I've been running a fairly basic setup in my self-build camper for about 18 months now — 200W of panels through a Victron BlueSolar PWM 20A into a pair of 110Ah AGM leisure batteries wired in parallel. The system works well enough day to day, but I genuinely cannot get a reliable state of charge reading out of it. The controller's display bounces around all over the place, and I've had the batteries sitting at what it reckons is 80% only to find them struggling to run my compressor fridge through the night.
I know PWM controllers aren't exactly famous for precise SOC tracking, and I've been wondering whether a proper battery monitor like the Victron BMV-712 would sort me out, or whether the root issue is that PWM charging itself isn't fully topping the AGMs up — meaning the SOC is actually legitimately lower than expected rather than just being misreported. From what I've read, AGMs really want an MPPT controller to hit that proper absorption stage, especially on cloudy UK days when you're barely scraping 60–70W through the panels.
Has anyone made the switch from PWM to MPPT mid-build and noticed a genuine difference in how well the batteries stay topped up? Or would fitting a BMV first at least tell me whether I've got a measurement problem or an actual charging problem? Torn between spending £50-odd on a monitor or going straight to a Renogy 40A MPPT for around £120.