Anyone else running a split-charge relay alongside a MPPT controller — or is a VSR actually pointless in that setup?

Just finishing the wiring in my Transit Custom build and I've been going back and forth on whether to keep the 140A VSR I bought or just ditch it. I've got a Victron SmartSolar 100/30 feeding a 200Ah lithium (a Fogstar Drift as it goes), and the solar's handling the bulk of charging fine when the sun's out. My worry is those grey November weeks parked up — hence wanting the alternator as a backup.

Thing is, I've read a few times now that a plain VSR can actually cause grief with lithium because it'll trigger on voltage alone and the alternator ends up fighting the BMS if the battery decides to disconnect. Some people are saying I need a proper DC-DC charger (B2B) like a Victron Orion-Tr Smart instead, which would isolate things properly and limit the current draw so I'm not flogging the alternator either. The Orion 12/12-30 is about £130-ish — not the end of the world but I'd rather not spend it if the VSR will do the job safely.

Has anyone actually run a VSR with lithium long-term without issues, or is the B2B the only sensible route? Keen to hear what setups people are actually using rather than just the theory.

FEE_Solar | 847 posts

@PYQ_Power Good question! The VSR isn't pointless but it does become a bit redundant depending on your priorities. Your MPPT handles solar charging beautifully on its own, but the VSR still earns its keep for those longer drives where the alternator can top up your leisure battery once the starter battery's satisfied.

The key thing to watch is that VSRs typically close around 13.7V — which your MPPT will trigger regularly, potentially cycling the relay more than ideal. Some folk disable the VSR entirely and just rely on solar, which works fine if you're not doing massive daily mileage.

What's your typical usage pattern? Mostly stationary or lots of driving? That'd change my recommendation honestly. If you're driving frequently, keep it. Mostly parked up with decent solar exposure? You could probably ditch it without much consequence.

@PYQ_Power the VSR isn't pointless but it's doing a different job to the MPPT — your solar handles daytime charging via the SmartSolar, but the VSR handles top-up from the alternator when you're driving. Two separate charge sources, both useful.

That said, with a 200Ah bank you might want to look at a DC-DC charger (B2B) instead of a plain VSR. The VSR just connects batteries directly, which can stress a modern Euro 6 alternator. A Victron Orion-Tr Smart 30A would charge properly and protect your van's electrical system.

I ditched my VSR on my static caravan setup for exactly this reason — the DC-DC route is cleaner once you factor in smart alternators. Depends whether your Transit is pre-2016 or newer, as that changes things significantly.

BordersSolar | 1,203 posts

Worth adding something the others haven't touched on yet — the VSR actually earns its keep when you're driving at night or in winter when your panels are producing next to nothing. Your alternator is then the only meaningful charge source, and the VSR is exactly what connects that to your leisure battery automatically. So rather than thinking of them as competing, they're really covering different scenarios. The MPPT handles solar, the VSR handles alternator charging — they complement each other nicely.

That said, with a 140A VSR and a 200Ah battery, just double-check your VSR's voltage threshold isn't going to interfere with how Victron manages absorption/float. Some cheaper VSRs can behave oddly there. What's the rest of your alternator setup like — standard Transit alternator or have you uprated it?

ST_Builds | 312 posts

Running almost identical setup in my motorhome — Victron SmartSolar feeding a Fogstar 100Ah lithium alongside a VSR off the alternator.

Honest take: the VSR became mostly decorative once I added the MPPT. But I kept it because driving any distance still tops the battery faster than solar alone, especially in winter up here.

One thing worth flagging though — if you've gone lithium, double-check your VSR is rated for it. Standard voltage-sensing relays can overcharge LiFePO4 if the BMS doesn't cut in cleanly. A proper DC-DC charger (B2B) like the Victron Orion-Tr Smart is a cleaner solution than a dumb VSR for lithium setups.

For AGM you're probably fine keeping it. For lithium, I'd rethink.

Ben1960 | 847 posts

One thing worth considering @PYQ_Power — with a 200Ah lithium I'd actually look at whether your alternator can handle sustained charging through the VSR at all. Lithiums will happily pull massive current when they're low, and some Transit alternators aren't thrilled about that for extended periods. Might be worth looking at a DC-DC charger (B2B) instead of the VSR — yes it's more money, but it properly regulates the charge current and protects the alternator. Your Victron MPPT handles the solar side beautifully regardless. The VSR made more sense in the AGM days when batteries had natural internal resistance limiting current draw. Just something to factor in before you finalise the wiring.

Sam1985 | 578 posts

Good thread this. One angle I'd add — if you're planning to use the Victron app to monitor everything, the SmartSolar already gives you really granular data on what's going in and out. The VSR won't report back to that ecosystem at all, so you end up with a bit of a blind spot on the alternator charging side unless you add a SmartShunt or similar.

Personally I ditched my VSR in favour of a Victron Orion-Tr Smart DC-DC charger and haven't looked back. Yes, it costs more, but it charges the lithium properly from the alternator and integrates into VictronConnect. For a Transit Custom with a smart alternator, a DC-DC is arguably essential anyway rather than optional — those variable-voltage alternators don't play nicely with a standard VSR. Worth checking which alternator variant your Custom has before committing either way.