Anyone else finding split-charge relays a bit useless for leisure batteries these days?

Been running a 200Ah Fogstar Drift LiFePO4 on my static caravan and recently started doing the same on the boat. Both setups originally had voltage-sensing split-charge relays to top up from the alternator when moving the boat around, but I keep reading that standard VSRs aren't really suited to lithium — something about the flat discharge curve fooling the relay into thinking the bank is always "full."

Swapped over to a Victron Orion-Tr Smart DC-DC on the boat and the difference is noticeable — it actually charges properly and I can monitor it through the app. But it feels like a lot of money (£150+) just to handle the odd 30-minute motor around the marina. Wondering if it's total overkill for my use case or whether the alternator protection side of things alone justifies the cost.

Anyone running a simpler setup with lithium that actually works, or is the DC-DC converter basically the only sensible answer now?

@Chopper42 yeah, relays are pretty much a dead-end solution once you go lithium. The fundamental problem is they can't do proper CC/CV charging — you're just dumping alternator voltage into the battery with zero control over current or absorption.

Switched mine over to a Victron Orion-Tr Smart DC-DC a while back and it's a different world. Proper charge profiles, Bluetooth monitoring, protects both the alternator and the lithium. On my motorhome setup it actually charges the LiFePO4 faster than the relay ever did because the DC-DC can run at rated current consistently rather than tailing off as voltages equalise.

The only scenario I'd still consider a relay is with a traditional AGM where the voltage curves are more forgiving. For LiFePO4 specifically, a DC-DC converter is just the right tool. Worth the extra cost.

@Chopper42 had exactly this with my static caravan setup — switched to a DC-DC charger (Victron Orion-TR Smart 30A) and the difference was night and day. The relay was just lumping unregulated voltage across the lithium and hoping for the best.

What pushed me over the edge was adding EV charging to the mix — the draw profiles got complicated enough that I needed proper charge management rather than a dumb relay closing whenever the alternator spun up.

One thing worth noting for the boat specifically: the isolated version of the Orion is worth the extra cost if your start and leisure banks share a negative. Common ground setups cause grief that people don't realise until something goes wrong.

Budget around £120-150 for a decent DC-DC unit but you'll recoup it in battery longevity easily.

Gibbo | Posts: 1,847

@Chopper42 worth noting that on the boat especially, the DC-DC route gives you another big advantage — proper isolation between your starter and leisure banks. Had a mate lose his starter battery because a dodgy relay let his lithium drain it overnight at the marina.

The Orion-TR Smart @VivaroDream mentioned is cracking kit, but if budget's tight the Sterling B2B chargers are solid too and well-regarded over here.

One thing people often overlook: check your alternator's output capability before sizing the DC-DC charger. Lithium will happily pull whatever you throw at it, and older marine alternators particularly can run hot if you're not careful. Might be worth a 20A unit rather than jumping straight to 30A depending on what engine you're running. What alternator have you got on the boat?

Good shout from @Gibbo on the boat angle — I'll add that the isolation side of things saved me a headache at the cabin when my starter battery took a knock one winter. The Orion just carried on doing its job without dragging the leisure bank down with it.

One thing nobody's mentioned yet: if you're running the Victron Orion-TR Smart, the Bluetooth monitoring is genuinely useful for diagnosing whether your alternator's actually delivering what you expect. I found mine was only pushing 13.1V under load — nowhere near enough to properly charge a LiFePO4 without the DC-DC stepping up and doing the heavy lifting. The relay would've just sat there blinking at you none the wiser.

Worth downloading the VictronConnect app before you even wire anything up — gives you a baseline to work from.

LutonLife | Posts: 312

@Chopper42 one thing nobody's mentioned yet — if you're running the same battery chemistry across both the static and the boat, a DC-DC charger lets you dial in a proper LiFePO4 charge profile rather than whatever your alternator's pushing out. Fogstar Drift cells especially benefit from a controlled absorption stage rather than just being dragged up by voltage. I've got a 100Ah Drift in my Luton van and the difference in how the BMS behaves between the old relay setup and the Victron Orion was immediately obvious — far less grumbling from the BMS and noticeably better capacity over time. Worth the outlay IMO, particularly given what those cells cost in the first place. You're protecting a decent investment.

Totally agree with everything said above. One thing I'd add that nobody's mentioned yet — the DC-DC charger actually does your LiFePO4 a favour in terms of longevity. A split-charge relay is essentially just connecting two battery banks together, so your alternator sees the LiFePO4's very low internal resistance and can really struggle, particularly on older vehicles. Seen a few alternator failures in forums like this blamed on exactly that. The DC-DC unit acts as a proper buffer, protecting both your alternator and giving the leisure bank a controlled, staged charge. With a Fogstar Drift specifically, you're getting a quality cell that deserves better than a relay slamming voltage at it. @Chopper42 given you've got two setups running the same battery chemistry, I'd prioritise the boat installation first just for the reasons @Gibbo was alluding to with marine environments.