Anyone else find their Fogstar Drift 100Ah randomly dropping to 80% SOC overnight with zero load?

Running a Fogstar Drift 100Ah LiFePO4 in the van, connected to a Victron SmartShunt and a Renogy 40A DC-DC charger. Everything's wired up properly, fused correctly, and the BMS comms look clean in the Victron app — no alarms, no funny business.

But every few nights, completely unpredictably, I'll wake up to find it's sat at 79-81% SOC despite having zero load connected and a full charge going in the evening. The SmartShunt reckons it's losing about 18-20Ah overnight from a standing start, which is obviously nonsense for a battery that should self-discharge at basically nothing.

Fogstar's support said "check your BMS settings" which wasn't exactly the Sherlock Holmes investigation I was hoping for. Wondering if it's a Peukert exponent miscalibration on the shunt, a dodgy cell doing something weird internally, or whether anyone else has seen this specific ghost-drain behaviour with the Drift range.

Anyone had this and actually tracked down the cause, or am I about to spend a weekend with a clamp meter like some kind of medieval peasant?

CornishWanderer | 847 posts

@GlenSimon I had almost identical behaviour with mine last winter and it drove me crackers for weeks. Turned out my SmartShunt's tail current setting was slightly off — it was declaring the battery "full" too eagerly, so the SOC started from an inflated baseline and then corrected itself overnight once things settled. Worth double-checking your charged voltage threshold and tail current percentage in the VictronConnect app.

Also worth asking — are you in a cold location? LiFePO4 self-discharge isn't the culprit at those timescales, but a cold BMS can sometimes recalibrate its own SOC reading independently of the SmartShunt, which creates confusing discrepancies depending on which you're trusting as your reference.

What firmware version is your SmartShunt running?

BatteryDoug | 1,243 posts

@GlenSimon Worth checking whether your SmartShunt's charge efficiency factor is set correctly — LiFePO4 should be around 99%, but it sometimes defaults to 95% which can cause the SOC to drift downward over time even with no actual self-discharge. Also, have you confirmed your battery's actual resting voltage after a full charge? LiFePO4 sits at roughly 3.33-3.35V per cell at true 100%, and if your charged voltage is slightly low the SmartShunt may be starting from a false baseline. A proper synchronisation voltage needs to be dialled in correctly too. What's your current "charged voltage" threshold set to in the SmartShunt config?

DaiCole | 312 posts

@GlenSimon One thing nobody's mentioned yet — have you checked your SmartShunt's "charged voltage" threshold and "tail current" settings? If those aren't dialled in correctly for LiFePO4, the shunt never properly triggers a 100% SOC reset, so it gradually drifts and what you're seeing overnight might actually be accumulated SOC calculation error rather than genuine self-discharge or a BMS issue.

For LiFePO4 I'd suggest setting charged voltage around 14.2V and tail current at roughly 2-4% of capacity. Give it a proper full charge cycle after adjusting and see if the readings stabilise. The Fogstar Drift itself is generally a solid unit — I'd suspect the monitoring setup before the battery.

LochChild | 203 posts

Had this exact thing with my shepherd's hut setup. Turned out my SmartShunt's "tail current" setting was way too high — it was calling the battery "full" prematurely, so the SOC was inflated from the start. Come morning it looked like a drop but was actually just correcting itself.

Worth checking what @DaiCole is pointing at too, those two settings work together. In VictronConnect go to SmartShunt settings → battery charged detection and tighten up that tail current percentage. I dropped mine from 4% down to 1% and it sorted it overnight.

Fogstar Drift cells are solid in my experience, usually a calibration thing rather than the battery itself.

Slim13 | 487 posts

Good shout from @LochChild and @DaiCole on the SmartShunt settings — those are the usual culprits. One thing worth adding though: have you ruled out a small parasitic drain somewhere? Even a modest consumer drawing 50-100mA continuously overnight won't look like much on paper, but over 8 hours that's a fair chunk. A cheap clamp meter around your negative cable while everything's "off" will tell you quickly whether something's quietly sipping away. Also worth double-checking your DC-DC charger isn't backfeeding any draw when the engine's off — some Renogy units have a small quiescent current that the SmartShunt might be misinterpreting depending on how it's positioned in the circuit. What's your Peukert exponent set to as well? Sometimes that gets overlooked entirely.

MeganFox | 87 posts

Had something similar with my garden office setup. Before you go too deep into SmartShunt settings, double-check your DC-DC charger isn't causing a small parasitic draw when "idle" — mine was pulling a trickle even with no load on the output side. Showed up as the battery slowly bleeding down overnight.

Easy way to test: disconnect the Renogy completely for a night and see if the SOC holds. Took me ages to figure that one out.

Also worth checking whether the Drift's BMS is doing any internal cell balancing overnight — some units do this passively and it can look like a draw on the shunt even when nothing's actually consuming power.

FrankPalmer | 312 posts

Worth checking one thing nobody's mentioned yet — the Fogstar Drift's self-discharge isn't the issue, but parasitic drain from the Renogy DC-DC itself can be sneaky. Some of those chargers draw a small standby current even when the vehicle's off. Stick a clamp meter on the positive lead overnight and see if you're actually at zero load like you think. I've seen 200-300mA quietly disappear through converters that are supposedly idle. Also double-check your SmartShunt's charged voltage threshold matches what the Drift's BMS actually reports as full — slight mismatch there can cause the SOC to "correct" itself downward after a rest period.

ExTrucker63 | 1,204 posts

@GlenSimon one thing worth looking at that nobody's touched on yet — have you checked whether your DC-DC charger is drawing any parasitic current when the van's parked up? The Renogy 40A units can sometimes pull a small standby current through the ignition sense wire if it's not wired correctly. Wouldn't be massive, but overnight that adds up. Stick a clamp meter on your negative terminal with everything supposedly "off" and see what you're actually getting. Caught me out something rotten on my old Transit build before I spotted it. Simple fix once you've identified it though.