Anyone else finding MPPT controllers wildly optimistic about their battery SOC readings?

So I've got a Victron SmartSolar 100/30 paired with a 200Ah LiFePO4 (Fogstar Drift) in my Transit conversion, and the SOC on the VictronConnect app seems to drift all over the shop. I'll come back after a cloudy day where the panels barely pushed 10W and the app is telling me I'm at 87% — doesn't match up at all with what my Bluetooth BMS is reporting (closer to 65%). Starting to wonder if I'm missing something obvious in the settings.

I've got two 175W panels wired in series giving me around 44Voc, so the controller itself seems to be working fine — pulling decent amps on a good day, hitting absorption without any issues. It's purely the SOC figure I don't trust. I've read that MPPT controllers aren't really designed to be your primary SOC monitor and that you need a dedicated shunt like the SmartShunt or a BMV-712 for anything reliable. Is that genuinely the consensus here, or have people found ways to get the MPPT reading accurate enough on its own?

Would love to know what others are running for SOC monitoring, especially anyone with LiFePO4 where that flat discharge curve makes voltage-based guessing basically useless anyway.

@ShaunKelly87 MPPT controllers are brilliant at chasing the sun and absolutely terrible at counting electrons — use a dedicated shunt like the Victron SmartShunt and suddenly your SOC goes from "creative fiction" to "mildly accurate novel."

LisaHunt | 847 posts | ⚡ Solar Obsessive

@ShaunKelly87 @MultiPlusFan is spot on — your SmartSolar really shouldn't be your SOC source of truth. I've got a similar setup in my Sprinter and the game-changer was adding a Victron SmartShunt. It talks directly to VictronConnect alongside your MPPT and gives you proper coulomb counting rather than voltage-based guesswork. The Fogstar Drift is actually a great battery but LiFePO4's flat discharge curve makes voltage readings almost meaningless across most of the range anyway. The SmartShunt is around £60-70 and honestly transforms how confident you feel about your remaining capacity. Just make sure you configure the battery capacity and Peukert exponent correctly in the settings, and let it do a full charge cycle before trusting it completely. Makes a massive difference day-to-day.

TracyGraham64 | 312 posts | 🔋 Van Life Convert

@ShaunKelly87 Had almost identical issues with my Fogstar Drift until I added a Victron SmartShunt — absolute game changer. The MPPT is just estimating SOC based on voltage which is notoriously unreliable with LiFePO4 because of that incredibly flat discharge curve. The SmartShunt does proper coulomb counting and syncs beautifully through the VE.Smart network with your existing SmartSolar. Everything shows up in one place on VictronConnect too. Worth every penny of the ~£60 it costs. One thing to watch — make sure you configure the battery capacity correctly in the settings, and let it complete a full charge cycle before trusting the readings. Mine took a few cycles to properly calibrate itself. @MultiPlusFan and @LisaHunt have clearly pointed you in the right direction already! 👍

Gill1982 | 203 posts | 🏕️ Shepherd's Hut & Cabin

@ShaunKelly87 The SmartSolar genuinely isn't designed to track SOC accurately — it's a charge controller, full stop. What sorted mine completely was adding a Victron BMV-712. It sits between the battery and everything else, counts coulombs properly, and the SOC figures are night and day compared to what the MPPT was estimating.

Worth setting the Peukert exponent correctly for LiFePO4 too (closer to 1.05 rather than the default) — makes a noticeable difference to accuracy on the BMV.

The VictronConnect integration means everything talks together nicely on one app as well, which is handy if you're checking things remotely like I do with the hut.

GemmaFisher82 | 156 posts | 🚐 Transit Van Dweller

@ShaunKelly87 Worth mentioning that LiFePO4 has an incredibly flat voltage curve through most of its discharge range, which makes voltage-based SOC estimation almost meaningless for the SmartSolar. It'll look rock solid at 13.2V whether you're at 80% or 30%.

Honestly your best bet is adding a proper battery monitor like the Victron BMV-712 or a Victron SmartShunt — they track actual coulombs in and out rather than guessing from voltage. The SmartShunt is slightly cheaper and does the same job. Once you've got one paired up in the VictronConnect app alongside your SmartSolar it makes a massive difference.

Also double-check your Fogstar's charge profile settings — incorrect absorption voltage can throw things off considerably too. @TracyGraham64 might have thoughts on that from their experience with the same battery.

PhilFox99 | 847 posts | ⚡ Off-Grid Narrowboat

@ShaunKelly87 The root issue is that your SmartSolar is a charge controller, not a dedicated battery monitor — it's essentially guessing SOC from voltage alone, and as @GemmaFisher82 is hinting at, LiFePO4's flat discharge curve makes voltage-based estimation almost meaningless across most of the usable range.

Genuine fix: grab a Victron SmartShunt and wire it in as your battery monitor. It tracks actual coulombs in and out (with efficiency factor applied), so the SOC figure becomes genuinely reliable. Bonus is it integrates beautifully with VictronConnect alongside your existing SmartSolar.

Make sure you set the battery capacity correctly and let it run a proper full charge cycle to synchronise before trusting the readings. Night and day difference on my setup.

WingAndPrayer88 | 312 posts | 🚐 Transit Van Dweller

@ShaunKelly87 Adding to what @PhilFox99 said — if you're serious about accurate SOC, a dedicated battery monitor like the Victron BMV-712 or SmartShunt is really the proper solution. It uses Coulomb counting rather than just voltage estimation, so it tracks actual charge in and out. Pairs beautifully with your existing Victron kit via the VE.Smart network too. I've got the SmartShunt on my Transit setup with a similar Fogstar battery and the difference in accuracy is night and day. Worth every penny honestly.