Anyone else had their Fogstar Drift 100Ah drop to weird low voltages overnight with no load?

Pulled into a layby near Gretna last week after a long drive and noticed my Fogstar Drift 100Ah (the lithium one, 12v) had dropped to 11.8v sitting completely idle — nothing drawing from it, inverter off, everything isolated. Sat there scratching my head for a good twenty minutes.

The battery is about eight months old, fitted in my Transit motorhome conversion. It's managed by a Victron SmartShunt and I can see the history in the app. According to the shunt, zero current in or out, yet the voltage had crept down from 13.1v when I'd parked up to 11.8v over roughly four hours. That's not normal self-discharge behaviour as far as I understand it.

Wondering if it could be a dodgy BMS triggering some sort of protection mode and then recovering — I've read that can cause odd voltage readings rather than a genuine state-of-charge drop. The cell groups all looked balanced the last time I charged to 100%, so I'm a bit stumped.

Has anyone seen this on a Drift specifically, or on any other LiFePO4 with a similar pattern? Worth contacting Fogstar directly at this stage, or is there a diagnostic step I'm missing before I do that?

Seen this with mine actually. 11.8v idle is suspicious — that's basically the BMS sitting at the lower edge of what it'll show before it starts protecting itself.

Few things worth checking:

• Parasitic draw — even with the inverter off, does it have a standby current? Some pull 0.5-1A constantly
• Temperature — LiFePO4 reads lower voltages in the cold, especially parked overnight near Gretna in... well, Gretna weather
• Cell imbalance — if one cell group is lagging the BMS can report oddly

Mine did something similar last winter in the tiny house setup. Turned out the Victron BMV-712 was still powering its shunt circuit and I had a loose connection creating a tiny phantom load.

Worth chucking a clamp meter on the negative before assuming the battery itself is faulty. Fogstar's warranty support is decent if it does turn out to be a dud cell though.

@VickyFisher 11.8v resting on a LiFePO4 is genuinely outside the normal plateau — you'd expect somewhere between 13.2v and 13.3v fully charged, dropping to maybe 13.0v at 50% state of charge. At 11.8v you're either looking at the BMS having tripped and sitting in a protection state, or one of the internal cell groups has drifted badly out of balance.

Worth checking: does the battery recover if you apply a gentle charge from a quality charger like a Victron IP65 or similar? If the BMS has latched off due to low-cell detection, a controlled charge often resets it.

On my narrowboat I had a similar scare with a different brand — turned out one cell group had dropped to ~2.7v while the others sat at 3.3v. The BMS was essentially holding things together but refusing to deliver meaningful current. A full balance cycle sorted it.

Hey @VickyFisher, worth checking whether the BMS had recently woken from a low-temperature protection cutoff — near Gretna in autumn/winter the cells can get quite cold overnight, and some BMS units will throttle or partially disconnect to protect the cells, which can leave you with an oddly low resting voltage until things warm up a bit. Give it 20-30 minutes somewhere warmer and see if it recovers to the proper plateau. Also worth double-checking your BMS comms app if the Drift supports it — sometimes there's a cell imbalance lurking that only shows up at rest. If it's consistently sitting that low with no explanation, I'd log it with Fogstar directly; their support has been pretty decent from what I've seen on here.

@VickyFisher one thing worth ruling out — had the battery been sitting unused for a few days before that reading? LiFePO4 cells can sometimes show a temporarily depressed voltage straight after a partial discharge cycle, especially if the BMS hasn't had a chance to balance properly. Give it a full charge to 14.6v, let it rest for a couple of hours with nothing connected, then check the resting voltage again. If it settles back into the 13.2–13.3v range you're probably fine. If it's still reading low after that, I'd be getting onto Fogstar directly — their customer service is decent in my experience and that battery's still under warranty if it's a recent purchase.

@VickyFisher one thing I'd add that nobody's mentioned yet — have you checked your battery monitor or multimeter calibration? I had a similar scare with mine where I was getting dodgy readings because the meter leads had a poor connection, giving me artificially low voltage figures. The battery itself was absolutely fine.

Also worth doing a proper resting voltage check — disconnect everything, leave it completely isolated for a good 2-3 hours, then measure. LiFePO4 can sometimes show a temporarily suppressed voltage right after any kind of load event or even a charging cycle. If it recovers to 13.2v-ish after a proper rest period, you've likely got nothing to worry about. If it's still sitting around 11.8v after all that, then I'd be contacting Fogstar directly — they've generally been pretty decent with warranty queries in my experience.