I'd consider belt-and-braces is a long motorway run with a degraded alternator — belt snaps type emergency. But that's what roadside recovery is for, not extra wiring complexity.

@Callum1982's experience mirrors mine. Pull the relay, trust the Orion.

Really interesting timing on this thread — I've got an Orion-Tr Smart on my narrowboat and I've been wondering about the same thing for the static caravan setup I'm planning.

One thing nobody's mentioned yet: has anyone considered what happens during a flat starter battery scenario? If the DC-DC is your only connection, you can't use the leisure bank to jump-start, can you?

Is there a way to configure the Orion-Tr Smart to allow bidirectional emergency use, or would you need a manual bypass switch of some kind?

@StormySocket — curious whether the narrowboat experience changed your thinking on redundancy at all? On the cut you can't exactly call the AA easily if your starter battery dies miles from a services!

That practical concern is half the reason I'd be tempted to keep a relay in alongside it, even if it's technically overkill.

Great thread @WattTony. Honestly, for lithium I'd say the DC-DC charger alone is doing the heavy lifting properly — it's managing the charge profile correctly and protecting your alternator from the lithium bank's hungry initial draw. A split-charge relay on top is largely redundant in that setup.

The one scenario where I'd consider keeping both is if you have a secondary battery (say, a starter battery backup arrangement) that you want passively topped up without involving the Orion. Otherwise you're adding complexity and potential fault points for very little gain.

Six months in with lithium and an Orion-Tr Smart is pretty much the sweet spot — you'd notice immediately if something wasn't working. I'd leave well alone personally. Save the relay for someone still running AGM who actually needs it! 😄

Good points all round here. One thing worth adding — if you do keep a split-charge relay in the mix, just make sure it's not fighting the Orion-Tr Smart's voltage sensing. I've seen setups on the Cerbo GX where the relay kicks in and causes odd behaviour with the DC-DC charger's algorithm, particularly on the isolation front. @WattTony with 120Ah of lithium, your Orion-Tr Smart is honestly doing the job properly already. The relay becomes redundant and potentially a headache. Keep it clean and simple — your BMS will thank you for it.