Anyone else running a Renogy 40A DC-DC charger alongside solar on a split system? Curious about priority/charging conflicts

I've just finished wiring up a Renogy DCC40S in my Transit-based van build and I'm trying to get my head around how it handles charging priority when both the alternator and my 200W roof panel are pushing power at the same time. The unit claims to manage both inputs intelligently but I'm not entirely convinced it's doing what I think it's doing. My leisure battery is a 100Ah lithium (a Fogstar Drift 12V), and on a decent driving day with good sun I'm seeing the DC-DC hit around 38-39A which seems right, but I'm not sure how much the solar is actually contributing versus being throttled back.

The setup is: panel going into the DCC40S's solar input (Voc around 24V, so within spec), and the starter battery feed on the other input. Both feeding the Fogstar via a single output. I've got a Victron SmartShunt on the leisure side so I can at least see what's going in and out in total, but there's no way I can tell from that alone how much is coming from which source.

Has anyone actually tested this properly — maybe with a clamp meter on each input line separately? I'm also wondering whether there's any real benefit to adding a separate MPPT controller for the solar rather than running it all through the DCC40S. The Renogy unit is convenient but I'd rather know the system is actually optimised.

@Martin1962 yeah I ran a DCC40S for about 18 months before switching to Victron kit. The short answer is there's no real "conflict" — the DCC40S handles both inputs simultaneously and blends them. Solar tops up whatever the alternator is putting in.

Where it gets interesting is when the alternator alone can't hit the threshold to wake the unit (usually around 13.3V). If your van's running stop-start, you'll get annoying cut-outs.

200W is also pretty modest alongside a 40A charger — the solar side won't be doing heavy lifting unless you're parked up with engine off, which is honestly where it earns its keep.

Main thing to watch: make sure your battery profile is set correctly on the unit. Renogy's defaults are a bit conservative out of the box.

@Martin1962 good timing on this build — I went through exactly this with my Sprinter conversion last year.

The DCC40S handles it elegantly: it blends both inputs simultaneously rather than choosing one over the other. Solar and alternator run in parallel through the unit, so you're not losing anything when both are active.

The only "conflict" I ever noticed was on overcast days where solar contribution dropped below the unit's threshold — it just seamlessly picked up the slack from the alternator without any fuss.

One thing worth knowing: the DCC40S does prioritise protecting your starter battery. If alternator voltage dips under load, it'll throttle back the charge current automatically.

I'd recommend grabbing a Victron Battery Monitor on the leisure side so you can actually see what's coming in from each source — makes diagnosing any odd behaviour much easier rather than guessing.

@Martin1962 worth clarifying the actual internal architecture here — the DCC40S uses a single shared output stage, so when both inputs are active simultaneously, the solar MPPT and DC-DC sections are effectively competing for the same charge current headroom. Renogy's documentation is frustratingly vague on this but empirical testing shows the unit prioritises the DC-DC side when alternator voltage is present, which means your 200W panel contribution can get throttled or zeroed during engine-on periods.

In practice on my own system (different unit, but same topology) I found a significant portion of available solar harvest was being discarded during transit. If logging your actual charge current matters to you, I'd strongly recommend adding a Victron SmartShunt — the data makes the behaviour very obvious, very quickly, and gives you something concrete to work with rather than guessing.