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

So I've been scratching my head over my setup for a few weeks now. Currently running a Sterling Pro Batt Ultra 60A B2B charger to top up my 200Ah lithium leisure battery from the van's alternator (2019 Ford Transit, 130A alternator). It works a treat when I'm driving, but I've also got an old 20A split-charge relay sitting in a box doing nothing from my previous setup.

My thinking was — could I wire the relay in parallel to dump a bit of extra current in during longer motorway runs? I've seen a few people mention doing this but I'm not sure if it causes any headaches with the B2B's sensing or protection circuits. Don't want to fry anything expensive. The lithium bank is a 12V 200Ah unit from Fogstar if it makes any difference.

Has anyone actually run both simultaneously, or is the B2B doing enough on its own that the relay would just be redundant ballast? I'm getting roughly 55-58A out of the B2B in practice, which seems decent, but on shorter hops of an hour or less I'm not always getting back to full before I'm parked up again.

Would love to know what others have done — especially if you've got a similar Transit-based setup or a comparable alternator size.

@CamperBaz I'd ditch the split-charge relay alongside a B2B — you're essentially fighting yourself. The B2B already handles the isolation and charge profile properly for lithium, so a relay just adds another failure point and can confuse the system depending on how it's wired.

That said, I do run a basic VSR on my boat purely as a low-voltage disconnect failsafe if the B2B ever throws a fault. Belt and braces, not active charging duty.

For a van setup though? Redundant. Your Sterling is doing the job properly — let it get on with it. Make sure your alternator-sensing wire is clean and you'll get reliable bulk charging without the relay muddying things.

What alternator are you running? Smart alternator changes the whole conversation — that's where B2Bs really earn their keep.

@WD40Wizard11 makes a fair point, but worth being precise about why the relay becomes redundant rather than just saying ditch it.

The B2B charger already handles isolation between starter and leisure batteries — that's literally its primary function alongside the voltage conversion. Adding a split-charge relay on top means you potentially have two devices both trying to manage that same connection, which could cause odd behaviour depending on your relay's trigger voltage and whether it's VSR or manual.

One scenario where I'd reconsider: if you're also charging a second separate bank (say, a bow-thruster battery on a boat — relevant to my narrowboat setup), the relay might serve a distinct circuit not covered by the B2B.

Could you clarify your exact wiring topology, @CamperBaz? Are both devices monitoring the same positive feed from the alternator, or are they on separate branches? That detail changes the answer considerably.

@RogerKnight is right that the why matters here. The B2B charger handles voltage conversion and current limiting itself — it's designed to protect your alternator regardless of battery chemistry. A split-charge relay just connects the two batteries directly, which on a lithium bank can pull far too hard on older alternators, and the B2B already prevents exactly that.

My static caravan uses a simpler DC-DC setup but the principle's the same — duplicate protection doesn't add safety, it just adds failure points.

One scenario where you might keep the relay: if you have a separate loads circuit you want isolating, not actually for charging. But for charging purposes alone? The relay's doing nothing useful that the Sterling isn't already doing better.

@GemmaCooper86 and @RogerKnight have covered the redundancy angle well, but there's a practical edge case worth flagging: if your B2B ever fails mid-trip, a latent relay gives you a rudimentary fallback to at least limp home with some charge transfer. Not ideal for lithium — you'd want to watch voltages carefully — but it's saved me once in Scotland with a dead Victron Orion-Tr.

That said, in normal operation the relay adds zero value alongside a proper B2B. The Sterling you're running already handles the current profile correctly for lithium; a relay in parallel just risks uncontrolled bulk current hitting the battery if it somehow activates simultaneously.

My advice: keep the relay wired but switched off via an isolator. Emergency insurance only, never day-to-day.

Adding to what @CamperGeek mentioned about edge cases — one thing I'd flag from my own setup is the ignition-sensing wire on the B2B. If yours is properly wired to sense the ignition rather than just voltage, that alone handles the "don't drain the starter battery" problem that a split-charge relay traditionally solves. So you're genuinely doubling up on protection that's already baked in.

That said, I kept a simple VSR in my Transit purely as a backup path for equalising voltage between batteries when the B2B is switched off and we're on hookup. Probably overkill, but it cost me a tenner and twenty minutes. Horses for courses really!