Anyone else running a split-charge relay off a leisure battery bank on the cheap?

Been faffing about with my setup this week and thought I'd share where I've got to. I've got two 100Ah AGM batteries wired in parallel (picked up secondhand off Facebook Marketplace for £40 the pair — not bad condition considering) feeding a 30A split-charge relay off the alternator. The relay itself was a fiver from a car boot sale, cleaned up the terminals and it seems to be doing the job fine so far.

My concern is that I'm only seeing about 13.8V at the leisure bank when the engine's running, which feels a bit low to me. The van's a 2005 Transit with the stock alternator, and I've used about 4 metres of 10mm² cable for the run from the engine bay. Not sure if I'm losing voltage along that cable run or if the alternator is just outputting a fairly modest charge.

I've also got a cheap 20A PWM controller in there for a single 175W panel on the roof — again, second hand, £25 from a bloke on eBay. Total budget so far is sitting around £130 including the panel, which feels decent to me but I'm aware I've possibly cut a few corners.

Has anyone else done a similar budget relay setup and run into voltage drop issues? Would love to know if upgrading to a VSR (voltage sensing relay) actually made a noticeable difference, or whether I should just be looking more carefully at my cable sizing first?

@DuctTapeDave60 secondhand AGMs are a gamble — worth load-testing them before you commit to the full setup. A cheap battery analyser (CT3000 or similar) will tell you their actual capacity pretty sharply.

On the split-charge relay itself — I ran a voltage-sensitive relay (VSR) on my garden office setup for about 18 months before upgrading. Worked fine for the money, but the main gotcha is that VSRs don't play nicely if your alternator voltage is marginal. If the van/vehicle idles low, it won't trigger properly.

One thing worth doing: fuse both sides of the relay, not just the feed from the starter battery. Learnt that the hard way when a wiring fault took out more than it should've done.

What vehicle are you running this off? That changes the answer quite a bit with modern smart alternators.

@DuctTapeDave60 parallel AGMs secondhand is already a headache waiting to happen — if they're not identical age and cycle history they'll fight each other and you'll kill the weaker one faster. That's before we even get to the split-charge relay side of things.

Also a basic VSR off eBay is fine until it isn't. They don't care about battery chemistry, they just slam charge in when voltage hits the threshold. AGMs want a proper 3-stage charge. Your alternator won't do that either.

For the price of two dodgy secondhand AGMs you could've got a single Fogstar 100Ah lithium and a decent Victron Cyrix-Li-Charge relay and actually had a setup worth running.

Done this properly on my shepherd's hut build. Cutting corners on the battery bank specifically is where people always regret it.

What voltage does your alternator actually output when the engine's running? Worth checking with a multimeter before assuming the split-charge relay is doing its job properly — some older alternators barely hit 14V and won't charge AGMs properly anyway.

On the narrowboat I'm slowly getting sorted, I found the relay itself wasn't the bottleneck, it was the cable run length causing enough voltage drop to make the whole thing a bit pointless. What gauge cable have you used between the starter battery and the leisure bank?

@BatteryTim makes a fair point about mismatched batteries, but if the budget's tight it's the reality for a lot of us. Just worth keeping an eye on individual cell voltages periodically rather than assuming the whole bank is behaving uniformly.

@DuctTapeDave60 worth flagging that a basic VSR/split-charge relay won't play nicely if you eventually want to add solar into the mix — you'll get backfeed issues. Spent a frustrating weekend learning that the hard way with my shepherd's hut setup before swapping to a Victron Orion-Tr Smart DC-DC isolator. Yes it's more money upfront, but the B2B charger actually conditions the charge properly rather than just dumping alternator voltage straight in.

For £40 secondhand AGMs I'd keep expectations realistic — treat them as temporary and start saving toward a proper LiFePO4 bank when budget allows. Fogstar do decent cells if you're willing to DIY a pack later down the line.

Been down this exact road with my motorhome build last summer. The split-charge relay itself is fine on a budget — picked up a voltage-sensing relay (VSR) off Amazon for about £14 and it's been rock solid.

The thing nobody mentions though: cable sizing is where people get caught out. Undersized cable between your alternator and that leisure bank will throttle your charging and get warm in ways you don't want. I run 16mm² minimum for anything over a couple of metres.

@DuctTapeDave60 what's the run length from your starter battery to the leisure bank? That'll change the equation considerably. Also worth knowing what alternator output you're dealing with — some of the older vans are only 70A and you don't want to be flogging it constantly trying to bulk-charge two AGMs.

Good thread, @DuctTapeDave60. One thing worth mentioning that nobody's touched on yet — if your AGMs are secondhand, do a proper capacity test before trusting your whole setup to them. Two 100Ah batteries claiming 200Ah combined might actually be giving you 120Ah in reality if they've had a hard life. A basic battery load tester costs under a tenner on Amazon and could save you a nasty surprise.

Also, @NeilBurns91 raises a fair point about solar compatibility — a DC-DC charger (B2B charger) is the proper solution long-term, especially with modern smart alternators. Not exactly "on a budget" upfront, but Victron do a 30A unit that'll do the job properly without frying anything.

For now though, a simple VSR is absolutely fine as a starter setup. Get the fundamentals right first.

Great thread @DuctTapeDave60! One thing worth adding to what @NeilBurns91 mentioned about solar compatibility — if you do eventually want to expand, fitting a proper battery-to-battery (B2B) charger rather than a basic VSR from the outset isn't always as expensive as people assume. Picked up a Sterling Pro Charge B unit secondhand for around £60 and it's transformed how efficiently my alternator charges the leisure bank. Also plays nicely with solar down the line. Might be worth keeping an eye on eBay rather than buying a cheaper VSR now and replacing it later.

Really relevant thread for me — running something similar on my boat with a pair of secondhand AGMs and a budget split-charge setup.

@IslandDweller raises something I've been wondering about too. My batteries came from a mate's caravan and I've no idea of their actual history. Has anyone used a proper capacity tester on secondhand AGMs before committing to a full build? I picked up a basic one from Amazon but not sure how reliable it is.

Also curious whether anyone's had issues with the relay staying engaged at low alternator output — mine seems a bit inconsistent when the engine's just ticking over at idle.