Anyone else struggling to get accurate SOC readings from their MPPT controller on cloudy UK days?

I've been running a 400W panel setup on my Renogy Rover 40A MPPT for about eight months now, mostly parked up in Wales and the Scottish Borders. The controller's state-of-charge display has started doing my head in — it'll show 85% on a dull overcast morning, then suddenly jump to 72% when the sun breaks through for ten minutes and the panels actually start pushing some decent current. I'm running two 100Ah AGM batteries in parallel, so 200Ah total, and the wiring's all properly sized.

From what I've been reading, the issue is that most MPPT controllers estimate SOC using voltage alone when there's no charge current flowing, and on grey British days where you're getting a trickle charge all day, the voltage never really settles enough for an accurate reading. I did try recalibrating the battery parameters in the Renogy app — set the correct battery type, absorption voltage at 14.7V, float at 13.6V — but it's made sod all difference.

Has anyone found a proper fix for this, or is the only real solution to fit a dedicated battery monitor like a Victron BMV-712 with a shunt? Wondering if it's worth the £80-odd or whether I'm just expecting too much from the controller's built-in SOC estimation.

@PanelDai yes, this drives me mad on my boat setup. The MPPT controller's SOC is basically guessing based on voltage — it doesn't actually track charge flow in and out. On a grey Welsh day when panels are dribbling in tiny amounts, the resting voltage never properly settles, so the controller misreads it badly.

Honestly the only reliable fix I found was adding a dedicated battery monitor — I run a Victron BMV-712 now and it's a completely different world. It uses a shunt to measure actual amp-hours, so cloudy days don't throw it off at all.

Worth asking: what battery chemistry are you running? AGM voltage curves in particular behave oddly when partially charged, which makes controller SOC even less trustworthy in low-light conditions.

Good shout from @KellyRobinson about voltage-based guessing. Worth adding that the problem gets worse in winter because battery temperature affects resting voltage quite significantly — a cold battery in a Scottish Borders November will read lower than it actually is. If you're not already using a dedicated battery monitor with a proper shunt (Victron BMV-712 is the go-to recommendation here, though pricier options exist), that's really your only reliable fix. The MPPT's onboard SOC is always going to be approximate. Also worth checking whether your Renogy has the correct battery type programmed — wrong charge profile settings will throw the calibration off further. After eight months of use your batteries will also have drifted from factory specs somewhat, so a manual recalibration when you get a full sunny day and a proper full charge might help temporarily.