Anyone else running a Renogy 40A DC-DC alongside solar on a split system?

So I've got a Renogy DCC40S wired between my van's alternator and a 200Ah lithium leisure battery, and it's been solid for the most part. Running it alongside a 200W roof panel through a Victron 75/15 MPPT, both feeding the same bank. The idea was that the DC-DC handles the driving charge and the MPPT handles the solar — seemed tidy in theory.

The bit I'm scratching my head over is priority. On a decent sunny day the MPPT is already pushing the battery close to full by the time I start the engine, so the DC-DC barely gets a look in. Fine. But on overcast days I'm getting both sources hammering the battery at the same time and I'm not entirely sure the BMS is seeing what I think it's seeing. The Renogy app is pretty basic and doesn't talk to the Victron at all, obviously.

Has anyone found a clean way to monitor both input sources together without spending a fortune on a Cerbo GX or similar? I've been looking at a Victron BMV-712 on the output side of the battery just to get a proper state-of-charge reading, but that still doesn't tell me what each source is doing individually. Wondering if a shunt on each input feed is the way to go, or if I'm overcomplicating this.

Running almost the exact same setup in my Transporter – DCC40S paired with a 200W panel through a Victron MPPT. One thing worth keeping an eye on is the combined input behaviour when both sources are active simultaneously. The Renogy handles it fine in my experience, but I noticed on longer motorway runs the solar input can occasionally confuse the charge stage detection if your battery is already near full from a sunny day. Not a dealbreaker, just something to monitor.

What leisure battery are you running? If it's a proper LiFePO4 with its own BMS, make sure the Renogy's lithium profile settings match your battery's charge parameters – the defaults are a touch conservative but that's no bad thing really. Curious what issues you've been seeing with yours @12VNerd?

Curious about something similar on my narrowboat — I've got a Renogy DCC40S paired with 300W of panels through a Victron SmartSolar 100/30, and I've been wondering whether the two charge sources ever "argue" over absorption/float transitions.

Has anyone noticed the DCC40S holding the battery in bulk longer than it should when the MPPT is already pushing it toward absorption? I'm seeing occasional odd behaviour on my BMV-712 where the SOC reading bounces around during combined charging.

Also — does the DCC40S on your setup get noticeably warm when both sources are active simultaneously? Mine runs quite hot to the touch on a long cruise with decent solar input, though it's not throwing any fault codes.

@12VNerd what battery profile are you running on the DCC40S — the lithium preset or custom values?

@TrevorHughes narrowboat application is worth a specific mention — your engine alternator output will vary considerably depending on whether you're ticking over at tick-over or cruising at proper revs. The DCC40S input voltage range (8–32V) handles most alternator scenarios fine, but worth confirming your alternator isn't a smart/variable voltage unit, as some modern ones drop output voltage deliberately when the ECU decides battery is "full" — the DC-DC will then throttle back unnecessarily.

On the combined solar+DC-DC question generally: both sources feeding the same battery simultaneously is absolutely fine with this unit. The DCC40S handles priority internally. Where people occasionally get caught out is undersized cable runs on the alternator input side — at 40A continuous, voltage drop there directly reduces your charge current into the leisure bank.

What's your engine make, @TrevorHughes? Beta and Bukh installations I've seen handle this well.

RetiredSquaddie92 | 847 posts | ⭐ Trusted Contributor

@TrevorHughes worth flagging that on a narrowboat you'll also want to keep an eye on your alternator's duty cycle — those older BMC or Perkins units weren't designed with sustained DC-DC loads in mind. Had a mate on the Shropshire Union who cooked his alternator running a similar setup flat out through long cruising days.

@DevonDweller makes a fair point about output variation, and I'd add that engine RPM on tickover in locks or whilst manoeuvring will drop your alternator output noticeably — the DCC40S handles it gracefully enough but your charge profile will be inconsistent.

One thing nobody's mentioned yet: have you all got adequate fusing on the input side close to the source battery? Seen a few nasty situations where that's been overlooked. Non-negotiable in my book.

Good thread this. @12VNerd I'm running almost the exact same setup in my camper — DCC40S plus a Victron MPPT feeding a 200Ah lithium bank. One thing I noticed that nobody's mentioned yet is the priority behaviour when both sources are active simultaneously. The DC-DC and solar controller can occasionally "argue" over charge state if your BMS communication isn't properly sorted. Worth checking your battery's charge profile is consistently applied across both devices rather than letting them work independently. I spent a frustrating weekend wondering why my SOC readings were all over the place before I realised the two controllers had slightly different absorption voltage settings. Small discrepancy, but enough to cause confusion. @TrevorHughes the narrowboat application does add another layer with engine runtime being less predictable — you'd want to make sure the DC-DC is doing proper heavy lifting during those shorter cruising stretches.

@12VNerd one thing worth watching with that combo — the DCC40S and a Victron MPPT can occasionally "argue" over the battery if your BMS starts load-shedding. Had a similar setup in my motorhome and found the MPPT would briefly go into absorption while the DC-DC was still pushing bulk current. Nothing catastrophic but it confused the battery state readings.

What BMS are you running on the 200Ah? If it's a Fogstar Drift or similar with decent comms you can at least see what's actually happening.

Also — are you using the ignition sense wire on the DCC40S properly? Sounds obvious but I've seen installs where it's just floating and the unit runs off parasitic voltage. Kills alternators slowly.