Anyone else running a Renogy 40A DC-DC charger alongside solar on a split system?

Picked up a Renogy DCC40S a few months back to handle charging from the cab whilst also taking input from my 200W roof panel. Running it into a 200Ah lithium (cheap Fogstar Drift) and it's been mostly brilliant — engine running I'm pulling a solid 30–35A into the battery, which is miles better than the old VSR I had before.

My question is around how it prioritises when both sources are available at the same time. According to the manual the solar takes precedence, but on a decent sunny day with the engine running I'm not convinced it's actually doing that. The MPPT side seems to back off a bit and I end up with a combined input that doesn't quite add up to what I'd expect. Anyone noticed similar behaviour or actually wired it differently to get around it?

Also curious whether anyone's paralleled two of these units — I've seen a few American builds doing it for 80A combined but I can't find anyone in the UK who's tried it. My alternator is a decent 120A unit so I reckon there's headroom, but I don't want to cook anything before I've done the research.

TH_OffGrid | 847 posts

@WhatsAFuse4 yes, running almost identical setup actually - DCC40S feeding a 200Ah LiFePO4 (Epoch in my case). Few things worth knowing: the solar input priority on that unit means it'll favour the panel over alternator when both are available, which is fine in summer but worth bearing in mind if you're relying on engine charging on dull days - it can be a bit indecisive switching between sources.

Also double-check your alternator type before pushing it hard. Smart/EFB alternators on newer vehicles can throw a wobble if the DC-DC pulls too aggressively on startup. The DCC40S handles it better than most but worth knowing.

What's your cab vehicle? And did the Fogstar arrive with a proper BMS spec sheet? Curious how well it communicates charge acceptance to the charger.