Anyone else finding their MPPT controller underreading in cold weather?

Noticed something odd this week — my Victron SmartSolar 100/30 has been showing panel voltage about 8-10V lower than what my multimeter reads directly at the array terminals. We've had a few frosty mornings here in the Peak District, overnight temps dropping to around -3°C, and the discrepancy seems worst first thing before the panels warm up.

Running three 200W panels wired in series (Renogy mono, Voc 24.3V each so roughly 73V open circuit at STC). The controller is mounted inside the van body so it's staying reasonably warm, but the cable run from the roof is about 4 metres of 6mm² twin. I've checked the connections at both ends and they look solid — no corrosion, good torque on the terminals.

Has anyone seen this before? I'm wondering if it's a genuine voltage drop issue under even tiny load, a calibration thing with the Victron, or something to do with how the MPPT samples the input. Bit puzzled because I thought cold panels were supposed to increase Voc rather than drop it. Happy to share the VictronConnect screenshots if that helps anyone diagnose it.

@Chopper72 this is actually the opposite of what you might expect — cold temps increase Voc on crystalline panels, typically around +0.3-0.4V per °C below STC (25°C). So your multimeter reading higher than the controller is the suspicious part.

Worth checking:

• Cable voltage drop between array and controller terminals (measure under load, not open circuit)
• VE.Direct connection — if you're reading via the app rather than directly, there can be display lag
• What's your cable run length and cross-section?

On my setup (400W array, 6mm² cable over ~4m) I actually see Victron reporting higher than STC spec on cold bright mornings. The SmartSolar's internal measurements are generally reliable — I'd suspect your measurement methodology or a loose terminal before blaming the controller itself.

@Chopper72 actually the opposite problem is more common — cold panels produce higher Voc than rated, sometimes dangerously so. Worth double-checking which direction your discrepancy is running.

That said, 8-10V drop between array terminals and controller input is significant regardless of temperature. I'd be looking at:

• Cable losses — what cross-section are you running and over what distance?
• MC4 connector condition — corroded or poorly crimped connectors are notorious for this
• Fuse/isolator voltage drop — measure either side of every inline component

On my cabin setup I found a dodgy MC4 that was dropping nearly 6V under load. Looked perfect visually but the crimp was partial. Replaced it and everything came back into line immediately.

The Victron measures at its terminals, not at your panels — so the discrepancy is almost certainly in the cable run rather than the controller itself.

Hey @Chopper72, just to add something slightly different to what @CrafterDream and @IslandVanLifer have touched on — have you checked your cable connections at the array end recently? Frost and temperature cycling can cause terminals to work loose or develop a bit of oxidation, which would show up as exactly that kind of voltage drop between the array and the controller. Also worth checking if you've got any MC4 connectors that might have moisture ingress — that's a sneaky one in winter. The discrepancy you're describing (8-10V) does seem quite significant for just temperature effects alone, so I'd be looking at resistance losses in the wiring first before assuming it's a controller issue. What's the cable run length between your panels and the MPPT?

Hey @Chopper72, worth checking your cable run from the array to the controller. A 8-10V drop like that sounds suspiciously like a voltage drop issue in the wiring rather than anything to do with the cold itself. What gauge cable are you using and how long is the run? Undersized cable combined with higher current output (which you do get in cold, bright conditions) can cause exactly this. Also worth checking all your connectors and MC4 joints — a loose or corroded connection can drop voltage noticeably. Grab your multimeter and measure at the controller input terminals rather than up at the array and see if the discrepancy narrows. That'll help isolate whether it's a wiring/connection issue or something else going on. 🔧

@VanHolly makes a fair point about cable losses, but 8-10V feels steep even for a dodgy run — worth ruling out first though.

Something I haven't seen mentioned: when exactly are you taking the readings? On the boat I've caught myself comparing multimeter Voc (panels at open circuit, no load) against what the MPPT shows while it's actively tracking. Those two figures will never match — the controller is pulling current and seeing a lower operating voltage by design. That's not a fault, that's just MPPT doing its job.

Try this: disconnect the MPPT, let the panels sit idle for two minutes in steady light, then measure at both ends of your cable run simultaneously if you can manage it. That'll tell you whether it's the controller, the cabling, or just a comparison methodology issue.

@VanHolly yeah I did wonder about cable losses but I ran the numbers — it's a fairly short run, 6mm² twin core, maybe 4 metres. Voltage drop at typical current shouldn't be anywhere near 8-10V.

What's interesting is it only happens on cold clear mornings. By midday when panels have warmed up a bit the readings converge almost exactly.

Starting to think it might be a measurement timing thing — Victron sampling the voltage at a slightly different moment than when I'm poking the multimeter in? Or possibly the SmartSolar's internal compensation doing something weird.

Going to log it properly over the next few days using the VictronConnect app history and compare against manual readings at the same timestamps. Will report back.

Good shout from @Chopper72 on ruling out the cable losses first. One thing worth considering here — in cold weather your panels will actually produce higher voltage than their rated Voc, not lower. So if anything, the discrepancy should go the other way. That makes me wonder whether you're measuring at the array under load vs open circuit? The MPPT will be pulling the panels down to their working voltage (Vmp), whereas your multimeter reading at the terminals with nothing else connected gives you Voc. That'd easily account for an 8-10V difference and is completely normal behaviour.

Great point from @ExJoiner6 — and just to add to that thread, cold temps actually push Voc up on panels, not down, so if anything your multimeter should be reading higher than normal, not lower. That makes the 8-10V discrepancy even more interesting. @Chopper72 worth checking whether your Victron has the temperature compensation sensor connected — if it's trying to compensate based on an assumed temperature rather than an actual reading, it can do odd things to the display values. Also double-check you're comparing like-for-like: is the multimeter reading taken at the same moment as the controller display?