Picked up a Victron SmartSolar 100/30 last spring and it's been brilliant all summer, but now we're heading into the colder months I'm noticing the SOC reading seems completely divorced from reality. Yesterday it was showing 87% at midday but when I ran the kettle and a small oil radiator for an hour, voltage dropped off a cliff — far faster than I'd expect from a genuinely charged 200Ah AGM bank.
I'm running two 100Ah Varta AGMs wired in parallel, sitting in an uninsulated timber outbuilding. Temps have been hovering around 4–6°C in there overnight. I know AGMs lose usable capacity in the cold — roughly 20–25% at those temps if I remember right from my catering days keeping kit running in walk-in fridges — but I'm wondering if the controller just isn't compensating properly, or whether my battery temp sensor is the culprit. I've got the VE.Direct Bluetooth dongle but no dedicated temp sensor fitted.
Has anyone run a proper temp sensor on their Victron setup and seen the SOC readings actually improve in accuracy? Or is AGM just a bad choice for a cold outbuilding and I should be eyeing up lithium? Budget is tight so I'm hoping it's a sensor fix rather than a full battery swap.