Anyone else running a split charge relay instead of a B2B charger to save money — how's it actually working out?

I've been running a 20A Sterling B2B for a couple of years but recently got chatting to someone at a meet who swore blind his old-school split charge relay setup was "doing fine" on his 100Ah AGM. Got me wondering whether I've been overthinking it and spending more than I needed to.

The obvious issue I know about is that a basic VSR just dumps alternator voltage straight into the leisure battery, so with a 14.4V charge from the alternator you're never really getting a proper bulk/absorption/float cycle. For a cheap AGM that might not matter too much short-term, but I'd worry about battery longevity over a few years. My B2B cost me around £160 and the relay route would've been maybe £15-20 for a decent VSR like a Ring RSCDC.

Has anyone actually done a long-term comparison, or tracked battery health over time running just a relay? Curious whether the savings are genuinely worth it depending on how much driving you do versus solar or hookup. I do mostly weekend trips with a fair bit of driving, so alternator charging matters to me — but I can see the sums working differently for someone parked up most of the time.

@Davo — "doing fine" is doing a lot of heavy lifting in that sentence, isn't it.

Had a split charge relay on my old static before I knew any better. AGM sitting behind it looked healthy enough on a multimeter. Swapped it out for a Victron Orion-Tr Smart and the thing charged to a proper 14.7V absorption for the first time in God knows how long.

The relay just follows the alternator voltage — which is 13.8V on a good day, often less. Your battery never properly cycles, sulphation builds up quietly, and one winter you've got a dead 100Ah that "was doing fine."

It's fine the way eating nothing but toast is fine. You're alive, but you're not thriving.

If budget's the real issue, a Fogstar B2B is decent enough without the Victron price tag.

@OhmsLaw makes a fair point. The thing people miss with relays is they're voltage-sensitive — once your starter battery drops below the threshold during charging, the relay opens and your leisure bank stops charging entirely. Fine on a short motorway run, not so fine if you're mostly doing town driving or short hops between sites.

Also worth considering what your alternator sees. A B2B gives it a controlled load; a relay just throws your depleted leisure bank straight across it. On older vehicles that's usually fine, but on anything with a smart alternator it can cause all sorts of headaches.

For a healthy AGM doing occasional weekend trips it might genuinely be "fine." But if you're full-timing or running heavier loads, I'd stick with the B2B personally. Saves money until it doesn't, if you follow me.

Really depends what you're asking the system to do, honestly. If someone's just topping up a partially discharged AGM after a few hours driving, a relay probably won't cause obvious problems in the short term — the battery might even seem fine for ages.

The real issue shows up with lithium or if you're doing serious cycling. A relay can't do bulk/absorption/float stages, so you're basically just throwing voltage at the leisure battery and hoping for the best. AGMs can tolerate that more than lithium can, but you're still leaving capacity on the table and potentially shortening battery life without realising it.

@SY_Marine is right about the voltage sensitivity too — at low alternator output the relay barely conducts properly anyway.

For occasional weekend use with a healthy AGM? Maybe acceptable. As a long-term strategy? You're false-economising.

@Davo I think the missing context here is how discharged his leisure battery typically gets before he drives. If he's only ever pulling it down to 50-60% and doing decent motorway stints, a relay can muddle through on AGM. It's when you start expecting it to do real recovery work — say after a couple of stationary nights running a fridge — that you'll notice the voltage-based cutoff leaving you with a frustratingly partial charge. The alternator just sees "near enough" and stops pushing. Your Sterling meanwhile will actually finish the job. I'd be curious whether his mate has ever actually measured state of charge properly or is just going off "lights work, seems fine." Not the same thing at all! Horses for courses, but I wouldn't ditch a working B2B to save a few quid personally.

@RockyCaptain makes a really good point about depth of discharge. I'd add that the duration of the drive matters just as much. A relay setup on a 100Ah AGM might look fine on the surface because the voltage recovers quickly and the relay owner thinks "job done" — but AGMs really want a proper absorption stage to hit full charge, and a relay just can't deliver that once the voltage differential drops off.

His battery is probably sitting at 80-85% most of the time and slowly sulfating without him realising. Won't notice for a year or two then wonder why capacity has tanked.

If budget's the issue, a used B2B off eBay is still miles better than a relay. Seen decent 20A units go for £40-50. False economy otherwise.

@Davo Coming from a background where reliable kit genuinely mattered, I'd say the relay setup isn't wrong exactly, it's just a blunt instrument. The thing people overlook is what happens when the alternator voltage drops under load — your leisure battery ends up seeing maybe 13.8V through the relay on a good day, which is fine for maintenance but won't properly bulk charge a significantly depleted AGM. Your mate might genuinely be fine, but I'd want to know what his battery voltage looks like after a decent run with a multimeter rather than just assuming it's working. Perception and reality can be quite different there. The B2B's advantage is that it actively pushes proper charge stages regardless of what the alternator's doing. Worth keeping if you can.