Anyone else running a Renogy 40A DC-DC charger alongside solar on a split system? Curious how you're balancing the two

I've been running a 200W panel (two 100W Renogy flexibles in series) feeding a Victron SmartSolar 75/15 into a 100Ah lithium (Battle Born) in my Transit Custom conversion. Works a treat on sunny days, but I've just added a Renogy DCC50S 50A DC-DC charger to top the leisure battery up from the alternator while driving. First couple of runs seem fine but I'm getting my head around how they interact when both are active at the same time.

The MPPT and the DC-DC are both pushing into the same battery, and I know they'll each regulate themselves, but I'm wondering if anyone's actually measured the combined input in real conditions. On a recent run I had the solar showing around 8A and the DC-DC doing maybe 30A — so roughly 38A total going in. The Battle Born is rated for 50A continuous charge, so I'm technically within spec, but it felt worth double-checking with people who've actually lived with a similar setup.

Has anyone had any thermal issues, BMS trips, or anything unexpected running a setup like this? Also wondering whether it's worth adding a battery monitor like the Victron BMV-712 to get a clearer picture of what's actually happening — or is the SmartSolar app giving you enough data day-to-day?

@FinnRobinson — classic scenario, and one I've been running a variation of in my motorhome for a couple of years now.

The Renogy DC-DC 40A and a SmartSolar will happily coexist on the same battery bank, but the key thing to watch is priority contention — on a decent solar day, your MPPT may already be in absorption/float when the DC-DC kicks in on the drive. Not a problem as such, but worth checking your Victron app to confirm they're not fighting each other on charge voltage setpoints.

Renogy's DC-DC tends to run a fixed lithium profile (14.6V/13.6V float from memory), whereas Victron lets you tune everything precisely. As long as they're within ~0.2V of each other you're fine.

What's your alternator — standard 90A Transit unit? That'll influence how hard you can actually push the 40A charger in practice.

Been running almost the exact same setup but with a Renogy 40A DC-DC into a 200Ah Fogstar Drift 12V — genuinely curious whether you're seeing any conflict between the two charge sources fighting over the battery's preferred charge curve?

My main question is around priority — when the engine's running and the sun's out, does the Victron SmartSolar just back off gracefully, or are you seeing the two controllers essentially competing? I've read conflicting things about whether lithium BMS units handle dual simultaneous inputs cleanly.

Also — did you need to do anything specific on the Renogy DC-DC programming side to match the Battle Born charge profile? I've seen a few people just leave it on the generic lithium preset and hope for the best, which makes me nervous on a £600+ battery.

Hey @FinnRobinson, great setup! One thing worth knowing is that both your SmartSolar and the DC-DC will negotiate with the BMS independently, so you shouldn't get any conflicts — they'll both just back off when the Battle Born signals full.

What I would suggest though is keeping an eye on your total charge current. If the DC-DC is pushing 40A and your MPPT is chucking in another 15A on a bright day, you're potentially hitting 55A into a 100Ah battery. Battle Born recommend no more than 50A continuous, so on a genuinely sunny day whilst the alternator's also running, you could be nudging their limits. Might be worth capping the DC-DC to around 30A in the Renogy app just to give yourself some headroom. Has worked well for @NeilEdwards from the sounds of it with a similar arrangement.

Great thread this. @FinnRobinson one thing I'd add that nobody's mentioned yet — keep an eye on your charge priority sequencing. On my Sprinter I found the DC-DC was occasionally "fighting" with the solar controller when both were pushing hard simultaneously, not in a damaging way, but the lithium BMS was seeing slightly erratic charge curves.

Worth logging a session via the Victron app on a decent driving day with good sun and just seeing what the combined input looks like in practice. My Battle Born handled it fine but I adjusted my drive thresholds slightly so the DC-DC backs off a touch earlier near full charge.

Also — are your flex panels wired series or parallel? Series into the 75/15 should be fine but worth double-checking your Voc doesn't creep up too high on cold mornings. 😅

Hey @FinnRobinson, great setup! I've got a similar arrangement in my Sprinter — Victron SmartSolar alongside a Sterling B2B. One thing I'd flag that hasn't been touched on yet: watch your combined charge current against your battery's maximum acceptance rate. A 100Ah Battle Born is rated for 50A continuous charge, and if your DC-DC is pushing 40A while the MPSS is also contributing on a sunny day, you could theoretically nudge past that on good days. Unlikely with only a 15A MPPT ceiling, granted, but worth keeping in mind if you ever upsize the solar. The Battle Born BMS will protect itself regardless, but it's better not to be repeatedly hitting those limits. 🙂

Running almost the same setup in my Sussex-based Defender conversion, except I went full Fogstar Drift 100Ah and paired a Renogy 40A DC-DC with a Victron SmartSolar 100/20.

The thing that caught me out initially — and nobody seems to talk about this — is that on a long motorway run the DC-DC can hammer the battery so aggressively that by the time you park up, the solar has virtually nothing left to do. Sounds ideal, but I found my BMS was cycling into absorption repeatedly throughout the day as the panel kept topping off small loads.

Worth logging a full charge cycle via the Victron app before assuming everything's balanced. The data told a very different story to what I thought was happening. @Kingy72 makes a good point about priority sequencing — that's where I'd start troubleshooting first.

On my boat I run a Renogy 40A DC-DC alongside a Victron MPPT and honestly the key thing nobody's flagged yet — watch for both sources hammering the battery simultaneously when it's deeply discharged. My Fogstar was taking a right beating before I throttled the DC-DC back a bit via the Renogy app.

@FinnRobinson with lithium you've got good headroom but keep an eye on total combined input vs your BMS's max charge current spec. Battle Born is rated 100A charge so with 200W solar and the DC-DC on a long motorway run you could be pushing close.

Good point from @LDVWanderer — worth adding that on a Transit Custom specifically, the alternator load can be a real consideration. Ford's smart charging systems on the Transit will drop voltage when it detects low demand, which can confuse a DC-DC charger into thinking the engine isn't running. I sorted mine by wiring the ignition sense wire properly rather than relying on voltage detection alone. Also worth checking your chassis cable runs — undersized cabling between the starter and leisure batteries will throttle your DC-DC output regardless of what it's rated at. Happy to share my cable sizing if useful.

Great thread! @FinnRobinson, one thing worth considering with that setup is communication between your DC-DC charger and the MPPT — if both are pushing into the battery simultaneously without coordinating, you can end up with neither operating at its proper absorption voltage. With Victron kit you can sometimes use the VE.Direct or BMS signal to have one yield to the other. Also worth checking your cable sizing handles the combined potential current — people often size for one source and forget they could theoretically have both going flat out together. What's your cable run length from the battery to the solar controller?

Had almost identical headaches with my motorhome build last year. What sorted it for me was treating the DC-DC and the MPPT as completely independent charging sources — let the BMS arbitrate rather than trying to coordinate the two chargers manually. The Renogy 40A will back off naturally once the battery voltage climbs from the solar side anyway, so you rarely get genuine conflict in practice.

One thing nobody's mentioned yet: time your driving to coincide with low SOC mornings if you can. Sounds daft, but pulling away when the battery's at 40% means the DC-DC does meaningful work rather than just floating uselessly.