Anyone else running a Victron SmartSolar 100/30 alongside a split-charge relay — or is it a daft idea?

So I've got a fairly standard setup in my Transit van — 200W of solar through a Victron SmartSolar MPPT 100/30, feeding into a 100Ah AGM leisure battery. That side of things works a treat. The issue is I've also got a basic split-charge relay on the alternator circuit, and I'm wondering if the two are fighting each other without me realising.

What I've noticed is that after a long drive the battery reads around 14.1V on the Victron app, but then within a few minutes of stopping it drops back to 13.1V pretty sharpish. I'd expect some surface charge drop-off, but this seems quite steep. The solar is doing its job fine on sunny days and I'm not really struggling for power day-to-day, just want to make sure I'm not quietly wrecking the AGM.

Has anyone found that a VSR and an MPPT controller together cause any weird charging behaviour? I've read a bit about the relay potentially triggering and cutting out at odd voltages when the MPPT is active, but I can't find anything definitive about it. Wondering if I'd be better off swapping the VSR for a DC-DC charger like the Victron Orion-Tr Smart — seems like the cleaner solution but that's another £120 or so I hadn't budgeted for.

Hey @ShaunJohnson, looks like your post got cut off there — we're all waiting to hear the rest! 😄

But to answer the title question anyway — no, it's absolutely not a daft idea. Running a split-charge relay alongside your SmartSolar is pretty common in van setups. The MPPT handles your solar charging beautifully, and the relay tops the leisure battery up from the alternator while you're driving. They play nicely together as long as your AGM's charge profile is set correctly on the Victron side.

One thing worth bearing in mind is that a VSR (voltage sensing relay) can sometimes get a bit confused if the MPPT has already brought your leisure battery up to float voltage — the relay might not engage thinking the van battery needs protecting. Worth knowing about!

Finish your post and we can get into the specifics! 👍

@ShaunJohnson yeah your post did vanish mid-sentence there, but I can take a stab at where you're heading.

I ran almost exactly this combo at my cabin for a while — MPPT on solar, split-charge relay off a vehicle alternator, both feeding the same battery bank. The short answer is it works, but there's a subtlety worth knowing: your relay will happily dump alternator charge in bulk regardless of what the SmartSolar is already doing. They don't "talk" to each other.

In practice I found the Victron just gets on with it and manages its own absorption/float cycles. No fireworks. The AGM took it fine.

Where it gets messier is if you upgrade to lithium later — a dumb relay becomes a real problem then. Worth thinking ahead.

What were you actually going to ask before the post ate itself? 😄