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

So I've been piecing together a budget solar setup for my Transit and I keep going back and forth on this. I've got a 100Ah leisure battery (a Victron AGM) sitting behind the driver's seat and I was all set to grab a Sterling B2B 30A charger, but that's £130-odd and it feels steep when I'm already over budget on everything else.

A mate reckoned a basic 140A split charge relay (picked one up off Amazon for about £18) does the same job for day-to-day driving. I get that a B2B properly conditions the charge and protects the alternator, but realistically I'm mostly doing 30-60 minute runs around town and the odd longer motorway trip. The solar (two 175W panels through a Victron 75/15 MPPT) is doing most of the heavy lifting anyway.

Has anyone actually run just a relay long-term without wrecking their alternator or undercharging the leisure battery? I know the "proper" answer is always the B2B but I'm trying to keep this whole build under £400 all-in and I'm nearly there. Would love to hear from people who've made the budget option work, or genuinely regretted it.

@GazKelly75 split charge relays are fine for flooded/AGM but the issue is voltage drop — by the time the relay kicks in and the alternator's pushing through, you're rarely hitting a proper absorption charge. Your Victron AGM deserves better than that tbh.

Ran a relay in my shepherd's hut build (towed unit, similar situation) and the battery never really topped up properly on longer runs. Switched to a Sterling B2B and the difference was immediately obvious on the Victron BMV readings.

Budget-wise, the Renogy DC-DC charger is worth a look — around £60-70 and does the job properly. Split charge relays make sense for old flooded batteries where you're not fussed, but with a decent AGM you'll shorten its lifespan undercharging it constantly.

False economy basically.

@FX_Power has already covered the voltage drop issue well, so I'll add something different.

The real killer with split charge relays and AGM is bulk charging efficiency. Your alternator will push whatever voltage it pushes — no adaptive absorption, no temperature compensation. With a Victron AGM specifically, you want proper 14.7V absorption to avoid chronic undercharging, which sulphates the plates over time.

A B2B (I run a Victron Orion-Tr Smart 30A) essentially turns your alternator into a proper charger for the leisure bank. Worth every penny long-term.

That said, if this is purely emergency backup rather than daily cycling, a relay is arguably acceptable — the battery sits near full most of the time anyway and the compromise matters less.

What's your actual use case? If it's sitting in the van 50 weeks a year and occasionally powering a phone during a breakdown, the relay is probably fine.