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

Been umming and ahhing about this for a while. I've got a 100Ah AGM leisure battery in the van and currently using a basic 140A split-charge relay off Amazon (paid about £18 for it). It does the job when I'm driving — battery gets topped up and I've not had any issues starting the van. But I keep reading that a proper DC-DC B2B charger like a Victron Orion or even a cheaper Sterling unit would charge the leisure battery properly rather than just float it off the alternator.

The thing is, a decent B2B is £80-£150 depending on brand, and I'm genuinely skint at the minute. The relay cost me pennies by comparison and it's been working for about eight months. I did notice last winter the leisure battery didn't seem to bulk charge fully — voltage was sitting around 13.1-13.2v after a 90-minute motorway run, which doesn't feel right to me. Could be the relay dropping voltage, could be the AGM just being tired, not sure.

Has anyone done a proper side-by-side on relay vs B2B in terms of actual battery health over time? I've got a solar panel doing most of the heavy lifting in summer (200W on the roof, EPever MPPT controller) so the relay is really just a backup for winter top-ups. Wondering if the £100 upgrade is genuinely worth it for my use case or if I'm overthinking it.

@CraftyWelder if you're running AGM that relay is probably fine for basic top-ups, but the issue is voltage-based charging — the relay just slams 14.4V straight across whenever the alternator's running, no absorption stage, no proper tail current cutoff. AGM can handle it short-term but you're likely shortening battery life noticeably.

I ran a similar setup for about 8 months before switching to a Victron Orion-Tr Smart 30A. Night and day difference — proper 3-stage charging, configurable profiles, and the battery sits at a much healthier state of charge.

The maths is brutal but honest: £18 relay + replacing your AGM every 2-3 years vs £120-ish B2B and a battery that actually lasts. If you're planning to add solar or LiFePO4 down the line, the relay becomes a liability pretty quickly.

@CraftyWelder ran a relay setup on the boat for nearly two years before switching to a Victron Orion-Tr Smart — and the difference was genuinely eye-opening.

The relay's lazy plateau around 13.4-13.6V meant my AGMs were perpetually stuck at roughly 70-80% state of charge. They looked "full" on a basic meter but weren't. Over time that partial-state cycling quietly murdered the plates.

The B2B properly conditions the charge — absorption, float, the lot — and crucially it isolates your leisure bank from the starter battery rather than lashing them together when the alternator's working hard.

Saved £18 on the relay, spent maybe £200 replacing batteries 18 months early. The maths doesn't flatter the budget option once you run the full numbers.

If a new Orion's too steep, keep an eye on eBay — used units come up regularly and Victron kit is genuinely bulletproof secondhand.

Ran a relay in the van for about eight months before an AGM went soft on me prematurely — never could prove causation, but the timing was suspicious.

The thing nobody mentions is that a relay just shadows your alternator voltage. Once the starter battery reaches ~14.4V and the alternator backs off, your leisure battery is sitting there receiving maybe 13.8V thinking it's fully charged when it's genuinely at 70–80%. Over a Kent winter that compounds fast.

Switched to a Victron Orion-Tr Smart 30A and suddenly my Fogstar AGM was actually reaching 100% SoC regularly. The difference was measurable — not theoretical.

The £18 relay works, but you're slowly starving the battery of that final absorption charge every single cycle. Over 3–4 years the cumulative degradation probably costs more than the B2B would have.

Good thread this. One thing worth adding that hasn't been mentioned yet — think about what your alternator is actually seeing with a relay setup. Modern smart alternators (fitted to most vehicles from roughly 2015 onwards) can behave oddly when a depleted leisure battery suddenly gets slapped across them. You can end up stressing the alternator or triggering fault codes. A B2B handles that load management properly.

That said, @CraftyWelder if your van's older with a traditional alternator and you're not doing big daily discharges, the relay might genuinely serve you fine for years. It's really about matching the solution to your actual use case rather than assuming the expensive option is always necessary. What year is the van and how deeply are you typically cycling that AGM?

Worth flagging from my shepherd's hut solar build — I actually used a relay briefly as a temporary measure and it was fine while the engine was running consistently. Problem is short runs. Relay dumps bulk voltage straight in with no real current control, and if you're doing lots of stop-start driving your AGM never gets a proper charge cycle completed.

@KentVanLifer's soft battery issue sounds familiar. Hard to prove but the pattern's there.

A Victron Orion-Tr Smart 12/12-30 is around £120 odd — hurts upfront but it's doing proper 3-stage charging and plays nicely if you ever add solar later (which you will 😄). The £18 relay isn't saving you money if it's quietly killing a £80-100 battery every 18 months.

Really depends on your use case @CraftyWelder. The relay will technically charge your AGM, but the problem is it can't taper the charge properly — you're essentially just paralleling the batteries and hoping for the best. With AGM specifically, that uncontrolled bulk charging can stress the battery over time, which might be what @KentVanLifer experienced.

A B2B also protects your alternator from the leisure battery acting as a big drain on startup, which matters more on older vehicles.

That said, if you're only doing occasional weekend trips with some solar topping up the rest of the time, an £18 relay might genuinely be fine for years. It's continuous daily driving commuters who probably notice the difference most.

What's your typical usage pattern like? That'd help work out whether upgrading is actually worth it for you specifically.