Anyone else struggling to get accurate SOC readings from a cheap PWM controller with AGM batteries?

I've got a fairly basic setup in my shed — 200W panel going through a Renogy Wanderer 30A PWM controller into two 110Ah AGM batteries wired in parallel. Been running it for about eight months now and the state of charge display on the controller seems to be all over the place. It'll show 100% after an hour of decent sun, then drop to 60% within ten minutes of running a 240V inverter with a modest load on it.

I know PWM isn't the gold standard and I've read that AGMs need a fairly precise charging profile to stay healthy, but I'm wondering if the SOC reading is just fundamentally unreliable on these cheaper units. I don't have a dedicated battery monitor — no Victron BMV or anything like that — so I'm basically flying blind beyond what the controller tells me. Resting voltage this morning after a full night was 12.5V across both batteries, which I think suggests they're not actually at 100% at all.

Has anyone found a practical workaround short of buying a proper shunt-based monitor? Would adding something like a Victron BMV-700 actually give me a reliable picture, or are there other options at a lower price point that people have had decent results with?

MountainJackie | 847 posts

@JimWilliams71 — looks like your message got cut off there, but I reckon I can guess where you're heading!

PWM controllers and AGM voltage readings are notoriously unreliable, especially under load or immediately after charging. The Wanderer uses voltage-based SOC estimation, which is already a rough approximation, but AGMs have a flatter discharge curve than flooded batteries, so the percentage readout can be wildly optimistic right up until you're nearly flat.

A couple of things worth trying: always check voltage readings with no load and after the batteries have rested for at least an hour — that'll give you a much truer picture. Also, Renogy's default battery type setting sometimes needs adjusting specifically for AGM.

Honestly though, if accurate SOC matters to you, a dedicated battery monitor like a Victron BMV-712 is worth every penny. Coulomb counting beats voltage guessing every time.