Anyone else running a split-charge relay alongside a B2B charger — or is it overkill?

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 200), and I recently fitted a Victron Orion-Tr Smart 30A B2B charger to properly charge it from the alternator. Works brilliantly — pulls around 360W when it gets going and the leisure battery is topped up properly by lunchtime on a decent drive.

Here's the thing though — I've still got the old 80A split-charge relay sitting in there from before the upgrade. A mate reckons I should yank it out entirely since the B2B does the job properly and the relay could theoretically cause issues. But another mate says leave it in as a backup path. Neither of them actually knows what they're talking about, if I'm honest.

My main concern is whether running both simultaneously could cause any odd behaviour — voltage conflicts, false readings, or just unnecessary wear on the alternator. The Orion is set up correctly with the ignition sense wire, so it only kicks in when the engine's running. The relay is a basic voltage-sensing type that cuts in around 13.3V.

Has anyone actually run this combination, or did you ditch the relay once you went B2B? Would love to hear from people who've been through the same faff rather than just guessing at it.

Hey @DevonCamper, great setup with the Fogstar Drift! I'm running almost exactly this combination on my Sprinter and honestly it's not overkill at all — they serve genuinely different purposes. The B2B handles the proper controlled charging that lithium needs, whilst the split-charge relay acts as a useful backup and can help bridge initial connection before the B2B kicks in. One thing worth checking though — make sure your split-charge relay is actually lithium-compatible, as some older voltage-sensing relays get confused by lithium's flatter discharge curve and either won't activate or cut out too early. A dedicated lithium-aware relay or a simple manual override switch alongside your B2B gives you proper redundancy without any conflicts. What Victron unit did you go for? The Orion-Tr Smart is brilliant if you're not already on that one.

Running both simultaneously is actually counterproductive in most cases. The split-charge relay was designed for lead-acid chemistry — it'll dump alternator voltage directly at whatever your vehicle bus presents, typically 14.4V or lower, which is inadequate for properly charging lithium to capacity.

Your B2B (assuming Victron Orion-Tr Smart or similar) already handles the chemistry-appropriate charge profile and protects the alternator from the lithium battery's aggressive acceptance rate. That's the entire point of B2B technology.

The relay becomes redundant and introduces a conflict — two devices attempting to manage the same charging path.

My recommendation: disable or remove the relay entirely, let the Orion do its job. If you're worried about charge speed, size up your B2B — the 30A unit gives you roughly 360W input, which is respectable for transit driving.

What alternator output does the Transit Custom have? That's the limiting factor worth examining.

@BrookRunner makes a fair point about lead-acid origins, but I'm curious about a specific scenario that hasn't been covered yet — what happens during engine start when the alternator is cold and voltage is unstable?

My concern with emergency backup setups is whether the B2B charger handles those initial voltage spikes cleanly, or whether there's a brief window where a relay would actually offer some protection. Has anyone monitored their alternator output with a data logger during those first few seconds?

I'm also wondering whether the relay adds any meaningful redundancy if the B2B unit fails entirely — at least you'd still have some charging capability rather than nothing. For those of us prioritising backup reliability over optimum efficiency, does that argument hold water?

Not suggesting running both simultaneously necessarily, just asking whether a relay as a failsafe fallback makes sense rather than as a primary charging path.

@Clive1978 curious what scenario you're thinking of — though I'd guess it's something like a secondary alternator feed or maybe a chassis earth issue?

From my own experience on a Transit, I'd actually say there's one legitimate case where keeping both makes sense: using the relay purely as a low-voltage disconnect to protect the starter battery, while the B2B handles all the actual charging. Essentially the relay never really does any meaningful charging work, just sits there as a safety backstop.

That said, on most modern setups it's genuinely redundant extra complexity. More connections, more potential failure points. I eventually pulled my relay out entirely once the Victron Orion was doing its job properly. Simplified the whole loom considerably.

What voltage settings have you got programmed on your B2B, @DevonCamper? That's often where people run into bother with lithium rather than the topology itself.

Hey @DevonCamper, great question and one I wrestled with myself on my Sprinter build!

One thing worth mentioning that hasn't come up yet — the condition of your alternator matters quite a lot here. B2B chargers like the Victron Orion draw a controlled, steady current which your alternator actually handles really well. A split-charge relay on a lithium bank, by contrast, can demand a sudden bulk charge that some older or smaller alternators really struggle with, potentially causing them to run hot or even fail prematurely.

If your Transit Custom is fairly new the alternator should be fine, but it's worth checking the rated output before deciding on your setup. Personally I'd lean towards the B2B alone and ditch the relay — it's doing the proper job already. 😊