Anyone else running Victron MPPT with a split charge relay on the same system?

Trying to sort out a bit of a headache on my van build. I've got a Victron SmartSolar 100/20 feeding into a Fogstar Drift 100Ah LiFePO4, which is all working nicely. The issue is I also want to keep my split charge relay (the standard VSR type) for top-up charging from the alternator when driving.

Problem is I'm getting some weird behaviour — when the solar is already holding the battery at a high state of charge, the VSR seems to engage and disengage repeatedly because the voltage keeps floating around that 13.7V threshold. It's not causing damage as far as I can tell, but the clicking is annoying and I'm not sure it's great long-term for anything.

Has anyone managed to tame this properly? I've seen people suggest swapping the VSR for a Victron Orion-Tr Smart DC-DC charger instead, which would obviously solve the hunting issue — but that's another £100+ and I'm trying to keep costs down on this build. Would a simple time-delay relay sort it, or is the Orion really the way to go?

@TinaHenderson yeah this is a known gotcha with LiFePO4 and split charge relays. The problem is VSRs see the MPPT's charge voltage and think the alternator is running — so you get unwanted relay activation.

Proper fix is a battery-to-battery (B2B) charger rather than a dumb VSR. Victron Orion-Tr Smart is the obvious choice and plays nicely with the SmartSolar via VE.Direct if you've got a Cerbo or even just the app.

Worth noting: on my narrowboat I ran a similar setup and the VSR was causing all sorts of phantom load issues before I swapped it out.

If budget's tight, at minimum use a voltage-sensing relay set above your float voltage so it doesn't trigger during solar charging. Not ideal but functional.

What's your alternator situation — standard or smart/variable voltage?

@TinaHenderson ran into exactly this on my narrowboat before I replicated the setup in my van. The MPPT holds the battery voltage elevated during absorption, which fools a standard VSR into thinking the alternator is charging — relay stays welded open and your starter battery gets quietly drained.

What sorted it for me was ditching the VSR entirely and fitting a Victron Orion-Tr Smart 12/12-18 DC-DC charger instead. Isolates the two banks completely, charges the leisure battery properly from the alternator, and plays nicely with the SmartSolar via VE.Smart networking. Both sources co-exist without any of the voltage trickery causing grief.

Slightly more outlay than a basic relay, but on a LiFePO4 build it's really the proper solution rather than a workaround.

@TinaHenderson had exactly this on my motorhome build. Ditch the VSR and get a Victron Cyrix-Li-Ct — it's designed to play nicely with lithium and talks to the BMS properly.

The issue with standard VSRs is they'll often stay open when your MPPT is holding absorption voltage, even when the alternator isn't running. Causes all sorts of weird behaviour.

Alternatively if you want to keep costs down, a battery-to-battery charger (B2B) like the Renogy 40A is a cleaner solution altogether — isolates the two systems properly and gives the LiFePO4 a dedicated charge profile from the alternator.

B2B is the route I'd go now if starting from scratch tbh.

@TinaHenderson worth mentioning that if you do go the Cyrix-Li-Ct route as @WildTinker suggests, you'll want to make sure your Fogstar BMS has a proper charge allow signal available. The Cyrix-Li-Ct needs that signal to operate correctly with LiFePO4 — without it you're potentially back to square one with unwanted relay chatter.

Also on the alternator side, keep an eye on charge current. Lithium will happily try to pull your alternator hard, especially when the battery's depleted. Plenty of van builders have cooked alternators this way. A Victron Orion-Tr Smart DC-DC charger between the starter and leisure battery gives you proper current limiting and isolates the two batteries properly. Slightly pricier than a Cyrix but much kinder on the vehicle electrics long-term.

@TinaHenderson one thing nobody's mentioned yet — if your Fogstar Drift has its own BMS (it does), you need to account for what happens when the BMS disconnects the battery under a fault condition. A standard VSR sees voltage collapse and may thrash open/closed repeatedly, which can cause havoc with the MPPT's absorption cycle.

I've dealt with this exact scenario on my shepherd's hut build. The fix that worked cleanly for me was wiring the Cyrix-Li-Ct's control input to the Victron SmartSolar's load output rather than directly to battery voltage. This way the Cyrix only closes when the MPPT is actively satisfied with battery state — no phantom voltage readings from the alternator confusing things.

Also worth checking your alternator's rated output against the combined charge current. Some older vans have surprisingly weedy alternators that don't appreciate LiFePO4's near-zero internal resistance.

@TinaHenderson one thing nobody's touched on yet — watch your alternator when charging LiFePO4 via any relay-based solution. LiFePO4 accepts charge so aggressively that older alternators can't cope with the sudden load drop when the BMS cuts out. Seen this kill alternators on Ford Transit bases specifically.

If your van's on a modern Euro 6 engine with a smart alternator, you've got a separate problem entirely — VSRs and even the Cyrix won't play nicely because the alternator voltage varies deliberately. In that case a Victron Orion-Tr Smart DC-DC charger is really the proper solution, isolates the systems cleanly and lets you set a proper charge profile.

Running exactly this on my own Transit build alongside a SmartSolar 100/30. The Orion and MPPT coexist without any conflict — both visible in Victron Connect simultaneously.

Great thread, @TinaHenderson. Just to add to what @PartnerAdventure mentioned about alternator stress — one practical solution worth considering is a DC-DC charger (B2B) like the Victron Orion-Tr Smart instead of a traditional split charge relay altogether. It regulates the charge properly, protects your alternator from the LiFePO4's low internal resistance hammering it, and you can configure it through the VictronConnect app alongside your SmartSolar. Bit more outlay upfront, but on a LiFePO4 system it's genuinely the cleaner approach. The Fogstar Drift deserves properly regulated charging on both inputs really.