Anyone else running a split-charge relay alongside a DC-DC charger — or is it one or the other?

Been scratching my head over this one for a few weeks now. I've got a 2019 Ducato-based van build with a 200Ah lithium (a Fogstar Drift 200) and I'm upgrading from a basic VSR split-charge setup to something that'll actually charge the battery properly while driving. The VSR was fine with AGM but it's pretty useless with lithium as I'm sure most of you already know.

I've ordered a Renogy 40A DC-DC charger (the DCC50S) and was planning to just swap it in place of the VSR. But a mate reckons I should keep the relay in the circuit as well — something about protecting the alternator from the lithium's BMS cutting in and dropping load suddenly. I've seen a few threads on other forums going back and forth on this, nothing conclusive.

My alternator is the standard 180A Ducato unit and I've not had any issues with it so far. The DC-DC charger is supposed to handle all that isolation itself, but now I've got doubt creeping in and I don't want to fry a £400 alternator finding out the hard way.

Has anyone actually run a DCC50S or similar B2B charger on a Ducato or similar van, with or without a relay in the mix? Would love to know what you're actually doing in your own builds rather than just the theory.

Hey @Rodney3, good timing on the upgrade! The short answer is ditch the VSR entirely once you've got a DC-DC charger in — they don't play nicely together with lithium. The problem is VSRs just see voltage, and lithium sits high enough that it can fool the relay into thinking the starter battery is fully charged when it isn't. Your DC-DC (a B2B charger) handles the whole job properly — it'll regulate the charge profile correctly for your Fogstar and protect your vehicle's alternator from the lithium's aggressive charge acceptance. Running both simultaneously is just adding complexity and potential for problems without any real benefit. What size alternator has your Ducato got? That'll help narrow down what rating B2B you'd want — the 2019 Ducatos can be a bit sensitive with higher-draw chargers.

@SparkySpanner covered the VSR side well, but worth adding — does anyone run both simultaneously during a transitional period, or is there a risk of them fighting each other?

Asking because I'm in a similar boat with my static caravan setup where I've got a Victron Orion-Tr Smart already doing the DC-DC work, and I'm wondering whether leaving an old relay in circuit causes any issues or whether it's essentially just redundant/harmless.

Also @Rodney3 — what alternator are you running? I've heard the smart alternators on newer Ducatos can be temperamental even with a proper DC-DC charger, curious whether yours has thrown up any issues there.

@IvyWalker raises a genuinely interesting edge case. I ran both simultaneously for about three months when I was mid-build on my shepherd's hut backup system — basically while I was still figuring out cable routing for the DC-DC.

What actually happened: the VSR kept trying to "help" and the two systems argued over voltage thresholds. Nothing catastrophic, but the Fogstar's BMS was throwing occasional low-priority alerts that only made sense once I traced it back to that conflict.

The Victron Orion-Tr Smart I fitted eventually made the decision easy — once it's properly profiled for lithium, it does everything the VSR was attempting but correctly. No guessing, no threshold fights.

Running both long-term isn't really a transitional strategy, it's just two systems undermining each other. Pick your DC-DC, set it up properly, and pull the VSR out entirely.