Anyone else running a split-charge relay alongside a B2B charger — is it even worth keeping both?

So I've been scratching my head over this for a few weeks now. I've got a 2019 Transit Custom conversion with a 200Ah lithium leisure battery (Fogstar Drift), and I've been running a Sterling B2B charger (BB1230, 30A) for about a year. Works brilliantly on longer drives — gets the lithium properly charged rather than just trickling. But I also still have the original 70A split-charge relay in the system from when I had a leisure AGM, and I've just left it in because I wasn't sure whether to pull it out.

The thing is, I'm not entirely sure the relay is actually doing anything useful anymore. From what I understand, the B2B takes care of the charging profile properly, and the relay sitting in parallel might actually be causing some confusion — or at worst, allowing the alternator to try and dump current into the lithium without proper regulation? I'm honestly not 100% sure on this, which is why I'm asking. I've read a few contradictory things on various forums.

Has anyone run both simultaneously and had issues, or is the relay genuinely harmless left in place? I'd rather not rewire the whole lot if I don't need to, but equally I don't want to be slowly cooking my alternator or doing something daft to the battery. Happy to pull the relay if there's a proper reason to.

RockyMaker | 847 posts

@GemmaFisher82 Short answer — ditch the split-charge relay if you're running a proper B2B. The relay made sense in the lead-acid days but your Fogstar Drift will confuse it something rotten. Lithium's flat voltage curve means the relay often won't even know the leisure battery needs charging, or worse, it'll think it's full when it isn't.

Your Sterling is doing the heavy lifting properly — it's actively managing the charge profile your lithium actually needs. The relay sitting alongside it is just a redundant complication waiting to cause a headache.

Only exception I'd consider is if you're doing very long motorway stints and want raw bulk current on top, but honestly for a 200Ah bank your B2B should be more than adequate. Simplify the system — you'll thank yourself later! 🔧

Brummie86 | 234 posts

Same setup in my Transit, pretty much. Had both running for a while thinking it was "belt and braces" but the relay was actually confusing things — kept getting weird voltage readings on the Fogstar.

@RockyMaker is right, the B2B does the job properly. Split-charge relays were designed for lead-acid, they don't really get lithium's charging profile.

Only reason I'd keep a relay is if your B2B ever packs in — handy backup while you're stuck in a layby somewhere. But day-to-day? Disconnect it from the circuit and just leave it as a fallback if needed.

What B2B are you actually running? Makes a difference. My Victron Orion-Tr Smart has been solid for two years now, proper CC/CV charging.

Wez1961 | 1,203 posts

@GemmaFisher82 The relay is doing nowt useful if your B2B is properly sized. Split-charge made sense with lead-acid because voltage-based isolation worked fine — lithium's flat discharge curve fools it completely.

One thing worth checking though: what's your alternator rated at? Older Transits can struggle with a decent B2B hammering it constantly. A 30A Sterling unit on a tired alternator isn't a great combination. Seen a few people fry diodes that way.

If your B2B has a load-sensing input, use it — lets the alternator warm up before charging kicks in properly.

Keep the relay only if you want a dead-simple backup for emergencies. Otherwise it's just unnecessary complexity and another thing to fault-find when something goes wrong at the side of a road in the dark. Ask me how I know. 🙄

T5Wanderer | 412 posts

Worth adding — the split-charge relay isn't just redundant with a B2B, it can actually cause problems with lithium. If your Fogstar BMS trips under load and disconnects the leisure side, that relay could still be trying to push alternator voltage through without the current limiting the B2B provides. Seen a few people fry their alternators that way, particularly on Transits where Ford's smart alternator is already temperamental with lithium charging.

Keep the B2B (Sterling do solid units), remove the relay, job done. The B2B handles the voltage step-up from the smart alternator properly — the relay simply can't.

Only caveat: make sure your B2B is rated high enough. Running EV pre-conditioning off leisure batteries means I need consistent charging on the move, so undersizing it kills you.

Nobby30 | 867 posts

Good points from @Wez1961 and @T5Wanderer already. One thing nobody's mentioned yet — if you do decide to keep the relay in place, make sure it's actually isolated rather than just disconnected at the terminal. Had a mate with a similar hybrid setup who left his relay wired but dormant, and it was causing intermittent voltage drop readings that had him chasing a fault for weeks. Turned out the relay itself was introducing resistance into the circuit even when supposedly inactive. With a Fogstar Drift specifically, the BMS is sensitive enough that you'll likely notice any funny business on your monitoring. My honest advice — remove the relay cleanly, label everything up properly, and let the B2B do its job. That's what you paid for it. Keep the install tidy and you'll thank yourself later.

Barry1962 | 2,847 posts

Good thread this. @Nobby30 raises an interesting point wherever he was going with that! One thing I'd add — with a Fogstar Drift specifically, you really want the B2B managing that charging profile properly. Lithiums are fussy about bulk/absorption stages and a dumb relay just chucks voltage at it without any real control.

The only scenario I'd argue for keeping the relay is as a dead-simple emergency fallback if your B2B packs up miles from anywhere — though honestly even then you'd want a low-current fuse in that circuit to protect the alternator.

What's your B2B rated at, @GemmaFisher82? A 30A unit on a 200Ah bank is going to feel a bit pedestrian if you're doing shorter runs. Might be worth checking whether that's your actual bottleneck before worrying about the relay.

VanRhys | 1,203 posts

Not a van setup but I've got a similar relay-plus-B2B situation on my static caravan build using a Victron Orion-Tr Smart — and honestly the relay just sits there doing nothing useful most of the time. The B2B handles current limiting properly for the alternator, which the relay absolutely cannot do with lithium. One thing worth flagging that nobody's touched on yet — the relay can create a back-feed path depending on how it's wired, particularly if you've got any solar in the mix. Caught me out briefly. Isolate it or remove it entirely, I'd say.

CumbrianWanderer | 412 posts

@VanRhys curious to hear more about the static caravan setup — I've got something similar on a shepherd's hut up near Penrith.

My honest experience: the split-charge relay became almost ceremonial once the Victron Orion-Tr Smart went in. It technically still does something, but the B2B is doing the actual heavy lifting and protecting the lithium properly with a tailored charge profile.

The relay's only real remaining job for me is a crude failsafe if the B2B ever packs up. Whether that redundancy justifies the extra complexity in your wiring loom is genuinely down to your own risk appetite.

Megan1996 | 89 posts

Not a van either but I've got a Victron Orion-Tr Smart doing all the heavy lifting for my garden office setup — runs off the house battery bank via a long cable run. Ditched the split-charge relay pretty early on and honestly haven't missed it. The Orion handles the voltage conversion properly and doesn't hammer the source battery. Only reason I'd keep both is as a backup failsafe, but even then it's extra complexity for not much gain. What's your cable run like @GemmaFisher82? That made a bigger difference to my setup than anything else.