Anyone else's MPPT controller lying about harvest numbers in winter?

My Victron SmartSolar 100/30 is telling me I pulled 800Wh yesterday but my Fogstar battery barely moved — went from 60% to 68% on a 200Ah 12V, which is closer to 192Wh if my maths isn't completely broken.

Running 2x 200W panels facing south-ish (slightly east, cheers previous owner), and I know December's grim, but an 800Wh claim on a grey Sheffield day smells like someone's been creative with the numbers.

Anyone else seeing this kind of discrepancy, or have I got a dodgy shunt somewhere making everything lie in slightly different directions at once?

@SaltyMechanic your maths looks right to me. Worth checking whether the controller's reported figure includes energy that went straight into loads during the day rather than just what landed in the battery. If you had anything running — fridge, phone chargers, whatever — that consumption gets harvested through the MPPT but never touches the battery SoC.

On my narrowboat I noticed the same apparent "discrepancy" until I realised my 12V fridge was pulling maybe 400Wh across the day straight from the panels via the controller. Battery barely moved but harvest numbers were legitimate.

Check your load output if you're running one, or stick a clamp meter on the battery positive for a day. Might not be the Victron lying at all.

@SaltyMechanic I've had exactly this with my SmartSolar 75/15 — worth checking whether the Victron is reporting cumulative yield rather than yesterday's figure, it's easy to accidentally read the wrong screen in the VictorConnect app. Also, has your battery's SoC been calibrated recently? My Fogstar Drift 100Ah reads wildly off in cold weather until it's done a proper full-cycle reset.

What's the temperature in your battery enclosure? LiFePO4 capacity drops noticeably below about 10°C, so 800Wh going in might genuinely only show as 192Wh usable if internal resistance is high and the BMS is throttling charge current.

@SaltyMechanic one thing worth checking that nobody's mentioned yet — what's your wiring like between the controller and the battery? Voltage drop across undersized or lengthy cable runs can cause the MPPT to "see" a higher state of charge at the battery terminals than actually exists, and the harvest figure it logs is based on what it output, not what the battery actually absorbed. Also, at low winter temperatures your Fogstar's usable capacity will be somewhat reduced, so percentages shift a bit oddly. Try logging the battery voltage directly at the terminals with a separate meter during charging and compare it to what VictronConnect is reporting — if there's a noticeable gap, your cabling's likely the culprit.