Anyone else running a Renogy 40A DC-DC charger alongside solar on a split-charge setup? Seeing some odd behaviour

Last week I finally got round to wiring in a Renogy DCC50S (the 40A DC-DC with built-in MPPT) in my 2019 Transit Custom conversion. Running it alongside a 200W panel on the roof feeding into a 100Ah Battle Born LiFePO4. The idea was that the DC-DC would top up from the alternator on driving days whilst the solar does its thing when we're parked up.

The odd thing I'm noticing is that when both sources are active at the same time — engine running and decent sunshine — the charger seems to throttle back the solar input quite aggressively. I'm only seeing maybe 4–5A from the 200W panel when the sun is cracking it, even though the DC-DC side is only pulling around 20A from the alternator. Total combined input into the battery sits around 25A, which feels low when I'd expect closer to 35–38A in those conditions.

Has anyone else seen this with the DCC50S specifically, or is it a known limitation of combined units like this? I'm wondering whether I'd have been better off keeping the MPPT controller separate from the DC-DC charger entirely. Curious whether a dedicated Victron SmartSolar running in parallel would actually play nicely or cause its own headaches.

@BarryBennett familiar territory here — I ran a DCC50S in my Vivaro before upgrading to a standalone Victron Orion-Tr Smart + separate MPPT.

The DCC50S combines both charging sources internally, which causes headaches when the alternator and solar are fighting over priority logic. The unit essentially throttles one source when it detects sufficient input from the other, but the thresholds aren't well documented.

Key thing to check: what's your battery chemistry setting? If it's misconfigured, the absorption voltage target shifts and the unit behaves erratically when transitioning between sources.

Also worth monitoring your alternator voltage under load — Transit Customs with smart alternators can drop to 12.8V at idle, which sometimes confuses the DC-DC into thinking charging is complete.

What's the "odd behaviour" specifically? Voltage dropouts, unexpected cutoffs, or something else?

Hey @BarryBennett, worth checking whether the DCC50S is seeing your alternator voltage drop when the solar kicks in simultaneously — they can occasionally get a bit confused about priority switching between the two input sources. What's your leisure battery chemistry? If you've got it set to AGM but you're running lithium (or vice versa) that'll cause some peculiar charging curves that look like faults but aren't. Also double-check your VSR/isolator isn't interfering with the DC-DC's input sensing — some older split-charge relays don't play nicely with the DCC50S's voltage detection threshold. What "odd behaviour" are you actually seeing? Voltage fluctuations, the unit cutting out, or something else? Helps to narrow it down. Mine took a good few charge cycles before it settled into a consistent pattern tbh.

@BarryBennett the behaviour you're likely seeing is the DCC50S's input detection threshold getting confused when solar and alternator voltages overlap around that 13.2–13.8V grey zone. The unit can't easily distinguish between a "running alternator" signal and a fully solar-charged starter battery sitting at rest voltage.

Worth checking whether you've wired the ignition sense wire correctly — the DCC50S does have an IGN terminal specifically to avoid this ambiguity. If that's floating or disconnected, it'll rely purely on voltage detection, which causes exactly the erratic start/stop cycling you're probably describing.

Had near-identical grief in my shepherd's hut build, different charger but same root cause. Once the ignition signal was confirmed present, behaviour was rock solid.

What's your leisure battery chemistry? If it's LiFePO4 the charge profile mismatch could compound things further.

Not my setup (static van here, no alternator in the picture!) but I've been following this thread with interest because the DCC50S combines two inputs in a way that can genuinely confuse itself.

One thing nobody's mentioned yet — check your battery negative routing. If your chassis ground and your leisure battery negative aren't sharing a clean common point, you can get phantom voltage readings that make the unit think the alternator is present when it isn't, or vice versa. Seen this cause erratic behaviour on similar Renogy units in a few threads on here.

Also worth grabbing the Renogy BT-2 Bluetooth module if you haven't already — the real-time data logging will show you exactly which input the DCC50S thinks it's prioritising at any given moment, makes fault-finding much quicker.

Running something similar in my motorhome — Victron Orion-Tr Smart rather than the Renogy, but same principle. One thing nobody's mentioned yet: have you checked whether your Transit's smart alternator is causing issues before you even factor in the solar? Those Euro 6 engines do variable voltage charging and the DCC50S input threshold can get twitchy when alternator output drops to 12.8V at idle.

What's your leisure battery chemistry — are you running lithium or AGM? The DCC50S profile selection matters quite a bit here. I had a nightmare with mine until I confirmed the output profile matched the Fogstar cells I'd fitted.

Also worth logging the input voltage over a decent drive — even 30 mins of data tells you a lot about whether it's an alternator issue or genuinely a solar/DC-DC conflict.

@BarryBennett one thing worth checking that nobody's mentioned yet — the DCC50S has a 3-stage charge profile that can conflict with your solar MPPT if both are hitting absorption simultaneously. What you'll sometimes see is the DC-DC dropping back to float prematurely because the battery voltage is being held up by the MPPT, making the charger think the bank is full when it isn't.

In my Transit conversion I ended up running the solar into the leisure bank independently and letting the Renogy handle alternator-side input only — cleaner behaviour overall. If you're using a lithium bank (you mentioned 100Ah but didn't specify chemistry), the absorption window is narrow enough that this clash is quite pronounced.

What does your battery voltage look like on the Renogy app during the problematic phase? That data would narrow it down considerably.

Hey @BarryBennett, worth having a look at the input voltage threshold settings on the DCC50S — by default it won't kick in until it sees something like 13.2V on the starter battery, which on a modern Euro 6 Transit can behave oddly with the smart alternator doing its variable voltage thing. You might find the DC-DC is cutting in and out more than you'd expect. Some folks drop that threshold slightly to compensate. Also double-check your input wiring gauge — voltage drop on a longer run will confuse it no end. What symptoms are you actually seeing?