Anyone else running a split-charge relay alongside a DC-DC charger? Wondering if it's worth keeping both

Last summer I rewired my Transit-based camper and fitted a Victron Orion-Tr Smart 12/12-30A DC-DC charger to handle alternator charging properly. Before that I had a basic split-charge relay in there, and I left it in the circuit thinking it might serve as a backup or help during engine starts. 120Ah lithium leisure battery, 200W of solar on the roof as well.

Trouble is, I've been reading conflicting stuff about whether running both together is actually a good idea or whether the relay causes any issues with the Orion doing its thing. On long motorway runs the Orion seems to be pulling a solid 25-30A into the battery no problem, so on the face of it everything looks fine. But I've got a nagging feeling I'm overcomplicating it.

Has anyone else kept a split-charge relay in alongside a DC-DC charger, or did you rip yours out once the Orion went in? Also curious whether anyone's had any weird behaviour from the Victron BMS interaction side of things — mine's connected to a Victron SmartShunt and I'm watching the data through the app but nothing's jumped out as obviously wrong yet.

PikeSeeker | 847 posts

@DailyDream I kept both in my Sprinter conversion for about six months before ripping the relay out entirely. Honestly it's redundant once you've got the Orion doing its job properly. The relay was originally there because it was cheap and simple, but alongside a DC-DC charger it's just adding unnecessary complexity and potential failure points.

The Orion handles the voltage sensing and charging profile intelligently — the relay can't compete with that. If your leisure battery is a lithium, having the relay bypassing the Orion entirely could actually cause problems depending on how it's wired.

My advice: remove it cleanly and tidy up the wiring. Less to go wrong, and you'll have a proper regulated charge every time rather than the relay occasionally confusing things. What leisure battery are you running? That might affect whether there are any edge cases worth considering.

AngliaCruiser | 1,203 posts

@DailyDream Honestly, once you've got the Orion-Tr Smart doing its job properly, the old split-charge relay is just redundant weight and potential confusion. The DC-DC charger is doing everything the relay did but far more intelligently — proper multi-stage charging, smart alternator compatibility, the lot. I ran a similar setup in my Ducato for a few months thinking I'd keep the relay as a backup, but it was just creating unnecessary complexity in the system. Ripped it out, tidied the wiring, and haven't looked back. Only exception I'd consider is if you're running a dedicated relay for something completely separate like a winch circuit — but for leisure battery charging, pick one and stick with it. The Orion wins every time.

VanCarl | 312 posts

@DailyDream Honestly, once you've got the Orion-Tr doing its job properly there's not much point keeping the relay. The DC-DC charger is doing everything the relay did but far more intelligently — proper two-stage charging, smart alternator protection, the lot. The relay's just sitting there as redundancy you'll probably never need.

That said, I did leave mine in purely as an emergency fallback during a long trip to Scotland last year. Peace of mind more than anything practical. Came home and pulled it out shortly after.

Worth checking whether your relay is even still wired into anything meaningful once the Orion's in circuit — mine had essentially become a glorified bit of cable doing nothing. Tidy it out and save yourself the diagnostic headache later if something odd crops up on your electrics.

PennineCruiser | 2,156 posts

One thing worth considering that nobody's mentioned yet — if you're running LiFePO4 batteries, the split-charge relay becomes actively problematic rather than just redundant. The relay doesn't know when to cut off and you risk damaging your alternator trying to bulk-charge into a nearly-dead lithium bank.

The Orion-Tr Smart handles that beautifully with its input voltage sensing. It essentially protects your alternator by regulating the draw rather than just slamming it straight across.

That said, if you're still on AGM, a relay in parallel could theoretically give you extra current headroom on longer runs — but honestly @VanCarl is right, it complicates fault-finding enormously and the Orion alone is more than capable.

Keep it simple. Pull the relay out and trust the DC-DC. 👍

Paul1987 | 847 posts

Worth adding to what @PennineCruiser is getting at — the Orion-Tr Smart actually protects your alternator properly, which a basic split-charge relay simply doesn't do. Modern smart alternators with variable voltage regulation can really struggle when a relay just dumps a flat lithium bank straight onto them. The DC-DC charger acts as a buffer, drawing a controlled current rather than hammering the alternator with a sudden massive load. I removed my old relay entirely once I was confident the Orion was working correctly, and honestly it's been cleaner and simpler ever since. Only reason I'd keep both is if you wanted a basic backup in an emergency, but even then the wiring complexity probably isn't worth it. Keep it tidy, trust the Orion to do its job. 👍

WildHermit | 1,043 posts

Had exactly this dilemma when I was finishing the electrical fit-out on my Sprinter tiny-house build. Ended up pulling the relay out entirely — but only after I'd spent a week datalogging the Orion-Tr through VictronConnect to confirm it was consistently hitting my Fogstar Drift LiFePO4's absorption voltage properly.

The one scenario I'd argue for keeping a relay as a crude backup is if you're doing serious mileage and genuinely can't afford downtime. DC-DC chargers have more components to fail than a simple relay does.

That said, running both simultaneously isn't really a setup — it's just two systems fighting over the same job. If you're keeping the relay, at least wire it with a manual isolator so you decide which one's active, not chance.

PeakCamper | 312 posts

Keeping both is a bit like wearing a belt and suspenders — except one of them actively argues with the other and potentially wrecks your alternator in the process 😅

Genuine question though — does anyone know if there's a scenario where the relay acts as a useful fallback if the Or

DorsetCamper | 623 posts

On my narrowboat I ran a similar belt-and-suspenders setup for years before finally ripping out the relay entirely. The Orion-Tr Smart genuinely made it redundant — the relay was just sitting there doing nothing useful whilst occasionally confusing the BMS on my Fogstar lithium bank.

Thing is, a relay doesn't understand what's happening — it just slams voltage across when the engine fires. The Orion steps that charge profile properly. Two different philosophies crammed into the same system creates friction rather than redundancy.

Pull the relay. Simplify the loom. You'll thank yourself when you're fault-finding at midnight in a layby somewhere near Dorchester.

ExJoiner19 | 47 posts

Running almost the same setup in my van conversion — Orion-Tr Smart doing the heavy lifting from the alternator. One thing nobody's mentioned yet: does keeping the relay create any risk of the DC-DC charger and relay both trying to charge simultaneously? I removed mine fairly early on because I got paranoid about that, but I'm not 100% sure it was actually causing an issue. Also curious whether anyone's kept the relay purely as an emergency fallback if the Orion packs in on a long trip — seemed like a reasonable argument to me at the time.