Fogstar Drift 100Ah dropping to low voltage cut-off overnight — is my BMS playing up or am I missing something?

Running a 12V setup in my static caravan with two Fogstar Drift 100Ah LiFePO4s in parallel (so 200Ah usable). Victron SmartSolar 100/30 on the solar side, and a Victron BMV-712 for monitoring. No load running overnight except a small 12V fan and the BMV itself — maybe 3-4A draw at most.

Problem is the batteries are hitting low voltage cut-off around 3-4am, showing roughly 10.8V when they trip. That's nowhere near what I'd expect — even at 200Ah I should have hours of headroom at that draw rate. Checked the BMV history and the SOC was reading 94% when I went to bed. Something doesn't add up.

Wondering if one of the BMSs is tripping independently and effectively cutting me down to 100Ah without warning, then the remaining battery is getting hammered? Both packs are the same age (bought together six months ago) but I haven't load-tested them individually. Has anyone seen this with parallel LiFePO4 setups, and is there a sensible way to diagnose which BMS is the problem without a proper load tester?

Hey @LisaStewart71, with no load overnight that's genuinely odd. A few things worth checking: firstly, have you set your BMV-712's Peukert exponent and charge efficiency factor correctly for LiFePO4? Incorrect settings can cause the BMV to misreport state of charge, making it look like the batteries are draining when they're not. Secondly, are the batteries actually hitting low voltage cut-off confirmed by the BMS, or is that just what the BMV is showing? Worth distinguishing between a real voltage drop and a monitoring quirk. Also double-check your SmartSolar charge profile — if it's set for AGM/lead-acid absorption voltages it could be surface-charging without properly topping the cells. What does the actual resting voltage read on the batteries directly with a multimeter?

@LisaStewart71 worth checking whether the two Drifts are actually balanced with each other before you dig too deep into the BMS. When you put LiFePO4s in parallel, if they're at different states of charge they can spend all night slowly equalising between themselves — the BMV-712 might be reading this as a voltage drop when it's really just cell balancing between the two batteries. Had something similar with my garden office setup.

What does the BMV show for current overnight? If you're seeing a small but consistent draw (even 0.1–0.2A) with supposedly no load, you might have a parasitic drain from the Victron kit itself — the SmartSolar and BMV both sip a little power constantly.

What firmware are the Drifts on? Fogstar pushed an update a while back that adjusted the BMS cut-off thresholds.