Anyone else running a split charge relay instead of a B2B charger to save money — worth it long term?

Picked up a cheap 140A split charge relay off eBay for about £18 to keep my leisure battery topped up from the alternator while driving. Works fine on the face of it — engine on, relay clicks, battery charges. Job done, or so I thought.

The issue I'm running into is that it's just dumping whatever the alternator puts out straight into a 100Ah AGM with no regulation. From what I've been reading, that's fine for flooded lead acid but potentially not great for AGM, and absolutely not suitable if I ever switch to lithium. A proper B2B like a Victron Orion (even the non-isolated 30A one) is sitting at around £90–£120 new, which is a big jump from £18.

I've had the setup running for about six months now, mostly weekend trips, and the AGM seems okay so far. But I'm wondering whether I'm slowly cooking it without realising, or whether the worry is overblown for occasional use. Has anyone stuck with a basic relay long-term and had no issues, or did you end up wishing you'd just bought the B2B from the start?

@VanBarry The relay works, but it's a blunt instrument. It'll connect your leisure battery directly to the vehicle battery, which means if your leisure bank is heavily depleted, you're hammering the alternator and potentially dragging down the start battery too.

The bigger issue with modern vehicles — anything with a smart/variable voltage alternator (pretty much post-2015) — is the relay becomes near-useless. The ECU sees a charged start battery and drops alternator output, so your leisure bank barely gets a look-in.

For a static caravan or garden office setup where you're driving to site occasionally, a Victron Orion-Tr Smart B2B makes a genuine difference. Not cheap, but it regulates the charge properly and protects both batteries.

The £18 relay will cost you in battery replacements sooner than you'd expect, especially with lithium.

@VanBarry genuine question — what battery chemistry are you running in the back? Because if you've gone lithium like half of us on here seem to have done, that relay is basically trying to charge a Ferrari with a hosepipe 😅

The real kicker with a relay vs a B2B (I use a Victron Orion-Tr Smart, which yes, was not exactly "budget") is that your alternator doesn't know when to stop. Lithium wants a proper CC/CV charge profile — a relay just lobbing 14.4V at it and hoping for the best makes me slightly nervous.

If you're on AGM or lead-acid though, honestly? Probably fine for casual use. How many miles are you typically doing between stops?

@EcoFlow_Nerd raises the critical point. To elaborate bluntly: a split charge relay is essentially just a glorified switch. It sees your alternator voltage, goes "close enough," and slams your lithium directly onto it.

Problem is, lithium wants a proper CC/CV charge profile. A relay gives it neither. You'll spend months wondering why your Fogstar or whatever you're running never quite hits full capacity — it's because bulk charging via relay into lithium is basically pouring water into a bucket with a hole.

A Victron Orion-Tr Smart B2B will run you £120-180 depending on the amps, but it'll actually charge your battery rather than just vaguely wave voltage at it. The maths on battery longevity makes it a no-brainer over 2-3 years.

On flooded lead-acid? Relay is probably fine. On lithium? It's false economy dressed up as cleverness.

@VanBarry worth adding a practical concern that hasn't been mentioned yet — even with a lead-acid leisure battery, that relay is connecting two batteries of potentially very different states of charge directly together. If your starter battery is low after a cold start, it'll actually draw from your leisure battery initially rather than charge it. You might be unknowingly running your starter flat in certain scenarios.

The £18 saving looks less attractive when you factor in a knackered starter battery or a roadside callout. A decent B2B like a Victron Orion (non-isolated if budget's tight) will properly regulate the charge and protect both batteries. Yes, it's £80-100 more upfront, but it earns that back pretty quickly in battery longevity alone.

What's your alternator rated at? That matters too.

@VanBarry ran almost this exact setup in my motorhome for two seasons — relay from a well-known auction site, felt smug about saving forty quid.

The problem nobody tells you about is what happens to your alternator on a modern vehicle. My Fiat Ducato's smart alternator was designed to reduce charging when it detects a full battery. Slap a big depleted leisure bank directly across it with a relay and you're essentially asking it to work flat-out indefinitely. Mine started throwing fault codes within six months.

Swapped to a Victron Orion-Tr Smart 30A B2B — properly isolated, current-limited, talks nicely to the alternator. Night and day difference, and the alternator's been fine ever since.

The £18 saving cost me a diagnostic session and considerable stress on a layby near Exeter.

@VanBarry the relay doesn't care that your Fogstar lithium is screaming internally — it's just out here connecting wires like a sociopath while your BMS quietly plots its revenge.

@VanBarry worth flagging from my shepherd's hut setup — when I was mobile with a trailer, I tried a relay briefly before switching to a Victron Orion-Tr Smart B2B. The relay was oblivious to battery state entirely, just dumping current regardless. With lithium especially, you really want proper CC/CV charging rather than whatever the alternator happens to throw at it.

The £18 saving evaporates pretty quickly if you're stressing the cells or cooking an alternator that wasn't designed for the sustained load a hungry lithium presents when deeply discharged.

Hey @VanBarry — the bit nobody mentions is what happens when your lithium is nearly full. A relay just keeps shovelling current in because it can't communicate with the battery at all. A decent B2B like a Victron or Sterling will taper the charge properly and genuinely extend your battery's cycle life. On a 100Ah lithium that's potentially hundreds of extra cycles. The relay feels like a bargain until you're replacing a £200+ battery prematurely. Might be worth costing it out over a few years rather than just the upfront saving.

@VanBarry had a relay on my cabin's backup charging circuit for a while — the hidden killer for me was voltage drop under load masking the actual charge state. The relay sees "engine running = charge" regardless of whether your battery actually wants it.

The B2B (I use a Victron Orion-Tr Smart) isn't just about lithium protection — it also regulates input, so a tired alternator or long cable run doesn't murder your charge efficiency. That £18 saving evaporates fast if you're replacing cells or dealing with a cooked BMS.

For a basic AGM setup the relay is fine. Lithium? False economy.

Good point from @Simon1988 on the lithium charging issue — worth adding that your alternator health is another casualty here. Modern smart alternators (most post-2015 vehicles) use variable voltage regulation, and a dumb relay connecting a depleted lithium pack essentially presents a dead short to that system. Seen a few people burn out alternator diodes this way. A Victron Orion-Tr Smart B2B costs around £90-120 but properly current-limits and handles the smart alternator protocols. Running my motorhome with one for three years, zero issues. The £18 relay "saving" evaporates fast if you're replacing an alternator at £400+.