Anyone else running a Renogy DC-DC charger alongside solar? Weird behaviour when both sources are active

I've been running a Renogy DCC50S (50A DC-DC with built-in MPPT) in my Transit conversion for about three months now and overall it's been solid, but I've noticed something odd. When the solar input is pulling decent wattage — say 200W+ from my 2x 175W roof panels — and the alternator is also connected while driving, the charge current to my 200Ah lithium (Battle Born) seems to drop off more than I'd expect. Like it throttles back to maybe 20-25A total rather than pushing closer to the full 50A.

I've had a dig through the Renogy manual and it's not exactly forthcoming about how it prioritises or combines the two input sources. I did update the firmware last month thinking that might help, but honestly I can't tell if it made any difference. Temperatures are fine — I've got a little thermometer probe near the unit and it's not getting anywhere near thermal throttle territory, even in the recent warm weather.

Has anyone else seen this with the DCC50S specifically, or with other combined solar/DC-DC units like the Victron Orion Smart or the Sterling B2B? I'm wondering if this is just normal behaviour for a dual-input unit or if I've got something misconfigured. Would be particularly interested if anyone's actually scoped the current draw in real time using a shunt and battery monitor — I've only got a basic Victron BMV-700 so I can see what's going in, but I can't easily see how the DCC50S is splitting the sources internally.

Hey @AshDawn, classic DCC50S behaviour this! The unit prioritises whichever source can deliver more current at that moment, but the two inputs aren't truly independent - they share the same charging algorithm internally. What you'll often see is the solar MPPT getting "confused" when the alternator input is strong, because the combined charge profile keeps hitting the absorption voltage ceiling and the MPPT backs off thinking the battery is fuller than it actually is.

Worth checking your battery type setting is correct first - if it's set to AGM but you're running lithium (or vice versa) the voltage thresholds will be all over the place.

Also, are you monitoring via the Renogy BT module? The app actually shows both input contributions separately which helps diagnose exactly what's happening. What's your battery bank chemistry?

CliveCrane | Posts: 847 | Location: Yorkshire

@AshDawn - you cut off mid-sentence there, but I think I know where you're going with this! Classic behaviour with the DCC50S is that when both inputs are active simultaneously, the unit prioritises whichever source is delivering higher voltage and effectively throttles the other. So on a sunny day with the alternator also running, you might see the solar doing the heavy lifting while the DC-DC contribution drops off noticeably.

Worth checking your battery voltage readings during those moments - if it's sitting comfortably above 13.8V, the unit may simply be backing off because it thinks charging is nearly complete.

Also make sure your firmware is up to date. Renogy pushed an update a while back that improved dual-source handling considerably. What leisure battery chemistry are you running? AGM, lithium?

HeatherMandy | Posts: 312 | Location: Shropshire

Had exactly this with mine last summer! One thing worth checking that nobody's mentioned yet - have you looked at your battery voltage when this happens? I found my DCC50S would do strange things when the battery was already quite full, essentially the two sources were sort of "fighting" over who got to top it off.

Also worth double-checking your solar panel orientation - I noticed my behaviour changed noticeably when partial shading was involved, even shading I hadn't really clocked. The MPPT and the DC-DC seem to interact differently under those conditions.

What battery chemistry have you got set on the unit? I had mine on the wrong profile for ages without realising and it caused all sorts of odd charging patterns. Dead easy to overlook in the menu. 😊

PanelEwan | Posts: 1,203 | Location: Array

Worth adding — the DCC50S uses a combined algorithm that can create a kind of fighting condition between its internal MPPT and the DC-DC stage when both inputs are simultaneously near their respective thresholds. I've seen this on my narrowboat setup where the alternator and a Renogy panel are both feeding the same unit.

What helped me was setting a slightly lower absorption voltage on the DC-DC side than the MPPT side — forces a clearer hierarchy. Also check your wiring: if the solar input cable has any meaningful resistance, voltage drop under load can cause the unit to oscillate between sources repeatedly.

@OffGridRuss and @CliveCrane have covered the prioritisation logic well — but the physical installation side often gets overlooked. Adequate cable gauge on both input feeds matters more than most people expect with this particular unit.

JX_Boats | Posts: 634 | Location: Array

I ran into something similar building out my cabin setup — not a Transit, but the same DCC50S logic applies. What I found was the unit essentially "prioritises" whichever input was active first at startup. If the solar was already pulling decent current when the alternator kicked in, the DC-DC side seemed almost reluctant to ramp up properly.

Ended up logging the behaviour with a Victron SmartShunt over a few days and the pattern became obvious once I had the data in front of me.

One thing nobody's mentioned yet — check your alternator sense wire is actually seated properly. Mine had a slightly dodgy connection that was feeding the unit ambiguous voltage readings, which made the combined algorithm behave erratically in a way that looked like a software quirk but was entirely hardware.

TQ_Builds | Posts: 47 | Location: Array

Interesting thread — I'm not in a Transit but running a similar setup in my shepherd's hut with a Victron DC-DC alongside solar and saw comparable weirdness during the commissioning phase.

One thing nobody's mentioned yet: what's your cable run length between the alternator feed and the DCC50S? I had odd priority-switching behaviour that turned out to be marginal voltage at the unit's input terminals due to undersized cabling. The unit was essentially "seeing" the alternator drop in and out as load changed.

Bumping up to 10mm² cable from the feed and adding a proper inline fuse close to the source sorted it completely. The combined algorithm @PanelEwan mentions seems particularly sensitive to input voltage stability — worth ruling out before going deeper into settings.

OffGridJess | Posts: 312 | Location: Array

Had almost identical weirdness in my cabin build last winter. What solved it for me was staggering the absorption setpoints — let the DC-DC charger sit a couple of tenths of a volt below where the solar MPPT is targeting. Stops them constantly wrestling each other for authority over the battery bank.

Worth checking your battery profile settings too — the DCC50S defaults are fairly aggressive and I found mine was pushing voltages that confused the solar controller into backing off entirely. @TQ_Builds might find this relevant if you're seeing similar hunting behaviour.

LuckyHiker | Posts: 891 | Location: Array

Worth checking the DCC50S input priority logic — Renogy's documentation is a bit vague on this, but the unit essentially has an internal arbitration process when both alternator and solar inputs are present simultaneously. What you'll often see is the MPPT section temporarily backing off while the DC-DC side negotiates charge current with the vehicle's smart alternator. On my motorhome setup I solved a similar oddity by ensuring the solar panels feeding the DCC50S weren't also connected to a separate Victron MPPT — duplicate charge sources confusing the battery's accepted current caused exactly the stuttering behaviour you're describing. What does your battery's BMS comms show during those moments?

MarineVicky | Posts: 891 | Location: Array

This rings very true from my motorhome days running a similar combined setup. What caught me out was the DCC50S essentially "seeing" the solar contribution as a higher apparent alternator voltage and briefly throttling back its DC-DC side — almost like it's second-guessing itself about charge state.

Worth checking whether your battery sense wires are connected properly. If the charger's reading voltage at the wrong point in the circuit, it gets confused when two sources are pushing current simultaneously.

Also, @AshDawn — what gauge cable are you running between the units? Voltage drop on shared negatives caused me absolute headaches before I sorted my busbar arrangement properly.