Swapped out my split charge relay for a B2B charger — was it worth it?

Been running a 40A Sterling B2B for about three months now after getting fed up with my old VSR setup barely topping up my leisure battery on short drives. Old split charge relay was doing next to nothing on anything under an hour, which is pretty much every trip to the shops or a quick weekend jaunt. Picked up the Sterling Pro Batt Ultra 40A for around £180 and fitted it myself in a Saturday afternoon.

The difference has been pretty noticeable, to be honest. Even on a 25-minute drive I'm seeing a proper bulk charge going into my 100Ah AGM, whereas before I'd arrive at a campsite at maybe 70% and be rationing the kettle. The B2B also seems to keep the alternator from getting hammered by limiting the draw, which I'd read was a concern on newer Euro 6 vehicles with smart alternators — I'm running a 2019 Transit Custom.

One thing I didn't expect was how warm the unit gets. It's mounted on the bulkhead behind the cab and after a longish run the case is pretty hot to the touch — not dangerously so, but enough to make me wonder if I should've given it a bit more breathing room. Anyone else running one of these in a tight spot?

Also curious whether anyone's made the jump to a higher amperage unit — the 60A or even 80A versions. Given I'm only on 100Ah of AGM, is 40A already the ceiling I should be working to, or would there be any benefit upgrading the battery bank first?

@TransitProject similar story with my shepherd's hut build, except I went the static route rather than a van.

What sold me on B2B was watching the voltage logs on my Victron BMV — the VSR was essentially doing nothing useful once the alternator load dropped off. The B2B just works, proper constant current regardless of what the engine's doing.

One thing worth watching on the Sterling units specifically — the temperature compensation setting. Mine was defaulted a bit aggressively for LiFePO4 and I nearly cooked a Fogstar cell before catching it. Worth double-checking your battery profile matches exactly.

Three months in, you'll be wondering why you waited. The difference on a 20-minute school run versus an hour's motorway stint becomes almost irrelevant, which is the whole point really.

Great timing on this thread @TransitProject — I made the same swap about eight months ago on my Transit Custom build and honestly it's one of the better upgrades I've done.

The thing people don't appreciate until they've actually run a B2B is how well it handles partial state of charge recovery. My old VSR would limp the leisure bank up to maybe 70-75% on a typical run, then just... stop doing much. The Sterling properly pushes through that absorption phase regardless of what the alternator voltage is doing.

One thing worth mentioning — if you've got a smart alternator (most post-2014 Transits do), the B2B becomes pretty much essential rather than just a nice upgrade. The VSR simply can't cope with the variable output those alternators produce.

@CumbrianWanderer curious whether your static setup uses grid hookup as the primary charge source or solar?

@TransitProject the B2B upgrade is genuinely one of the best pound-for-pound improvements you can make to a van build. Three months in you'll really notice the difference once you hit winter — alternator voltage drops with cold-start compensation and a VSR simply can't track that properly.

Worth checking your B2B's absorption voltage is matched to your battery chemistry. If you're running lithium (Fogstar Drift cells are popular right now), make sure the Sterling's profile is set correctly — some units default to AGM profiles out of the box.

One thing @ValleySolar's post probably covers, but I'll add anyway: cable sizing to the B2B matters enormously. I ran 25mm² on my Transit build and the difference in charge acceptance versus the undersized original wiring was immediately obvious on the Victron shunt readout.

What battery chemistry are you charging into?

Slightly off-topic since I'm not in a van, but I've been wondering whether a B2B makes sense for a garden office setup where the feed comes from a long cable run from the house rather than an alternator.

The voltage drop over ~25m means my Fogstar 100Ah LiFePO4 never seems to hit a proper absorption charge. Would a B2B actually help in that situation, or is it solving the wrong problem? Should I just be looking at a proper MPPT from a small solar array instead?

@CumbrianWanderer — curious how you're handling charging on your shepherd's hut given you mentioned going static. Are you on mains or purely solar?

Really interesting thread — I made this same switch last year and haven't looked back.

@Macca97 your garden office question isn't as off-topic as you think! A B2B could absolutely make sense depending on how you're feeding it. If you've got a solar panel on the office roof feeding a battery, a B2B between that and a secondary battery bank would give you much better charge management than a basic PWM controller arrangement. Though honestly for a static setup like that you might get more mileage from a decent MPPT solar controller instead — depends what your actual power source is.

Back on the van side of things — @TransitProject did you notice any difference in battery longevity yet? I'm curious whether the proper multi-stage charging profile is making a visible difference to your battery health over time. Mine seemed to improve noticeably after a few months.

@Macca97 the garden office scenario is actually a decent use case for a B2B, but only if your supply cable run has significant voltage drop or you're running a smart alternator vehicle as the source. If it's a straightforward fixed supply feeding the office, you'd almost certainly be better served by a proper mains charger — Victron Blue Smart or similar — which will give you proper multi-stage charging without the unnecessary DC-DC conversion losses.

What's your supply situation exactly? Mains spur, or genuinely pulling from a vehicle?

@TransitProject on the original question — the real gain with B2Bs on modern Euro 6 vans is that they actually work with variable-voltage smart alternators, whereas a VSR just sees the suppressed voltage and barely opens. Three months of decent charging versus three months of surface charging is a meaningful difference for battery longevity.

Hey @Macca97 — to add to what @Bazza60 was getting at, the other thing worth considering for a garden office setup is whether you're running any decent solar alongside it. A quality B2B will properly manage charging from multiple sources, which a basic relay simply can't do. If you've got AGM or lithium leisure batteries in the office, you'll definitely want the proper charge profile control anyway. What battery chemistry are you planning to use? That might actually be the deciding factor for your situation more than the cable run length.