Anyone else running a split-charge relay alongside a DC-DC charger? Worth it or just redundant?

So I'm mid-build on a Transit Custom conversion and I've ended up in a bit of a rabbit hole. Started with a basic 140A split-charge relay from Ring to top up my 200Ah LiFePO4 leisure battery from the alternator while driving. Worked fine for the first few weeks but I kept reading about how unregulated charging isn't ideal for lithium, and that a proper DC-DC charger (B2B) would be kinder to the battery and the alternator.

Ended up fitting a Victron Orion-Tr Smart 30A DC-DC alongside the relay rather than replacing it. My thinking was the Orion handles the clever regulated charging, and the relay acts as a backup/overflow if I ever need to. But now I'm second-guessing whether having both in the circuit is causing any weirdness — the relay occasionally seems to be fighting the Orion, or at least that's what it feels like when I watch the current on my Victron BMV-712.

Has anyone actually run this kind of dual setup, or did you just rip the relay out once the DC-DC went in? I'm also curious whether the 30A Orion is even enough — on a full day's drive I'm pulling in maybe 40–50Ah according to the BMV, which seems low given the 30A rating. Is that normal, or am I losing something somewhere?

Chunk85 | Posts: 847 | Location: Derbyshire

@DodgyRigger I'd ditch the split-charge relay entirely mate. The problem is that LiFePO4 sits at a higher resting voltage than a lead-acid starter battery, so once your leisure bank is reasonably charged, the relay sees little enough voltage difference that it either doesn't engage properly or worse, tries to pull back from your starter battery.

A DC-DC charger like a Victron Orion handles all that properly - it actively regulates the charge regardless of voltage differentials and won't confuse your van's smart alternator either, which Transit Customs definitely have.

Running both is just adding unnecessary complexity and potential points of failure. Pick one, make it the DC-DC. The relay might feel like cheap insurance but it's really just dead weight in your wiring loom.

What size alternator are you working with? That might influence which Orion spec you go for.

RenogyKing | Posts: 1,203 | Location: West Yorkshire

@DodgyRigger @Chunk85 raises a fair point, but I'd actually keep the relay as a fallback if your DC-DC charger ever packs in - stranded without any alternator charging isn't fun! That said, make sure your DC-DC charger is doing the primary heavy lifting. With LiFePO4 you really need that proper CC/CV charge profile, which a basic split-charge relay simply can't deliver - it'll either undercharge or confuse the BMS depending on your setup.

What DC-DC charger are you running alongside it? If it's a decent 30A+ unit you're probably fine for a Transit Custom. Just wire the relay so it only kicks in if the DC-DC fails, perhaps via a manual override switch rather than automatically. Belt and braces without the redundancy causing headaches. 👍

XJ_Solar | Posts: 312 | Location: Array

Worth noting that a DC-DC charger (B2B) will properly profile-charge your LiFePO4 — the split-charge relay just dumps alternator voltage straight in, which your BMS will tolerate but it's not ideal long-term. Running both is genuinely redundant in most setups.

The one edge case I'd consider keeping something simple alongside is if you want a direct emergency fallback, but honestly on a Transit Custom the alternator output through a decent Victron Orion-Tr Smart 30A will serve you well enough solo.

I run a similar arrangement on my static cabin hookup — DC-DC handling all the clever stuff, no relay in the mix. Cleaner install, fewer points of failure.

What alternator are you working with? Some of the newer smart alternators on Transit Customs don't play nicely with relays anyway, which makes the B2B route basically mandatory rather than optional.

VictronMaster | Posts: 2,156 | Location: Array

One thing nobody's mentioned yet: with LiFePO4, the split-charge relay can actually damage your alternator. The battery presents such a low internal resistance when depleted that it essentially creates a dead-short load — some alternators don't cope well with that sustained demand.

The Victron Orion-Tr Smart (isolated version) is what I run on my narrowboat and my motorhome. Yes, it's £130-odd versus a tenner for a relay, but it properly protects the alternator via current limiting, and the Bluetooth monitoring through VictronConnect is genuinely useful.

@XJ_Solar is right about the charge profiling too — though I'd add that the Orion handles LiFePO4 absorption and float correctly out of the box once you configure it.

Keep the relay only if you're also running a VSR for a second alternator output. Otherwise it's redundant at best, harmful at worst.

GrahamCole | Posts: 2,156 | Location: Gloucestershire

@DodgyRigger one thing nobody's mentioned yet - with a LiFePO4 bank, running a split-charge relay alongside a B2B actually risks confusing your BMS depending on how it's wired. The relay can dump unregulated alternator voltage straight onto the lithium if there's any wiring fault or relay chatter, which your BMS won't thank you for.

Also worth considering your alternator health - modern Transits with smart alternators (and the Custom almost certainly has one) will behave very oddly with a direct relay connection. The B2B handles that variable voltage properly, the relay really doesn't.

If you're worried about charge speed, just get a decent-sized B2B - a 40A unit will outperform that relay setup in real-world conditions anyway. Keep it simple.

PennineSolar | Posts: 847 | Location: Array

Running exactly this setup in my Sprinter — Victron Orion-Tr Smart 30A alongside an old VSR I never bothered pulling out.

Honestly the VSR does basically nothing useful now. The Orion keeps the leisure battery properly charged with the correct LiFePO4 profile, and the VSR's voltage threshold never really triggers meaningfully because the B2B is already managing the relationship between the banks.

Only reason I've kept mine is laziness, not logic. 😅

If you're starting fresh @DodgyRigger, just fit the Orion-Tr Smart and be done with it. The isolated version is worth the extra few quid if your Transit's got any smart alternator/EFM nonsense going on — most newer Customs do. Don't bother with the relay at all, it's solving a problem that doesn't exist once you've got a proper B2B in there.