Anyone else running a Victron SmartSolar on a split-charge relay setup — seeing weird behaviour at low SOC?

Been scratching my head over this one for a few weeks now. I've got a Victron SmartSolar MPPT 100/30 feeding a 200Ah lithium (a Fogstar Drift 12V) in my Transit-based van build. Split-charge relay wired off the alternator for top-ups on driving days. All worked fine over summer but I'm starting to notice the relay seems to be cutting in and out almost randomly when the battery drops below about 20% SOC on grey days when the solar's barely putting out anything.

The relay itself is a decent one — a Sterling Battery-to-Battery charger actually, not a basic VSR — so it should be managing things sensibly. I've got it set to lithium profile. But I'm wondering if the SmartSolar and the B2B are somehow fighting each other over charge control when both kick in at the same time. The Victron Connect app is showing some odd absorption cycling that I wouldn't expect from a fresh-ish battery.

Has anyone run a similar dual-input setup and sorted this out? Particularly curious whether separating the charge inputs — maybe running them into the battery at different points or adding a BMS comms cable into the mix — actually makes a practical difference, or whether I'm overthinking it and it's something simpler like a dodgy earth somewhere.

@WattTony oh this took me ages to figure out on my motorhome build.

The culprit in my case was the split-charge relay dropping out when the Fogstar's BMS hit a low-voltage protection event — the relay sees the voltage collapse and disconnects, then the SmartSolar loses its load reference entirely and starts behaving oddly when it reconnects.

What sorted it for me was fitting a Victron Battery Protect between the relay and the leisure bank rather than relying on the relay alone. The BP handles the disconnect more gracefully and gives the MPPT something sensible to talk to.

Also worth checking in the VictronConnect app — if you're seeing absorption cutting short at low SOC, your tail current settings might need adjusting for lithium chemistry. Fogstar's own docs have recommended figures for the Drift cells specifically.

Really interesting thread — been lurking because I'm mid-build myself and haven't hit this yet, but it's got me worried I will.

Quick question for @Nessa68 and @WattTony: does the weird behaviour only show up when the Fogstar is genuinely low (say below 20%) or are you seeing it at higher SOC too?

Also — is the SmartSolar getting its battery voltage reading directly from the battery terminals, or somewhere further up the circuit? I've read that voltage drop across connections can confuse the MPPT's absorption/float decisions, which might compound whatever the relay is doing.

Would the fix be as straightforward as running a proper voltage sense wire directly to the battery? Or does that not play nicely with the split-charge side of things?