Anyone else running a split-charge relay alongside a DC-DC charger, or is that overkill?

So I've been scratching my head over this one for a few weeks now. I've got a 2018 Transit Custom conversion with a 200Ah lithium leisure battery (a Fogstar Drift 200 if anyone's wondering) and a Victron Orion-Tr Smart 30A DC-DC charger fitted. The van came with an old split-charge relay already wired in from the previous owner, and I've been leaving it in the circuit rather than ripping it out.

My thinking was it couldn't hurt to have both, but a mate reckons the relay is just confusing things — apparently when the DC-DC charger is doing its job the relay shouldn't even be kicking in, and if it is, you might be pushing unregulated alternator voltage straight into the lithium cells, which the Orion is supposed to be preventing in the first place. I don't fully understand the interaction between the two, if I'm honest.

Has anyone actually measured what's happening on the input side when both are present? I've got a basic Victron battery monitor so I can see what's coming into the leisure battery, and on a good run I'm seeing around 14.1–14.3V which seems right. But I've no idea if the relay is silently doing something daft in the background.

Would it be worth pulling the relay out entirely and running the Orion on its own, or is there a sensible reason to keep both? Interested to hear how others have set theirs up.

@ExTrucker32 Running both is largely redundant and potentially counterproductive. A DC-DC charger (assuming you've gone Victron Orion-Tr Smart or similar) already handles the isolation between alternator and leisure bank — that's literally its job. The split-charge relay predates lithium chemistry; it was designed for lead-acid where bulk dumping voltage wasn't an issue.

The problem with running both simultaneously is the relay will attempt to parallel your starter and leisure batteries directly, bypassing the DC-DC's carefully regulated charge profile. With a Fogstar Drift that has its own BMS, you'll likely just confuse the system rather than damage anything, but it's sloppy engineering.

Pick one. For lithium, the DC-DC charger wins every time — proper voltage regulation, temperature compensation, and it protects your alternator from a deeply discharged lithium bank dragging it hard.

@Paddy is right about redundancy but worth adding — the relay could actually cause issues with lithium. Old-school split-charge relays don't care about charge profiles, they'll just lump whatever the alternator's doing straight onto your Fogstar. DC-DC charger handles that properly.

I've got a Victron Orion-Tr Smart in my cabin setup and it does exactly what it should — proper CC/CV charging, protects the alternator too.

Ditch the relay, run just the DC-DC. Simpler wiring, better for the battery long-term.

Hey @ExTrucker32 — to add to what @Paddy and @DeclanKnight57 have said, there's also the alternator to consider. Your Transit Custom will almost certainly have a smart alternator that drops voltage when it detects a full chassis battery — a split-charge relay won't cope with that at all, whereas the DC-DC charger is specifically designed to handle the variable output. So not only is the relay redundant alongside a proper B2B charger, it's arguably the wrong tool for that van anyway. Ditch the relay, let the DC-DC do its job, and you'll get a healthier charge into that Fogstar. Cracking battery choice by the way — I've got the same one and it's been brilliant.

Good points all round, but one thing nobody's mentioned yet — BMS interaction.

Your Fogstar Drift has a built-in BMS that will disconnect under certain fault conditions (over-voltage, over-temp, etc.). If you've got a split-charge relay sitting in parallel with your DC-DC charger, when that BMS trips and reconnects, you can get a sudden direct alternator connection through the relay with zero current limiting. That's not a hypothetical — I had a near-identical setup in my Sprinter build before I ripped the relay out entirely.

The Victron Orion-Tr Smart handles reconnection events gracefully because it's isolated and current-controlled throughout. The relay doesn't care — it just dumps whatever voltage differential exists straight through.

Keep the DC-DC, remove the relay entirely. The complexity isn't buying you anything except additional failure points and potential BMS stress on reconnect events.