Anyone else finding MPPT controllers wildly optimistic about their voltage readings in cold weather?

Running a Victron SmartSolar 100/30 on my static van in Scotland. Panel array is 2x 200W Renogy polys wired in series, so nominal 48V open circuit going into the controller. During the recent cold snap we had (temps dropping to around -4°C overnight), I was seeing Voc spikes nudging 52-53V on the Victron app first thing in the morning — which is obviously expected given the temperature coefficient, but the controller was still reporting "bulk charging" states that seemed oddly brief given the actual battery state.

The batteries are two 100Ah Fogstar Drift LiFePO4s in parallel, and I'm fairly confident they were sitting around 30-40% SOC after a couple of overcast days. Should have needed a solid bulk phase. Instead the SmartSolar was declaring absorption after maybe 20 minutes. I've triple-checked my absorption voltage settings (14.2V, which Fogstar recommend) and the tail current is set to 2A. Wondering if the temperature sensor is somehow skewing the algorithm — I've got the Victron temperature sensor dongle fitted.

Has anyone else seen this behaviour on Victron MPPT units during cold conditions, or is this more likely a quirk of parallel LiFePO4 cells not balancing properly and one cell group hitting the voltage threshold early? I'm also not ruling out that one of the Fogstar units might have a duff BMS — it's only been in service since last spring but the van sits unoccupied for weeks at a time.

@Compo yes, cold temps actually increase Voc on crystalline panels - it's not the controller lying, the panels genuinely are pushing higher voltage when it's cold. Around -0.3%/°C temperature coefficient is typical for poly panels, so on a -10°C morning vs the standard 25°C test condition you're looking at roughly 10% higher Voc than the spec sheet suggests.

Worth checking your maximum input voltage rating on that SmartSolar - the 100/30 means 100V absolute max. Two Renogy 200W polys in series could theoretically breach that on a hard Scottish frost if your panels have a high Voc to begin with.

Had the same concern wiring up my narrowboat array last winter. Ended up running a parallel config rather than series just to keep headroom.

@Barry1988 has the right of it on the physics. Worth knowing your actual numbers though — most poly panels run around -0.35%/°C temperature coefficient on Voc.

In my van conversion I learned this the hard way: on a proper cold Scottish morning (say -5°C versus the STC baseline of 25°C), that's a 30°C swing pushing Voc up roughly 10.5%. On your series string that could mean knocking on 53V instead of 48V.

The Victron SmartSolar handles this fine — it's rated well above nominal — but it's worth plugging your exact panel spec sheet into Victron's MPPT calculator. You want to confirm you're not creeping toward the absolute maximum input voltage on genuinely brutal nights.

Not the controller lying at all. Just physics being physics in the cold.

@Liam1998:

@VitoBuild was about to give you the coefficient figures I reckon! To add to what @Barry1988 said — worth checking your panel spec sheet for the temperature coefficient of Voc, usually around -0.30% to -0.36% per °C for most polys. So if you're sitting at say -5°C instead of the standard 25°C test condition, you're looking at a 30°C swing which could push your Voc up by roughly 9-10% above nominal. With two panels in series that adds up pretty noticeably.

Main thing I'd double-check @Compo is that your actual input voltage isn't creeping towards the controller's 100V maximum on a really sharp frost. Scotland's been brutal lately and early morning temps combined with bright low sun can catch people out. Victron's VictronConnect app will log your peak PV voltage so worth having a look back through the history.

@PaulJackson:

Good point from @Barry1988 and @Liam1998 — the physics checks out. What's worth flagging though, @Compo, is that in Scotland during a cold snap you can genuinely push dangerously close to your controller's maximum input voltage. Your 100/30 has a 100V Voc limit, and with two Renogy polys in series during sub-zero temps, you could be nudging that ceiling more than you'd expect. Worth pulling up the actual spec sheet for your specific panels and running the numbers at, say, -10°C using the temperature coefficient. If you're regularly seeing readings that concern you, the Victron Connect app will log your peak Voc figures historically which makes it dead easy to check. Might also be worth considering whether parallel wiring gives you more headroom if you're cutting it fine. Better to know now than to fry the controller mid-winter!

@MaxFrost:

Relevant question for me — I've got a similar setup on my boat and also use a Victron SmartSolar, though mine's the 75/15 for a smaller array. What I've noticed is the Victron app's voltage history logs are actually useful here — you can scroll back and see your morning pre-dawn resting voltage versus the spike at first connection when panels are cold.

Have you tried disconnecting the array overnight and manually measuring open-circuit voltage with a multimeter at the coldest part of the morning? That'd tell you whether the controller's reading is off, or whether the panel output genuinely is elevated due to temperature. On my boat in winter I've seen notably higher VOC than the spec sheet suggests, which tracks with what @Barry1988 and @Liam1998 are describing about cold weather physics.

Worth ruling out a genuine overvoltage situation before assuming it's just a display quirk.