Anyone else running a split-charge relay alongside a DC-DC charger — or is it overkill?

Just finished rewiring the leisure setup in my Transit-based van and I've ended up with both a Sterling Pro Split R 12V relay and a Renogy 40A DC-DC charger running in parallel. The relay was already in from the previous owner and rather than rip it out I left it in, thinking it might help top things up faster on longer runs. Now I'm second-guessing myself.

The leisure bank is two 100Ah AGMs wired in parallel. On a decent motorway run of a couple of hours the DC-DC alone was pushing around 38–40A into the bank, which seems solid. But I'm wondering if having the split-charge relay dumping unregulated alternator voltage on top of that is actually causing more harm than good — particularly as the batteries get closer to full.

I've read a fair bit about DC-DC chargers being the "right" way to do it with modern smart alternators, and I get the logic there. My van's a 2019 Transit with the 2.0 EcoBlue, so it almost certainly has a variable voltage alternator. Leaving the relay in feels a bit like wearing a belt and braces made of different materials — not sure they're actually working together.

Has anyone tested whether the two systems interfere with each other, or found a neat way to use a relay as a backup/boost without it conflicting with the DC-DC? Keen to hear how others have approached this before I start pulling things apart again.

@SamKing not really my world (I'm cabin/garden office rather than van) but the principle's the same — a split-charge relay on its own won't do your leisure battery any favours if you've got a modern smart alternator. The DC-DC charger handles that properly.

Running both in parallel though does seem like you're doubling up unnecessarily. The relay will just dump whatever the alternator's throwing out unregulated, while the Renogy's doing the sensible controlled charge alongside it. Could even confuse things depending on your battery chemistry.

I'd either ditch the relay or at least wire it so the DC-DC takes priority and the relay's only a backup/emergency fallback. What battery type are you running? If it's lithium that relay could be a problem anyway.

Different world here (narrowboat not a van) but we run something similar — Victron Orion-Tr Smart doing the heavy lifting off the alternator, and I did briefly have an old relay in the mix too during a rewiring bodge.

Ended up pulling the relay out. The DC-DC handles the absorption/bulk stages properly, the relay just muddied things. On a boat you really don't want two things fighting over the same circuit.

One DC-DC doing it right beats two systems doing it badly, imo. What's your reasoning for keeping both — was it an existing install you didn't want to rip out?

Been down this exact rabbit hole with my motorhome build last spring. What pushed me over the edge was realising the relay was doing nothing useful once I'd fitted lithium — the voltage differential just isn't there to trigger it reliably when the DC-DC is already regulating.

Ended up pulling the relay out entirely. The Renogy DC-DC handles the alternator-side charging properly, and without the relay muddying the circuit there's far less heat and confusion.

@BreezyDrifter makes a fair point about the Victron Orion-Tr — I'd have gone that route if I were starting fresh, honestly. The Bluetooth monitoring alone would've saved me several confused evenings in a layby with a multimeter.

@SamKing — what battery chemistry are you running? That'll really decide whether keeping both makes any sense at all.

Not a van person myself — I've got a Fogstar lithium setup in my garden office fed from solar — but this question is nagging at me because I'm considering adding a DC-DC charger as a backup from the house battery bank.

Quick question for the thread: if you're running both simultaneously, is there any risk of the relay and the DC-DC charger essentially "fighting" each other, or does the DC-DC charger's internal logic sort that out? I'd assume the MPPT takes priority but I genuinely don't know how the voltage signalling interacts when both are active at once.

@VickyFisher — you mentioned the relay becoming redundant, which makes sense to me. Did you end up just leaving it in circuit as a failsafe, or did you physically bypass it?

Really interesting setup @SamKing — I've got a similar arrangement in my Sprinter, though I eventually pulled the relay out once I was confident the DC-DC was doing its job properly.

The relay made sense as a belt-and-braces approach during the build, but long-term they can actually work against each other depending on how you've wired it. If the relay kicks in and dumps unregulated alternator voltage straight onto lithium cells before the DC-DC can respond, you're potentially bypassing the protection you paid good money for.

What battery chemistry are you running? If it's lithium, I'd honestly retire the relay and let the DC-DC handle everything solo — that's what it's designed for. If you're on AGM, the relay alone would arguably do the job fine anyway.

Not overkill necessarily, just worth making sure they're not fighting each other.

Good timing on this thread — I ran almost the same combo in my Ducato for about six months before making a decision.

The thing worth considering that nobody's mentioned yet is alternator load management. Modern Euro 6 engines with smart alternators can get quite grumpy if the DC-DC charger and relay are both pulling hard simultaneously. The DC-DC handles that gracefully by design; the relay obviously doesn't care.

What I'd actually suggest @SamKing is keeping the DC-DC as your primary charger and wiring the relay purely as a low-voltage fallback — set its disconnect threshold high enough that it rarely triggers. Best of both worlds without the relay interfering with your day-to-day charging cycle.

@CrispySkipper curious what made you pull the relay entirely — was it the smart alternator issue or something else?

Good question @SamKing. I ran a similar combo briefly and honestly the DC-DC charger alone is the right tool for the job with lithium — it handles the charge profile properly and protects your alternator from being hammered. The split-charge relay alongside it is redundant at best, and at worst you're risking the relay dumping unregulated voltage into your leisure battery before the DC-DC kicks in properly. Unless you've got a specific reason to keep both (fallback perhaps?), I'd simplify and pull the relay. Cleaner install, fewer potential fault points. What battery chemistry are you running?