Renogy 40A DC-DC charger cutting out at high alternator voltage — anyone else had this?

Fitted a Renogy DCC50S a few weeks back in my Transit-based camper, wired between the starter battery and a 200Ah lithium (LiFePO4). Running a 130A alternator on a 2019 Ford Transit. All seemed fine initially but lately it's been cutting out after about 20–30 minutes of driving, particularly on longer motorway runs. LED goes red, unit gets warm but not scorching hot. Reset it by cycling the ignition and it fires back up, but same thing happens again.

I've checked my wiring — 6mm² cable with a 60A fuse close to the starter battery, grounds are solid, connections are tight. The Transit's alternator seems to be putting out around 14.6–14.8V under load which I'd have thought was fine. Ambient temp in the cab is probably 25–30°C on a warm day, unit is mounted on a wooden board under the passenger seat with maybe 5cm clearance around it. Wondering if that's the problem, but Renogy's own docs say it's rated to 40°C ambient.

Has anyone else run into this with the DCC50S or similar units? I'm debating whether to move it somewhere with better airflow, or whether there's something else going on — dodgy batch, firmware issue, something with Ford's smart charge system throwing odd voltages. Saw a mention somewhere that some Transits spike briefly to 15V+ during regenerative charging phases, which could be tripping the over-voltage protection.

Happy to get the multimeter on it properly and log some voltages if that'd help diagnose it. Just wanted to check if this rings any bells before I start pulling things apart again.

Hey @RM_Marine, sounds like it could be the over-voltage protection kicking in. Ford Transit alternators from that era can spike quite high under certain conditions — I've seen them push past 15V briefly, especially when the engine's cold or the smart charging system does its thing. The DCC50S cuts out at around 16V if I recall correctly.

Worth chucking a multimeter on the starter battery terminals while the engine's running and watching what it does over a few minutes. If you're seeing spikes, a small capacitor across the input terminals can help smooth things out, though that's a bit of a workaround.

Also double-check your input fuse and wiring gauge — voltage drop causing the unit to compensate oddly can sometimes cause weird behaviour too.

What does the unit's LED pattern show when it cuts out? That might narrow it down considerably.

Had this exact issue last year. Worth checking whether your Transit has smart charge management — those newer Fords can spike voltage quite high during regenerative charging phases, especially on cold starts or when the ECU decides to bulk charge.

The DCC50S cuts out above 16V if I recall correctly. Grab a cheap USB voltmeter or a Bluetooth battery monitor on the starter side and log what's actually happening during those cutouts — you might be surprised how high it spikes.

I ended up fitting a small capacitor bank across the input terminals which smoothed things out considerably. Some people swap to a Victron Orion-Tr Smart instead — it handles input voltage fluctuations more gracefully in my experience, though it's obviously pricier than the Renogy.

What's your alternator load looking like when it cuts out — heavy electrical use or light?

Good shout from @Mark1978 there. To add to that — the DCC50S has an input voltage cut-off around 16V, and Ford's smart alternator system can spike noticeably higher than that during regenerative charging events, especially on cold starts or when the engine's under load going uphill.

Worth grabbing a cheap USB voltmeter or a proper battery monitor and logging what your alternator rail is actually doing when the cutout happens. If you're regularly seeing above 15.5V on the input side, that's likely your culprit.

One fix some folks have had success with is fitting a small capacitor or a brief input delay, but honestly the cleaner solution is wiring the DCC50S off a dedicated leisure circuit rather than directly from the alternator B+ terminal. Have you checked the Renogy app logs at all? It does record fault codes which might confirm the over-voltage theory.

@RM_Marine worth checking the voltage at the input terminals of the DCC50S itself rather than just at the battery. I had a similar thing on my boat with a Victron Orion — looked fine at the alternator but there was enough of a difference at the charger input to tell a different story.

Also, the Renogy's over-voltage cutoff is around 16V if I recall right. If your Transit is pushing spikes above that during cold starts or heavy load switching, it'll drop out.

Grab a decent multimeter and log the input voltage while driving. If you're seeing spikes, a small capacitor or even a basic voltage clamp inline can help. @Mark1978 is right about the smart charge system too — those variable-voltage alternators can be unpredictable.