Anyone else running a split-charge relay alongside a DC-DC charger? Confused about whether I need both

I've been scratching my head over my van's charging setup for a few weeks now and could do with some outside input. I've got a 2019 Ford Transit Custom with a smart alternator, and I'm trying to properly charge a 100Ah lithium (LiFePO4) leisure battery. From what I've read, the variable voltage from smart alternators makes a standard split-charge relay pretty much useless — it can't hold the threshold voltage long enough to kick in reliably.

So I bought a Victron Orion-Tr Smart 12/12-30A DC-DC charger, which I've had running for about a month. It handles the smart alternator properly and I'm genuinely happy with it. But when I bought the van, it already had a 140A split-charge relay wired in by the previous owner. I've left it in place because I wasn't sure whether removing it was the right call, and part of me wonders if there's any benefit to running both together.

My gut says the relay is now just dead weight — or worse, potentially interfering with the Orion doing its job properly. Could it cause any issues having both in the circuit simultaneously, or is it genuinely just redundant now? The Orion is wired directly between the starter and leisure batteries, fused both ends (40A on the leisure side, 60A on the starter side).

Has anyone dealt with this exact setup, or does anyone know enough about the Orion's internals to know whether a relay upstream could cause it any grief?

VickyMurray | 847 posts

@RachelLamb Yes, this catches a lot of people out with modern Transits! Short answer: with a smart alternator you really don't want a traditional split-charge relay on its own - the alternator sees the connected battery as a load and can behave erratically trying to compensate.

A DC-DC charger (B2B charger) is what you actually need as your primary charging method - it works with the smart alternator's variable voltage output rather than against it.

Some folks run both, but the relay becomes largely redundant and can actually cause issues. The DC-DC charger handles everything properly on its own.

What size leisure battery setup are you running? That'll help narrow down which B2B charger capacity makes sense for your use case. Victron Orion-Tr Smart is popular for the Transit Custom if you're looking at options.

WelshSolar | 1,203 posts

@RachelLamb With a smart alternator you definitely don't want a split-charge relay — it'll either undercharge your leisure battery or confuse the alternator's voltage sensing. Been there with my previous Berlingo before I knew better.

A DC-DC charger (B2B) is the only proper solution here. The Victron Orion-Tr Smart is the gold standard, but Renogy and Sterling also do decent units at lower price points.

The DC-DC charger isolates your leisure battery completely from the starter circuit whilst still pulling a proper charge profile. Your alternator thinks it's just seeing a small, steady load — happy days.

• One or the other, not both
• B2B wins every time on a smart alternator
• Size it appropriately for your cable run (voltage drop matters)

What leisure battery chemistry are you running? That affects which unit suits you best.

StormyWelder | 312 posts

@RachelLamb Just to add a potentially daft question — have you checked whether your Transit Custom actually has the smart alternator or the fixed-voltage one? A mate of mine spent a fortune on a Victron Orion-Tr Smart DC-DC before realising his older spec van had the traditional alternator all along. 🤦

Worth plugging in an OBD reader or checking with Ford using your VIN before you commit to anything. The smart alternator variants do voltage stepping that'll confuse a basic relay into barely charging your leisure battery at all.

If it is smart alternator (likely on a 2019), then as @WelshSolar says, the DC-DC charger is your friend here — the Orion-Tr Smart is what I ended up fitting on my shepherd's hut build's donor vehicle. Absolute game-changer.