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

Been scratching my head over this one while planning the electrics for my Transit-based build. I've got a Renogy 40A DC-DC charger on order for the leisure battery (a 200Ah LiFePO4), which I know is the proper way to go for lithium. But I've also got an old 120A split-charge relay sitting on the shelf from a previous van, and I'm wondering whether there's any point keeping it in the circuit at all.

From what I can gather, running both simultaneously could actually cause problems — the relay might try to push unregulated alternator voltage straight into the lithium while the DC-DC is also doing its thing. That sounds like a recipe for confusion at best and a damaged BMS at worst. But I've seen a couple of builds online (admittedly mostly American forums) where people use the relay as a backup or for a separate load, so I'm not entirely sure what the accepted wisdom is over here.

My alternator is a standard 90A unit on a 2.2 TDCi, so the DC-DC charger on its own would be pulling around 550W — roughly 45A on the 12V input side — which feels like it's working the alternator fairly hard already. Would a relay even add anything useful on top of that, or would it just be surplus to requirements?

Happy to hear how others have wired this up, especially if you're running lithium. Concrete wiring diagrams or part numbers welcome — the more specific the better.

@BatteryDoug interesting timing — I'm mid-way through sorting something similar for a narrowboat setup but the principles overlap.

One thing worth considering: with a DC-DC charger already handling proper stage charging for your LiFePO4, a split-charge relay alongside it is largely redundant and could actually cause headaches. The DC-DC isolates your starter battery properly and prevents the alternator seeing the LiFePO4 as a massive load.

Curious though — what alternator are you running? On newer Transits with smart alternators (variable voltage output), a split-charge relay reportedly struggles badly, whereas a DC-DC charger handles that natively. That might make your decision for you regardless.

Has anyone on here run both simultaneously and actually seen a measurable benefit? I'd genuinely like to know before I finalise my own Victron Orion-Tr Smart installation.

Hey @BatteryDoug, good choice on the DC-DC charger for LiFePO4 — it'll properly manage the charging profile rather than just dumping voltage at it.

To answer your question directly: it really is one or the other. Running a split-charge relay alongside a DC-DC charger is redundant at best and potentially problematic at worst. The DC-DC charger already handles the isolation and charge regulation between starter and leisure batteries — adding a relay into the mix can create confusing parallel paths.

Just wire the DC-DC charger between your alternator feed and leisure bank and let it do its job. The Renogy 40A unit should handle your 200Ah LiFePO4 nicely, though bear in mind your Transit's alternator load — worth checking it can sustain that draw comfortably on longer runs.

@AshSeeker — narrowboat setup will be very similar logic, should translate well!

@BatteryDoug mate, running both is basically like wearing a belt and suspenders — except the belt is also slowly strangling your alternator 😅

The DC-DC charger is your split-charge solution — it isolates the banks, conditions the charge properly for LiFePO4, and won't cook your Transit's smart alternator like a traditional VSR would.

Ditched my old split-charge relay when I fitted a Victron Orion-Tr Smart 30A in my motorhome and never looked back — the Bluetooth monitoring alone is worth the upgrade.

Only reason to keep a relay alongside it would be a basic emergency bypass, but even then it's probably overcomplicating your install.