Anyone else running a split-charge relay alongside a B2B charger? Worth it or just redundant?

So I've been scratching my head over my setup in the Transit-based van. Currently got a 40A B2B (DC-DC) charger running off the alternator into a 200Ah lithium leisure battery, and it works a treat when I'm driving any decent distance. But I've also kept the old 20A split-charge relay in the circuit from when I had AGMs, and I'm wondering whether it's actually doing anything useful or just adding unnecessary complexity.

My thinking was that the relay might give a bit of extra charge current on top of the B2B, but I've read conflicting things — some say the B2B handles everything and the relay can actually cause issues, others say running both is fine as long as the wiring's sensible. I'm getting around 38–40A into the leisure battery when driving, which seems about right for the B2B alone, so I can't tell if the relay is contributing anything at all.

Has anyone actually measured whether a split-charge relay adds meaningful current on top of a B2B, or is it just dead weight at this point? I've got a Victron SmartShunt so I can log the incoming amps fairly accurately if someone wants me to run a proper test.

@Gazza22 Different world on a boat but the principle's the same — I ran a split-charge relay alongside a B2B for about six months thinking I was getting belt-and-braces protection.

What actually happened was the relay was doing precisely nothing useful. The B2B already handles the voltage conversion and protects your alternator from the lithium's greedy appetite. Slapping a relay in parallel just means you occasionally get unregulated direct charging bypassing all that clever electronics.

Pulled the relay out, never looked back.

The one scenario where you'd want both is if you're running a separate starter battery top-up circuit — relay for the lead starter, B2B exclusively for the lithium leisure bank. That's a legitimate split.

Otherwise you're just adding failure points for zero gain.

@Gazza22 The relay alongside a B2B is genuinely redundant in most cases — the B2B is your charge management. Where people get confused is thinking the relay adds capacity, but your Victron Orion (or equivalent) is already current-limited by design.

One scenario where it makes marginal sense: using the relay to power a non-charging DC load directly from the starter battery when the alternator's running, keeping that load off your leisure bank entirely. But that's a very specific use case.

On a boat I run a similar arrangement — B2B handles all lithium charging, full stop. No relay muddying the waters.

Worth checking whether your 40A B2B is the isolated or non-isolated variant. If your chassis earth and leisure bank share a common negative, non-isolated is fine. If not, you'll want the isolated version — learned that the hard way with a previous install.

@Gazza22 One scenario where I'd actually keep both is as a fallback — if your B2B packs in on a long trip, a relay gives you something getting into the leisure battery rather than nothing. It won't be pretty for a lithium without proper charge management, but in a pinch it beats sitting with a flat battery in the middle of nowhere. That said, day-to-day the B2B is doing all the heavy lifting and the relay adds complexity for minimal gain. If you're not bothered about the emergency backup angle, I'd honestly just leave the relay out entirely and keep the wiring clean. What's your alternator rated at? A 40A B2B on a smaller alternator can sometimes cause issues that make people think the relay is helping when it isn't.

@CliveCrane makes a fair point on the fallback angle, though in practice if your B2B dies you're probably not getting much useful charge through a relay into lithium anyway without proper voltage regulation.

My shepherd's hut setup is obviously static solar rather than alternator-based, but I've done a fair bit of reading on this for a potential van build. The Victron Orion-Tr Smart is really the standard now — it handles the intelligent charging side so a relay becomes dead weight in normal operation.

One thing I haven't seen mentioned: some newer Fords with smart alternators produce variable voltage specifically to reduce fuel consumption, which means a basic relay can actually damage lithium cells. The B2B is doing critical protective work there, not just convenient work. Worth checking your Transit's alternator type before making any decisions.

@Gazza22 Worth considering the wiring complexity angle here — adding a relay alongside your B2B means another potential failure point, more connections, and more to diagnose when something goes wrong. On a lithium setup especially, your B2B is doing exactly what it should: managing charge profile properly, protecting the alternator from being hammered. A relay chucking bulk voltage straight at lithium always made me a bit uneasy anyway without proper oversight. If redundancy is the goal, I'd personally put that money and effort toward a small solar input instead — gives you a genuinely independent charging path rather than two systems fighting over the same source. What alternator are you running? Some of the newer smart alternators complicate matters further regardless of which route you choose.

@Gazza22 One thing nobody's mentioned yet — check whether your specific B2B charger has a built-in load disconnect or low-voltage protection on the input side. Some of the better units will simply stop drawing from the starter battery if voltage drops below a set threshold, which is essentially doing the relay's job electronically and more precisely than a voltage-sensitive relay ever could. If yours does, you're genuinely doubling up on protection that's already there. Worth digging out the manual or ringing the manufacturer. That said, if you're doing serious mileage in remote areas, @CliveCrane's fallback argument does hold some weight — just make sure the relay is properly fused and rated for the cable gauge you're running, otherwise you've introduced a weak point rather than a safety net.