Anyone else running a Renogy 40A DC-DC alongside solar? Getting odd behaviour when both inputs are active

Last week I finally finished wiring up my narrowboat's secondary charging setup — a Renogy DCC40S (the 40-amp DC-DC with built-in MPPT) fed from a 200W panel on the roof, plus the alternator from the Beta 38 engine. The leisure bank is 200Ah of lithium (Epoch 12V 200Ah), and I've had the Victron SmartShunt on there for a couple of months now, so I've got decent data to look at.

The issue I'm seeing is that when I'm cruising and the sun's out, the unit seems to throttle back the alternator input more than I'd expect. I thought the DCC40S would just pull from both simultaneously up to its 40A ceiling, but on a bright afternoon doing 3 mph on the cut, I'm only seeing around 24–26A into the battery rather than anything close to 40A. The panel alone on a good day pushes 14–15A through the MPPT side, so I'd have expected closer to 38–40A combined.

Has anyone else noticed this? I'm wondering whether it's a thermal throttling thing (the unit does get fairly warm, mounted in a fairly tight engine bay), or whether there's something in the firmware that deliberately de-rates when both sources are live. Can't find anything definitive in the Renogy docs — they're not exactly verbose on the detail.

Hey @Ken1999, welcome to the fun world of dual-input DC-DC charging! The DCC40S does have some quirky prioritisation behaviour when both inputs are live simultaneously — from what I've seen it tends to favour the alternator input and can throttle the MPPT side in ways that aren't immediately obvious from the manual.

Worth checking whether your panel voltage is sitting comfortably above the minimum threshold when the alternator's running, as the unit can get a bit confused if the solar input is borderline. Also, what battery profile have you got selected? Some of the lithium profiles behave differently under combined input conditions.

Could you share what "odd behaviour" you're actually seeing? Voltage readings, unexpected cutoffs, that sort of thing? Makes it much easier to diagnose properly rather than us all guessing! 🙂

Mine does the same dance on my van — turns out the DCC40S essentially has an internal arm-wrestle between MPPT and DC inputs, and DC wins when the alternator's above ~13.3V, leaving your panel sat there twiddling its thumbs in the sun like a Victron rep at a Renogy convention.

Something I stumbled into with my garden office setup — the DCC40S gets genuinely confused during that awkward transition window around midday when my alternator input is just above the detection threshold voltage and solar is hammering in simultaneously.

What sorted it for me was fitting a small Victron Battery Protect on the alternator feed with the enable wire tied to a simple voltage trigger. Essentially taught the unit to only "see" the alternator once it was confidently in range, not flickering at the margins.

@WayneKnight's arm-wrestle analogy is spot on — you want one combatant arriving clearly after the other has settled in.

@Ken1999 a narrowboat environment probably makes this worse too — engine bay voltage can be all over the place depending on load, bilge pump kicking in etc. Worth logging your alternator output voltage over a full engine run before assuming it's the DC-DC misbehaving.

Great thread @Ken1999 — narrowboat installs are brilliant for this kind of dual-input testing because you've got such predictable engine run times to work with.

One thing worth checking that nobody's mentioned yet: have you looked at your alternator output voltage under load? If it's dropping below roughly 13.3V when the engine's working hard or the alternator's warm, the DCC40S can get a bit indecisive about whether it's even seeing a valid DC input. I had a similar head-scratching moment until I logged the input voltage properly.

Stick a cheap voltage logger on the alternator feed for a full day's cruising and you might find the "odd behaviour" correlates neatly with voltage dips. What symptoms are you actually seeing — is it dropping charge current, cycling on and off, or something else? That'd help narrow it down considerably.

Really useful thread, this one. @Ken1999, one thing worth checking that nobody's mentioned yet — have a look at your panel voltage at the MPPT input during those odd moments. On my setup I found the DCC40S would behave erratically if the solar input was sitting in that awkward mid-range, not quite high enough to be confident but not low enough to ignore. Shading from trees along the towpath could easily cause this on a narrowboat.

Also worth confirming your battery type is correctly set on the unit — I had mine defaulting to something wrong after a firmware reset and it threw all sorts of strange charging behaviour that I initially blamed on the dual-input conflict. Simple fix but easy to overlook when you're focused on the wiring side of things.