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

So I've been scratching my head over this for a few weeks now. I've got a Renault Master (2019, Euro 6) with a 200Ah lithium leisure battery and a Sterling BB1230 B2B charger already fitted. Does the job brilliantly on the road — pulls a proper 30A into the lithium regardless of what the alternator's doing. Happy days.

The thing is, the previous owner also wired in a basic split-charge relay, and I can't work out whether it's actually doing anything useful or just sat there taking up space. My understanding is that the B2B charger handles the isolation between the starter and leisure batteries on its own, so the relay is essentially surplus. But I've read a few threads elsewhere suggesting some people keep both for redundancy in case the B2B packs in on a long trip.

Has anyone actually had a B2B fail mid-tour and been grateful they had the relay as a backup? Or is that just overthinking it? I'm tempted to yank the relay out and tidy up the wiring, but I don't want to regret it somewhere in the Scottish Highlands with a flat leisure battery and a fridge full of food.

Also worth mentioning — the van has a smart alternator with variable voltage output, which is exactly why the B2B was fitted in the first place. Wouldn't a plain relay be pretty useless on this engine anyway, even as a fallback?

Nobby78 | Posts: 847 | Location: Derbyshire

@LesHarris yes, essentially redundant in your case mate. The B2B is doing the proper job — it's isolating the starter battery, regulating charge current, and handling the lithium profile correctly. A split-charge relay alongside it would just be bypassing all that good work and potentially dumping unregulated voltage straight into your lithium cells, which you really don't want on a Euro 6 with its variable alternator output.

The BB1230 is a solid bit of kit — let it do what it's designed for. Save yourself the faff (and the cost) of adding the relay.

Only scenario I'd reconsider is if you were running a second alternator setup, but that's a whole different conversation! 🙂

SaltyRigger | Posts: 214 | Location: Array

Worth adding — the split-charge relay also risks confusing the alternator load management on a Euro 6 engine. Those smart alternators vary voltage deliberately, and a relay just passes that straight through to your lithium. The B2B is doing the clever work of interpreting that signal properly.

I ran a similar debate when planning my garden office build with a dedicated leisure setup rather than vehicle-based, but I looked at plenty of van builds before deciding. Every installer I spoke to said if you've already got a B2B fitted, a relay alongside it is solving a problem you don't have — and potentially creating a new one around uncontrolled charging paths.

What gauge cable is your Sterling running on? If it's undersized that's a more worthwhile thing to check than adding a relay.

BatteryAlan | Posts: 1,203 | Location: Array

@LesHarris the BB1230 is already doing the intelligent bit — adding a split-charge relay is like fitting a second driver who only knows how to floor it, on a Euro 6 alternator that'll throw a wobbly if you do.

LindaLamb | Posts: 312 | Location: Array

@LesHarris I ran a similar setup in my Transit for about six months before ripping the relay out entirely. The B2B is doing all the clever work — controlling charge profile, protecting the alternator on those modern Euro 6 smart alternators that throw a fit if you dump a raw lithium load straight on them.

What actually convinced me was noticing my alternator getting unusually warm on longer runs with both devices active. They were essentially fighting each other for control.

Keep the BB1230, ditch the relay. Your 200Ah lithium will charge far more efficiently with the B2B managing everything properly rather than two systems pulling in slightly different directions. Think of it like having two drivers trying to steer simultaneously — technically possible, practically daft.

Trevor1975 | Posts: 847 | Location: West Yorkshire

@LesHarris Short answer — yes, it's redundant on your setup. The BB1230 handles everything the relay would do, but far more intelligently. One thing nobody's mentioned yet though: on Euro 6 engines like your Master, the smart alternator output can drop to almost nothing when it detects a "full" starter battery. A relay would completely miss that and leave your leisure bank starving. The B2B actually compensates for this by drawing a controlled load regardless. So you're not just avoiding redundancy by ditching the relay — you'd actually be going backwards in terms of charging efficiency. Save yourself the hassle and the extra connections. Fewer joins in the wiring means fewer potential failure points down the road. Crack on with what you've got. 👍

OakTom | Posts: 2,156 | Location: Shropshire

@LesHarris Worth adding one thing the others might not have covered — on your Euro 6 Master specifically, the smart alternator with variable voltage regulation is precisely why you need the B2B in the first place. A split-charge relay on its own would struggle to charge your lithium properly because the alternator voltage dips all over the place. The BB1230 handles that brilliantly.

So not only is a relay redundant alongside it, on your vehicle a relay alone would actually be the wrong tool for the job. You've already got the right solution fitted. Save yourself the faff and spend the money on something useful instead — solar controller upgrade perhaps? 😄