Anyone else noticed their Victron MPPT reading dropping in this cold snap?

Been watching my BMV-712 and SmartSolar 100/30 obsessively this week (as you do) and something's caught my attention. Panel voltage on my 2x 175W Renogy monos is actually higher than usual — panels are putting out around 48V open circuit vs the usual ~44V in milder weather. That's the temperature coefficient doing its thing, fair enough.

But my actual harvest is way down. I'm on the narrowboat so shading from trees and low sun angle is always a factor in December, but even on the clearer days I'm barely cracking 200Wh. July I'd see 800Wh on a decent day with the same setup.

Got 200Ah of Fogstar Drift LiFePO4 so capacity isn't the issue — it's just not filling up. I've had to run the engine alternator twice this week which I hate doing.

Is anyone else seeing this on their setups right now, or is it mainly a narrowboat/mooring-angle problem? Curious whether folk with fixed roof installs on tiny houses or static setups are doing any better for December harvest.

@WattVicky yep totally normal — cold panels are actually more efficient at producing voltage. The Voc on most panels is rated at 25°C so anything below that and you get a bonus bump. Seen mine hit proper silly numbers on a clear frosty morning, way above the rated spec.

Worth keeping an eye on if you're close to your MPPT's max input voltage though — on a really cold clear day those two 175W Renogys in series could spike higher than you'd expect. Check the datasheet for temperature coefficient and do the maths for your coldest likely temp.

The flip side is amps stay low when the sun's weak so your actual harvest is still rubbish this time of year regardless 😅

IslandWanderer | Posts: 847 | Location: Outer Hebrides

@WattVicky worth checking your temperature coefficient spec on those Renogy panels — it'll be in the datasheet, usually something like -0.30% to -0.35% per °C below STC (25°C). So on a crisp -5°C morning you're looking at a meaningful voltage bump above rated Voc.

The flip side I've noticed up here is that whilst the voltage is cracking, if you've got heavy frost on the panels you'll lose amps until it clears — so your wattage figure can still look disappointing despite the high voltage reading.

Make sure your MPPT charge profile accounts for those elevated voltages too — worth double-checking you're comfortably within the 100V input limit on your 100/30, especially with panels wired in series. Stay warm! 🥶

WattBaz | Posts: 1,203 | Location: Peak District

@WattVicky What @CornishCamper is getting at is spot on — but the flip side worth mentioning is that whilst your voltage climbs in the cold, your actual current output won't necessarily follow suit. So don't be surprised if your watt-hours for the day are still disappointing despite those tempting voltage figures. Short days and low sun angles are doing the real damage this time of year. Also worth double-checking that your MPPT's absorption and float voltages are set correctly for your battery chemistry — some folks forget that certain battery types benefit from slightly adjusted charge parameters in winter. What batteries are you running? That'd help narrow down whether your controller settings are working in your favour or against you.

OddJobBob22 | Posts: 34 | Location: Array

Slightly different angle — has anyone checked whether the charge current is actually dropping despite the higher panel voltage? I noticed on my cabin setup last winter that even with elevated Voc readings, the MPPT was pulling less overall wattage because of low sun angle and shorter daylight hours. So the numbers can look encouraging on the voltage side but the daily kWh totals told a different story. Are you running the VictronConnect app to see your history graphs? The weekly overview is really useful for spotting whether this is genuinely affecting your charge cycles or just a cosmetic quirk in the live readings.

CrafterBuild | Posts: 312 | Location: Array

Worth adding — higher panel voltage in cold is great but watch your Voc doesn't creep above your MPPT's input limit. On my cabin setup I've got 3 panels in series and on a proper frosty morning the Victron SmartSolar logs show voltage nudging uncomfortably close to the 100V ceiling.

Learned that the hard way last January 😅

If you're running Renogy monos, dig out the datasheet and check the temperature coefficient for Voc — typically around -0.30%/°C. A few degrees below the STC rating of 25°C adds up fast, especially with multiple panels in series.

Victron's MPPT calculator tool is useful for checking worst-case Voc before you fry anything expensive.

SmartSolar_Master | Posts: 2,847 | Location: Narrowboat, various canals

@OddJobBob22 — good point about charge curves, and worth a separate thread honestly.

@WattVicky welcome to the obsessive BMV-712 watching club, you'll fit right in here 😄

One thing nobody's mentioned yet — if you're seeing higher Voc but lower harvest overall, check your absorption/float times in VictronConnect. Cold batteries need longer absorption to actually accept the charge properly. I noticed this on my boat last January; panels were pumping out beautifully but the Lithium (Fogstar Drift) was sitting in float prematurely because the voltage threshold was hit before the cells were properly topped.

Worth logging a week's worth of yield data on VRM if you've got it connected — patterns become really obvious once you can see the graphs properly.