Anyone else running a split-charge relay alongside solar? Struggling to get the balance right

I've got a 200W panel on the roof of my Transit-based camper feeding into a Victron SmartSolar 100/20 MPPT, which then charges a 100Ah AGM leisure battery. Been really happy with the solar side of things — on a decent day I'm pulling 8–10A without any drama. The issue I'm having is with the split-charge relay I've got wired in from the alternator. It's a basic voltage-sensing relay (VSR), and I'm not sure it's playing nicely with the MPPT.

The problem is that when I'm driving and the solar is also doing its thing, the VSR kicks in and the alternator and the MPPT seem to be fighting each other a bit — or at least that's what I think is happening. The leisure battery voltage looks a bit erratic on the Victron app and I'm getting some odd behaviour where the MPPT drops into float earlier than I'd expect, even when the battery clearly isn't fully charged. I've only got a basic battery monitor (Votronic 100A shunt) so I'm not 100% certain what's going on.

Has anyone else had this with a VSR and a SmartSolar together? I've been reading a few threads suggesting I should ditch the VSR and go for a Victron Cyrix-ct instead, or maybe even a DC-DC charger (B2B) like the Sterling Pro Charge Ultra. The B2B option appeals because it would regulate the alternator input properly, but they're not cheap — the Sterling 30A unit is around £180–200 new.

Would love to know what setups people are running and whether the Cyrix is enough of an upgrade, or if a proper B2B is really worth the money for a standard leisure battery setup.

Hey @HeatherOllie! Sounds like a solid setup. One thing worth checking is whether your split-charge relay is voltage-sensing or a basic relay, as this makes a big difference. With a VSR, once the solar pushes the leisure battery above the activation threshold (usually around 13.3V), the relay can connect the starter battery unnecessarily and create a bit of a tug-of-war between the two charging sources.

What I'd suggest is fitting a battery-to-battery (B2B) charger instead of a plain relay - Sterling and Victron both do decent ones. It properly isolates the two banks and charges them independently, so your MPPT and the alternator aren't fighting each other. Makes a massive difference to how predictable everything becomes. What voltage are you seeing at the leisure battery when you're driving on a sunny day? That'd help diagnose what's actually happening. 🙂

Great setup @HeatherOllie! One thing I'd add to what @TimKnight is getting at — with a Victron MPPT in the mix, it's worth considering whether a DC-DC charger (like the Victron Orion-Tr Smart) might serve you better than a traditional split-charge relay long-term. The relay can sometimes cause confusion because the MPPT sees the alternator voltage and behaves oddly, particularly during absorption phase.

Also worth checking your Victron app to see if your battery is actually reaching full charge or just hovering around 80-85% — AGMs really benefit from a proper full cycle to avoid sulphation. What voltage have you set your absorption and float at? Stock settings aren't always ideal for AGM. 👍