Anyone else finding MPPT controllers wildly optimistic with their SOC readings?

Been running a 200W panel setup on my Transit conversion for about eight months now — two 100W Renogy panels in series feeding into a Victron SmartSolar 100/20, with a 100Ah lithium (a Fogstar Drift). The Victron app is generally brilliant but I keep noticing the state of charge jumping around in ways that don't quite add up. After a decent sunny afternoon I'll be showing 97% SOC, but then by morning it's dropped to 71% even though I've only run a 12V compressor fridge and charged a laptop. That's roughly 26Ah consumed, which should be more like 74% on a 100Ah battery.

I've gone through the app settings a few times and I'm fairly confident the battery capacity and Peukert exponent are set correctly for lithium. Charge efficiency factor is at 99% as most people seem to recommend for LiFePO4. The absorption and float voltages are dialled in to what Fogstar actually specify (14.2V and 13.5V). So I'm not sure if the controller is just never genuinely reaching a full calibration cycle, or if there's something else going on.

Has anyone had similar with a Victron MPPT and a lithium bank? I'm wondering whether fitting a Victron BMV-712 shunt monitor separately would just give me a much more reliable picture, rather than relying on the controller's built-in SOC estimation. Keen to hear what others have done.

@CamperRachel the SmartSolar's SOC estimation is genuinely unreliable without a dedicated battery monitor — it's essentially inferring state from voltage curves and charge current, which is notoriously inaccurate with lithium's flat discharge profile.

The Victron BMV-712 or SmartShunt transforms this completely. Coulomb counting via a shunt gives you actual amp-hours in/out rather than voltage guesswork. On my van build I run a SmartShunt 500A feeding into VictronConnect — the SOC reading is night-and-day compared to what the MPPT alone was reporting.

Also worth checking your battery profile settings in the MPPT. If absorption/float voltages aren't dialled in precisely for the Fogstar Drift's spec sheet, the controller will misjudge when the battery is "full" and skew everything downstream.

Fogstar do publish recommended charge parameters — cross-reference those against what's currently programmed.

@PaulaFisher91 is on the right track but worth being specific about why — the SmartSolar estimates SOC purely from voltage, which is notoriously inaccurate with lithium because the discharge curve is so flat. You'll sit at "80%" for ages then drop off a cliff.

The fix is a BMV-712 or SmartShunt paired via VE.Smart networking — it does proper coulomb counting and the SmartSolar then defers to that reading instead of guessing from voltage.

I run a near-identical setup on my static (Fogstar Drift 200Ah, SmartSolar 100/30) and before I added the SmartShunt the readings were essentially fiction. Afterwards, rock solid.

One pedantic point: make sure your Peukert exponent is set correctly for lithium in the shunt settings — default is usually calibrated for lead-acid.

@BayJason makes a fair point about voltage-based estimation being the root cause, but there's another layer to this story that catches people out.

I ran a similar setup for about six months thinking my Fogstar was perpetually sitting at 85-90% — turned out my absorption voltage was slightly misconfigured, so the controller kept thinking it had reached full charge without actually completing the cycle properly. The SOC counter resets on what it believes is a full charge event, which then cascades every subsequent reading into rubbish territory.

The fix that actually worked: a Victron SmartShunt wired directly at the battery negative. Coulomb counting rather than voltage inference. Within a week I had readings I could genuinely trust for planning overnight loads.

The SmartSolar is excellent kit, but it's a charge controller, not a battery monitor — treating it as both is where the trouble starts.

Really interested in this thread — I've got a similar setup on my narrowboat and the SOC drift drives me mad, especially when I'm relying on it for emergency backup power and need accurate readings before shore power becomes unavailable.

@TorJake curious what that "other layer" is — are you talking about temperature affecting the voltage-to-SOC curve? Because I've noticed my SmartSolar reads noticeably differently in winter mooring conditions vs summer.

Would a Victron SmartShunt paired alongside the MPPT actually solve this properly, or is it still making assumptions? Anyone running that combination on a boat where the battery bank sits in a cold bilge area?

@CamperRachel I had exactly this on my shed setup before I wired in a proper battery monitor. The SmartSolar's built-in SOC is honestly just a rough guide — what transformed things for me was adding a Victron BMV-712. It tracks actual current in and out rather than guessing from voltage, so the readings become genuinely trustworthy. Your Fogstar Drift should play nicely with it too since lithium's flat discharge curve makes pure voltage-based estimation particularly unreliable, as @TorJake is hinting at. Takes about an afternoon to fit and calibrate properly. Night and day difference for knowing where you actually stand, especially if you're wild camping and can't afford nasty surprises. Worth every penny of the ~£70 secondhand on eBay.