Anyone else struggling to get accurate readings from cheap MPPT controllers in winter?

My Renogy Rover 40A has been giving me grief lately — the SOC display is all over the place when temps drop below 5°C. Showing 80% then suddenly dropping to 55% within an hour of light load. No big draws, just a 12V fridge ticking over.

Running two 200W panels into a 200Ah Fogstar Drift lithium. Setup's been solid all summer but something's clearly off now the days are short and the sun angle is low. Wondering if the controller just can't handle the lower Voc or if the temp compensation is doing something weird.

Has anyone actually got reliable winter readings from budget MPPT kit, or is it worth stepping up to Victron at this point? The SmartSolar 100/30 is tempting but feels steep for what it is.

@BatterySam the Rover's SOC reading is essentially useless in my experience — it's just doing a rough voltage-based calculation with no temperature compensation worth speaking of. Mine did exactly the same last January parked up in Scotland.

The real problem is that battery voltage behaves very differently at low temps, so a voltage-to-SOC lookup table that works fine in summer becomes genuinely misleading once you're below 5°C.

What sorted it for me was adding a Victron SmartShunt — does proper coulomb counting and you get accurate SOC regardless of temperature. Yes it's another £70-odd, but the Renogy display is basically decorative at that point.

If budget's tight, at minimum make sure you've got the temperature sensor actually connected and set your battery type correctly in the Rover settings — a lot of people skip that.

@BatterySam @SteveWhite70 is spot on about the voltage-based calc issue, and cold temps make it far worse — internal resistance rises significantly in lead-acid (and even LiFePO4 to a degree), so resting voltage becomes a really unreliable SOC indicator. The Rover doesn't apply any meaningful temperature compensation to its SOC algorithm from what I've seen.

Honestly your best bet is a dedicated battery monitor like a Victron BMV-712 or even a cheaper Votronic unit — they use coulomb counting rather than voltage guessing, which is far more accurate regardless of temperature. You'll need a shunt wired in, but it's not a complicated job.

The MPPT side of the Rover is actually decent enough, just don't trust anything on that SOC display, especially in winter. Treat it as decorative! 😄

@BatterySam @SteveWhite70 is spot on about the voltage-based calc issue, but worth adding that cold temps make this even worse because battery internal resistance rises significantly, so voltage readings become even less reliable as a SOC indicator. Your battery's resting voltage at 5°C looks completely different to what it does at 20°C, and most cheap controllers don't have any temperature compensation built into their SOC algorithm — they're just using a fixed lookup table calibrated for warmer conditions. If you haven't already, fitting a proper battery monitor like a Victron BMV-712 or even a cheaper Renogy one that does coulomb counting will make a massive difference. Proper shunt-based monitoring tracks actual current in and out rather than guessing from voltage, so temperature swings stop throwing everything off. Night and day difference in winter reliability.