Anyone else finding MPPT controllers wildly optimistic about battery state of charge?

I've got a Victron SmartSolar 100/30 paired with a 200Ah lithium (LiFePO4) battery from Fogstar, and honestly the SOC readings through the VictronConnect app are all over the place. Yesterday it was showing 97% charged after a fairly cloudy morning, then dropped to 71% the moment I put the kettle on. Doesn't inspire much confidence when you're trying to plan whether you've got enough juice for the evening.

From what I've read, MPPT controllers aren't really designed to track SOC accurately — that's more the job of a dedicated battery monitor like the Victron BMV-712 or a Smartshunt. My setup doesn't have one yet and I'm wondering if that's the missing piece. The Fogstar battery does have a built-in BMS with Bluetooth that gives its own SOC reading, but that seems to disagree with the Victron figure half the time as well.

Has anyone else gone down this rabbit hole? Did adding a proper shunt-based monitor actually sort it out, or is there something else I should be looking at first — wiring, settings, calibration? Keen to hear what's worked for people before I spend another £80-odd on more kit.

@EwanScott mate, MPPT controllers aren't actually battery monitors — they're just guessing based on voltage, which is a bit like diagnosing a patient by looking at their shoes.

Get a proper Victron SmartShunt wired in and your SOC readings will suddenly make a lot more sense — mine went from "confidently wrong" to "actually useful" overnight.

LiFePO4 flat voltage curve makes voltage-based SOC estimation basically witchcraft anyway, so no amount of tweaking your charge parameters will fix that fundamental problem.

@AnneButler is spot on about the voltage-guessing issue, and it's especially problematic with LiFePO4 because the discharge curve is so flat. Voltage tells you almost nothing useful across a huge chunk of the capacity range.

What you really want is a proper coulomb-counting battery monitor like the Victron BMV-712 or the SmartShunt — they track actual current in and out rather than inferring from voltage. Pair that with your SmartSolar via VE.Smart networking and the whole system talks to itself properly. Makes a massive difference.

Also worth checking your battery's charge parameters are set correctly in the controller — wrong absorption voltage settings will confuse the SOC estimate even further. Fogstar should have recommended settings somewhere on their site.

The SmartShunt is probably the most cost-effective fix here, around £90-100. Night and day difference in accuracy.

To add to what @AnneButler and @Ollie1981 are getting at — the proper fix here is a dedicated battery monitor like the Victron BMV-712 or the SmartShunt. These track actual current in and out using a shunt resistor, so they're doing proper coulomb counting rather than guessing from voltage.

With LiFePO4 the voltage curve is so flat between about 20-90% SOC that voltage-based estimates are practically meaningless in that range. I had exactly this problem with my Fogstar 200Ah before I added a SmartShunt — night and day difference.

The BMV/SmartShunt also integrates with VictronConnect over Bluetooth, so everything sits in the same app. Worth every penny of the ~£60-70 for the SmartShunt. Once properly calibrated with the correct battery capacity and Peukert settings, the SOC readings become genuinely reliable.