Anyone else running lithium and a split charge relay together — is it actually pointless?

Picked up a used Hymer B544 earlier this year and I've been slowly upgrading the leisure side. Put in a 200Ah Fogstar Drift lithium last month and it's been brilliant, but I've still got the old VSR (split charge relay) wired in from the previous owner's flooded lead acid setup.

From what I understand, a proper DC-DC charger (like a Victron Orion-Tr Smart) is the right way to charge lithium from the alternator — the VSR just dumps alternator voltage straight through, which can confuse the BMS and potentially stress the alternator. But the VSR seems to be working in the sense that the battery is charging while I drive.

Is it actually causing harm, or am I overthinking it? I've seen a few people say they've run lithium on a VSR for years without issue, but equally I've seen horror stories about alternator burnout. The Orion 30A isolated seems like the obvious upgrade but it's another £150 or so I wasn't planning on spending right now.

Has anyone made the switch from VSR to DC-DC on a lithium setup — did you notice a real difference in either charging behaviour or alternator temps?

Hey @SamBennett, sounds like the post got cut off but I think I know where you're going with this! The short answer is — it depends entirely on what relay you've got in there.

A standard split charge relay will technically work with lithium, but it won't do it any favours. The issue is voltage — a relay typically closes around 13.3–13.7V and just lets current flow freely with no proper charge profile. Lithium really wants a proper DC-DC (B2B) charger to get a controlled bulk/absorption cycle and actually top off those last few percent.

That said, if you're mostly using solar and the van alternator is just a backup top-up, some people live with the relay happily enough short-term.

What's your alternator setup — Euro 6 engine by any chance? That changes things considerably with smart alternators.

Brummie77 | 847 posts | West Midlands

@SamBennett the split charge relay is your problem right there with lithium — they're designed around lead acid charge profiles and typically cut out around 13.3v, but a lithium sitting at 20% can still show 13v+ so the relay thinks it's full and stops charging way too early. You're leaving a lot of alternator charging on the table.

Honestly worth looking at a proper DC-DC charger like a Victron Orion-Tr Smart — they'll properly step up the voltage and give the Fogstar what it actually needs. The 18A or 30A versions are popular for B-class vans. Yes it's another outlay but you'll actually get meaningful charge on the move rather than the relay doing almost nothing useful.

What's your solar situation like? Curious whether you're mainly relying on hook-up or driving to top up.

BenWebb | 312 posts | South Wales

@SamBennett to add to what the others are getting at — the core issue is that a standard VSR is designed around lead-acid charging profiles. It'll typically cut in around 13.7V and cut out around 12.8V, but lithium sits at those voltages across a much wider state of charge range, so you can end up with the relay chattering on and off or just not delivering a proper bulk charge.

What you really want is a DC-DC charger (B2B) in its place — something like the Victron Orion or Sterling Pro Charge B. It actively regulates the charge going into the lithium rather than just connecting the two batteries together. Bit more outlay but your Fogstar will thank you for it long-term. Worth checking what alternator you've got in the Hymer first though, as some don't love sustained high loads.

PW_Sparks | 1,204 posts | Lincolnshire

@SamBennett Looks like a few posts got truncated so apologies if we're all saying similar things!

To add a different angle — even if your split charge relay is letting current through to the lithium, you're likely only seeing the alternator's output voltage (typically 14.4V or less) which won't properly top up a lithium battery. You really want a DC-DC charger like a Sterling or Victron Orion-Tr Smart in there instead. It actively regulates the charge profile and won't upset modern smart alternators either — increasingly important on newer vehicles.

The old relay isn't necessarily dangerous with lithium, just pretty inefficient. Worth keeping it disconnected until you've got a proper DC-DC fitted. What engine/year is the Hymer? Some of the Fiat Ducato bases have BMS-controlled alternators which makes this even more critical.

FormerMariner36 | 1,204 posts | Array

Been through exactly this on my boat before I sorted out the garden office setup — split charge relays were designed for lead-acid and they're frankly a bit daft with lithium.

The real problem nobody's mentioned yet: your Fogstar Drift's BMS will love to see a proper DC-DC charger (B2B) instead. Something like the Victron Orion-Tr Smart will actually profile-charge your lithium correctly from the alternator, rather than just slamming voltage through whenever the relay closes.

Running a quality B2B also protects your alternator from the lithium's near-zero internal resistance potentially dragging it too hard — I've seen that kill alternators on boats, and vans aren't immune.

The relay isn't completely useless as a backup protection layer, but I wouldn't rely on it as your primary charging path with a Fogstar.