Anyone else running a Renogy 40A DC-DC charger alongside a split charge relay? Worth it or overkill?

Picked up a Renogy 40A DC-DC (DCC40S) a few months back to replace the old VSR split charge relay in my Transit-based van. Honestly the difference has been night and day — the relay was barely pushing anything useful into my 200Ah lithium once the alternator voltage dropped under load, whereas the DC-DC is actively holding a proper charge profile all the way through. Long motorway runs now actually top the battery up rather than just tickling it.

Thing is, I've still got the old relay wired in parallel because I was nervous about pulling it out entirely. A mate reckons I'm just adding unnecessary complexity and potential fault points, and honestly he's probably right. The DC-DC handles the B2B charging on its own just fine and the relay sitting there doing nothing feels a bit daft.

Has anyone here run both together long-term, or just pulled the relay out and gone pure DC-DC? I'm also curious whether the 40A is actually worth it over the 20A unit — my alternator is a 150A on a 2.0 EcoBlue and I'm not too worried about overloading it, but I want to make sure I'm not being an idiot here. Van also has a 400W solar setup feeding the same lithium bank if that changes anything.

RogerHobbs | Posts: 847 | Location: Shropshire

@PYQ_Power completely agree on the difference being noticeable. One thing worth adding — if you've got a modern Euro 6 engine with smart alternator, the VSR is essentially useless anyway since the alternator voltage drops intentionally when the engine's warm. The DC-DC charger reads that as a charging signal and gets on with it regardless, which is exactly what you want.

Running a similar setup in my Sprinter with a 200Ah lithium bank, and the DCC40S handles the battery-to-battery charging properly rather than just throwing whatever the alternator produces at your leisure batteries.

Only caveat I'd say — make sure your cable runs are adequately sized for sustained 40A draw. A lot of people underestimate the voltage drop over longer runs and wonder why it's not performing as advertised.

GrumpyWanderer | Posts: 312 | Location: Array

Running the same unit in my Sprinter. The VSR was basically useless once I fitted lithium — relay doesn't know what to do with a flat LiFePO4, just sits there doing nothing useful.

The DCC40S actually charges properly with its multi-stage profile. Paired it with a Fogstar 200Ah and the alternator's not getting hammered either, which was my main worry.

Only gripe is the Renogy app is a bit rubbish, but the hardware itself is solid for the money. Victron's Orion-Tr Smart is the obvious alternative if budget allows — better integration with the ecosystem — but at nearly double the price you've got to really want it.

For anyone still on a relay with lithium, swap it out. Not optional imo.

MXM_OffGrid | Posts: 1,203 | Location: Yorkshire

Good timing on this thread — just finished helping a mate install the same unit last weekend in his LDV Convoy.

One thing worth flagging that nobody's mentioned yet: if you've got a modern Euro 6 engine with smart alternator charging, the DC-DC isn't just nice to have — it's pretty much essential. Those variable voltage alternators will confuse a standard VSR something rotten, whereas the Renogy handles the fluctuating input voltage without blinking.

@PYQ_Power what's your leisure battery setup? Curious whether you're running AGM or lithium, because the DCC40S really earns its keep when charging lithium — the proper CC/CV profile makes a proper difference to charge times compared to whatever a VSR was managing to push through.

Also worth checking your cable runs are properly rated for 40A continuous. Seen a few underspec jobs cause grief down the line.

Gazza | Posts: 2,156 | Location: Array

Ran a VSR for two years in my first van build before finally caving and fitting a Renogy DC-DC. The moment that clicked for me was a winter trip to Scotland — alternator was charging at something daft like 13.1V whilst my lithiums needed proper absorption up around 14.6V. The VSR just couldn't bridge that gap.

What nobody mentions enough is the alternator protection side. Modern smart alternators in Transits and Sprinters genuinely hate being hammered by a big lithium bank pulling hard. The DC-DC limits that draw intelligently rather than just slamming the alternator with whatever current it fancies.

@MXM_OffGrid curious what your mate's van setup was — whether he's paired it with solar too. Running mine alongside a Victron MPPT and the two play together nicely without fighting each other.

RobRobinson78 | Posts: 487 | Location: Worcestershire

Worth flagging something nobody's mentioned yet — if you're running a modern Euro 6 engine with smart alternator (variable voltage charging), a VSR is basically redundant because the alternator voltage drops too low to trigger it half the time. The DCC40S handles that properly with its DC-DC conversion regardless of what the alternator's doing. Had exactly this issue in my '19 plate Transit Custom before I swapped over. The relay would just sit there doing nothing on a run because the ECU was throttling the alternator output to save fuel. @MXM_OffGrid your mate will definitely notice the difference if he's got anything post-2017ish. Proper game changer once you've got it wired correctly with the ignition sense wire sorted.

ColPalmer | Posts: 834 | Location: Derbyshire

@RobRobinson78 Curious what you were about to flag there — don't leave us hanging!

To add something to the mix: one thing I'd highlight is that the DCC40S genuinely earns its keep if you're running lithium leisure batteries. The multi-stage charging profile makes a real difference compared to a VSR which just slaps whatever voltage the alternator produces straight across your bank.

That said, if you're still on AGM and doing mostly short runs, a decent VSR isn't terrible — the DC-DC just does the job properly. Also worth checking your alternator's output rating before assuming 40A is fine to pull continuously. Some of the newer Euro 6 Transits have smart alternators that behave oddly without a DC-DC anyway, so it arguably becomes less optional on those.