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

I've got a Victron SmartSolar 100/30 paired with a 200Ah LiFePO4 (Fogstar Drift) in my van build, and the SOC readings on the VictronConnect app have been all over the place lately. It'll show 97% after barely an hour of sun on a cloudy March morning, then drop to 72% the moment I switch the kettle on. Doesn't seem to matter how much I tweak the absorption and float voltages — it just feels like it's guessing half the time.

From what I've read, MPPT controllers aren't really designed to track SOC accurately on their own — you're supposed to pair them with a proper battery monitor like the BMV-712 or a SmartShunt. I've been putting off buying one because I figured the app would be "good enough," but clearly it isn't. Has anyone else run into this before switching to a dedicated shunt-based monitor?

Wondering if the Fogstar BMS is partly to blame as well — it does its own balancing and protection, so maybe the two are giving conflicting signals somehow. Would love to know what setups people are running with LiFePO4 specifically, and whether the SmartShunt actually sorted things out for you.

@ValleyAmy the MPPT isn't actually tracking SOC properly — it's estimating based on voltage, which is notoriously unreliable with LiFePO4 because of that flat discharge curve.

The fix is a dedicated battery monitor like the Victron BMV-712 or SmartShunt. These measure actual current in and out (coulomb counting) and give you a genuinely accurate SOC.

Few questions before you chase anything else:

• Have you set the charge parameters specifically for LiFePO4 in VictronConnect? Absorption voltage should be around 14.2V, not the default lead-acid settings
• Is the Fogstar Drift communicating via any BMS integration, or is Victron working blind?

I've got a SmartShunt on my narrowboat setup and the difference in SOC accuracy versus trusting the MPPT alone was night and day. Genuinely don't bother trying to diagnose anything without one.

RetiredNurse96 | 847 posts | ⭐ Trusted Member

@ValleyAmy I had exactly this with my Renogy setup before I switched to a dedicated battery monitor. The MPPT is really doing two jobs at once and SOC estimation isn't its strong suit - it's primarily a charge controller.

The Fogstar Drift should have decent built-in BMS monitoring, so have you tried checking whether Fogstar publish a companion app or Bluetooth readout for the battery itself? That'll give you a second opinion straight from the horse's mouth.

Honestly though, the proper fix for most van builds is a standalone coulomb counter like a Victron BMV-712 or a Smartshunt. They track actual amp-hours in and out rather than guessing from voltage, which makes an enormous difference with LiFePO4 given how flat the discharge curve is. Worth every penny in my experience.

@ExFirefighter is right about the voltage thing — MPPT controllers are genuinely rubbish at SOC on their own.

What actually fixed this for me on the narrowboat was adding a Victron BMV-712. Proper coulomb counting rather than guessing from voltage. Night and day difference, my SmartSolar readings were almost meaningless before that.

LiFePO4 makes it worse too — that flat discharge curve means voltage barely moves across a huge SOC range, so voltage-based estimates are basically useless.

The BMV talks to VictronConnect alongside your SmartSolar as well, so you get everything in one app. Worth every penny imo.

Barry1962 | 1,203 posts | ⭐ Trusted Member

@ValleyAmy the proper fix here is a dedicated battery monitor with a shunt — something like the Victron BMV-712 or the SmartShunt. These measure actual current flowing in and out via coulomb counting, which is miles more accurate than voltage-based guesswork.

The good news is both devices integrate directly with VictronConnect, so you'd see genuinely reliable SOC data alongside your existing solar info. Dead easy to set up if you're already in the Victron ecosystem.

One thing worth checking in the meantime — make sure your battery profile settings in VictronConnect actually match the Fogstar Drift's specs. Incorrect absorption/float voltages will make the voltage-based estimates even more unreliable than they already are. Fogstar publish their recommended charge parameters on their website, worth a quick look.

Foggy80 | 412 posts

@ValleyAmy worth checking whether your SmartSolar is actually syncing properly with the Drift's built-in BMS via VE.Smart networking — if it's not picking up the BMS data, it'll just be estimating SOC from voltage curves which is notoriously unreliable with LiFePO4 due to how flat the discharge curve is.

Also, has it been doing a proper absorption/float cycle recently? The controller resets SOC to 100% when it hits float, so if you're only getting partial charges (cloudy days, short drives etc.) it never gets a clean reference point and the drift just compounds over time.

As @Barry1962 says, a shunt-based monitor is the real answer, but sorting the charging profile might help in the meantime.

EllaDixon71 | 287 posts

@ValleyAmy I had almost identical issues with my Drift until I realised the SmartSolar wasn't receiving the battery's BMS data properly. Have you enabled the "Has DC system" option in VictronConnect? It made a noticeable difference for me. Also worth double-checking your battery capacity is set correctly in the charger settings — mine had defaulted to something daft like 50Ah.

One other thing: the Drift's BMS does broadcast SOC via Bluetooth, so if you've got a Cerbo GX or even a Raspberry Pi running Venus OS, you can pull that data directly rather than relying on the MPPT's voltage-based guesswork. Bit of a faff to set up but honestly transformed how accurate my readings are. What firmware version is your SmartSolar on? There were some SOC-related improvements in recent updates.