Anyone else running a Renogy DC-DC 40A alongside a leisure battery bank? Getting odd readings

I've just finished wiring up a Renogy 40A DC-DC charger (the DCC40S) between my van's alternator and a 200Ah LiFePO4 bank I'm building out. Everything seemed straightforward enough on paper — isolating the starter battery, feeding the charger from the B+ terminal via a 60A fuse, good earthing back to the chassis. Fired it up yesterday on a run down the M6 and the charger was pulling a steady 38-39A, which felt promising.

The issue is the input voltage I'm seeing on the app. When I'm cruising at 70mph the input sits around 14.2V which is fine, but the moment I drop to 30mph in traffic it dips to 13.4-13.5V and the charger seems to throttle back noticeably — sometimes dropping to 20A or less. I'm using 6mm² cable for the run which is about 2.5 metres each way. Wondering if that's just the alternator struggling under load at low revs, or whether I've got a cable sizing issue.

Has anyone else seen this behaviour with the DCC40S specifically, or with DC-DC chargers generally at lower engine speeds? I'm half tempted to step up to 10mm² cable but I don't want to rip everything apart if it's just normal alternator behaviour. Voltage drop calcs suggest 6mm² should be fine at that distance, but the real-world numbers are making me second-guess myself.

@LiamWalker you've cut off mid-sentence so hard to know exactly what readings you're seeing, but a few common gotchas with the DCC40S:

• Input voltage drop — the unit pulls hard on the alternator circuit, so if your input wiring isn't properly rated (6mm² minimum, ideally 10mm²) you'll get erratic readings as voltage sags under load
• BMS communication — if your LiFePO4 bank has a BMS with a low-temp cutoff, the DC-DC will sometimes throttle or cut out unexpectedly without any obvious error
• Sensing wire — double-check the battery voltage sense wire is connected; without it the charger's reading its own internal voltage, not the bank

Had similar weirdness on my static caravan setup before I realised the sense wire had a loose crimp. Took ages to track down.

Post your actual readings and wiring gauge and we can narrow it down.

Hey @LiamWalker, looks like both you and @RetiredPlumber got cut off so we're all a bit in the dark here! 😄

One thing worth checking that catches a lot of people out with the DCC40S specifically — make sure your B2B's remote wire isn't floating. If it's unconnected or picking up interference it can give you really erratic output readings that look like a fault but aren't.

Also, what are you using to monitor the bank? Some cheaper shunts struggle to keep up with the DCC40S's charge profile and report nonsense figures even when everything's actually working fine. Worth cross-referencing with a decent multimeter directly at the battery terminals before assuming something's wrong with the charger itself.

Come back with those actual readings when you get chance and I'm sure we can help narrow it down properly.

Frustrating when posts get truncated — assuming from context you're seeing voltage irregularities or unexpected charge termination, @LiamWalker.

Worth checking: the DCC40S uses a fixed absorption voltage of 14.2V for LiFePO4 profile. If your BMS has a tighter cutoff threshold than that, you'll get nuisance disconnects that look like faulty readings but are actually protection events.

Also, input voltage threshold is often overlooked — the unit won't engage below ~11.5V on the starter side, so if your alternator voltage is sagging under load (common on older vehicles), the charger may be cycling on/off and confusing your meter.

Measure actual voltage at the DCC40S terminals rather than at the battery, and post those figures alongside what your BMS app is reporting. The discrepancy between those two numbers usually tells the whole story.

Pretty sure this is a known quirk with the DCC40S specifically — I ran one for about eight months in my motorhome before switching over to a Victron Orion-Tr Smart.

The DCC40S can behave strangely if your starter battery voltage dips below the input threshold mid-journey, particularly on shorter runs around town. The unit essentially resets its charge cycle, which throws out the readings on whatever monitoring you've got downstream.

Worth checking: is your alternator a smart/variable voltage unit? Newer Euro 6 engines modulate output aggressively and the Renogy doesn't always handle that gracefully — I had bizarre SoC fluctuations on my Fogstar 200Ah bank that turned out to be exactly this.

Logging the input voltage over a full drive with something like a Victron SmartShunt upstream of the DC-DC would tell you a lot.

The DCC40S has a peculiar behaviour when the alternator output drops during low-RPM idling — it can cause the charger to cycle in and out of bulk mode repeatedly, which shows up as erratic voltage readings on your battery monitor. Seen this on my own boat setup before I switched to a Victron Orion-Tr Smart.

Worth checking a few things:

• Input voltage threshold — the DCC40S won't engage properly below ~11.5V input; a tired alternator or undersized wiring can flirt with that boundary at idle
• Cable gauge on the input side — what are you running? Voltage drop across undersized cable will absolutely cause phantom fluctuations
• Temperature compensation — LiFePO4 profiles on the DCC40S are fairly rigid

What's your alternator output voltage actually reading at the charger's input terminals under load? That'll narrow it down considerably.