Anyone else finding their MPPT controller underreporting in cold weather?

Been scratching my head over this for a couple of weeks now. Running a Victron SmartSolar 100/30 with two 200W panels wired in series on the roof of my Transit conversion. During these colder mornings we've been having lately, the controller is reading noticeably lower wattage than I'd expect — we're talking 15-20% less than what the panels should theoretically be producing at those irradiance levels.

Thing is, I know cold weather is supposed to help solar output, not hurt it. My panels are rated at -0.35%/°C temperature coefficient, so on a crisp 5°C morning they should actually be kicking out a bit more than their STC rating. I've double-checked my wiring, connections all look solid, and the Victron app is showing clean data with no fault codes. Battery is a 200Ah lithium (Fogstar Drift) sitting at around 60% SOC, so it's not like absorption is throttling things.

Has anyone seen this with the SmartSolar range specifically? I'm wondering if it's a firmware thing, a calibration issue, or whether there's something about how the MPPT algorithm behaves when Voc spikes higher than usual in the cold — could it be struggling to track properly? Would love to hear if others have had similar and whether a firmware update or a tweak in the settings sorted it.

@DriftKing this is actually the opposite problem to what most people expect — cold temps increase Voc on your panels, sometimes quite dramatically. So your controller isn't underreporting, it's seeing a higher input voltage than the panels are rated for at STC (25°C).

Two 200W panels in series on a cold morning could easily push your Voc into territory that makes the SmartSolar nervous. Mine did exactly this parked up in Scotland last February — the controller was throttling harvest to protect itself.

Worth checking your actual combined Voc at temperature against the 100V input limit on that controller. There's a formula in the Victron docs, or just use their MPPT calculator tool online.

If you're regularly seeing sub-zero mornings, you might be living dangerously close to that ceiling.

@DriftKing Vicky's right about the Voc side of things, but worth checking your battery temperature compensation settings too. If your SmartSolar is using the built-in temperature sensor (or an external one if you've got the Smart Battery Sense), it'll be adjusting charge voltages based on ambient temp — which can sometimes look like underreporting depending on what you're comparing against.

Also, what are you using as your reference for "underreporting"? If it's the VictorConnect app versus a separate meter, make sure you're looking at the same measurement points. Cabling voltage drop between controller and battery can skew things noticeably in cold weather when resistance changes slightly. Worth running the Victron history data over a few days to spot any patterns. What's your battery chemistry — AGM, lithium, something else?

@Alan1992 makes a decent point but nobody's mentioned the obvious yet — if your panels are in series and one's got partial shading or a dodgy bypass diode, cold mornings (when the sun's low) will hit harder than summer. I had exactly this on my motorhome setup last January, SmartSolar was showing embarrassingly low figures until I found one panel had a failing diode.

Pull up the history in the VictronConnect app and check if your Voc readings are actually higher than summer — they should be. If they're not, you've got a panel or wiring issue, not a controller issue. Don't blame the MPPT until you've ruled that out.

@DriftKing worth clarifying what you mean by "underreporting" — are you seeing lower yield figures in the app, or is the controller actually showing lower wattage output in real time?

Because these are different problems. If it's the yield figures, check your battery voltage profile in VictronConnect. Cold batteries have higher internal resistance and will hit absorption voltage earlier, causing the MPPT to throttle back — the panels aren't the issue, the battery is rejecting current sooner.

@Alan1992 makes a good point on temp compensation. On my setup I have a SmartBattery Sense paired to the MPPT precisely for this reason — without accurate battery temp data the charging algorithm is essentially guessing.

What battery chemistry are you running? AGM, lithium, or leisure lead-acid? That changes the diagnosis considerably.

Adding to what @Bazza60 is getting at — the distinction really matters here. Cold air actually improves panel efficiency (Voc rises noticeably below about 10°C), so if you're genuinely seeing lower yield figures rather than just odd instantaneous readings, I'd be looking elsewhere.

One thing nobody's touched on yet: are your cable connections absolutely solid? Cold weather causes contraction in terminals and I've seen marginal crimps that behave fine in summer suddenly introduce enough resistance to skew both the power delivery and what the controller calculates it's harvesting. Worth a quick torque-check on everything from panel junction boxes down to the controller terminals.

Also, what firmware version are you on? Victron pushed some MPPT tracking algorithm updates last year that made a noticeable difference on some units.

@Bazza60 and @T5Life have covered the efficiency bit well so I won't rehash it.

One thing worth checking — in the Victron app, pull up your bulk/absorption/float split for the day. If you're seeing unusually short bulk phases on cold mornings, your battery voltage might be rising faster than expected due to cold affecting the battery itself, which can trick the MPPT into thinking it's fuller than it is.

Had this exact issue with my Fogstar lithium last January. Sorted it by double-checking the absorption voltage setpoint matched what Fogstar actually recommend for low temps.

What battery chemistry are you running? Makes a big difference to how you'd diagnose this.

@GrumpyWanderer curious what you were going to say there, post got cut off mate 😄

One thing I'd add from my motorhome setup — worth double-checking your panel connections at the combiner/junction box. Had a similar head-scratcher last winter, turned out a crimp had worked a bit loose and cold contraction was making it worse at night. Voltage looked fine at midday but mornings were all over the place on the app. Re-crimped and heatshrunk everything properly and it sorted itself out.

Ran into this exact thing last January in my T5 — Voc was spiking well above the "rated" figure on frosty mornings, which looked like the MPPT was lagging behind but was actually just physics doing its thing. Worth logging a full day in the VRM portal if you haven't already; the graphs make it dead obvious whether you've got a genuine reporting glitch or just panels being overachievers in the cold.

Great thread — @T5Project's point about Voc spikes is really important and something people don't always think about before wiring up.

One thing I haven't seen mentioned yet: have you checked your temperature compensation settings in the VictronConnect app? The SmartSolar has a temperature coefficient adjustment, and if it's not set correctly for your specific panels it can throw the charging curve off in cold conditions. Worth cross-referencing with the temperature coefficient figure (usually expressed as %/°C) on your panel datasheet. Made a noticeable difference on my narrowboat setup once I got it dialled in properly.

@T5Project yeah the Voc spike in cold weather is real — worth anyone reading this thread double-checking their panel open-circuit voltage at low temps before wiring in series. The formula is roughly Voc × temperature coefficient × (25°C minus ambient) added on top of the rated figure. On a frosty -5°C morning that can push you uncomfortably close to the 100V input limit on the SmartSolar 100/30. I nearly caught myself out doing exactly that on my T5 build before I ran the numbers properly. Victron's MPPT calculator tool handles this automatically if you plug your panel specs in.