Victron 200ah lithium dropped to 20% overnight with barely any load — BMS showing nothing dodgy

Seen this exact thing with my setup — turned out the Victron BMV-712 was lying through its teeth about state of charge because it hadn't done a proper full cycle to recalibrate since installation.

A few things worth checking before you start panicking:

• Peukert's exponent — if it's misconfigured in your BMV settings, the SOC calculation goes properly sideways
• Parasitic drain — even with "barely any load," inverters on standby, Cerbo GX, and that one random USB charger you forgot about can collectively munch through more than you'd expect overnight
• Cell drift — BMS showing "nothing dodgy" doesn't mean individual cells aren't quietly drifting apart; worth pulling the actual cell voltages if your BMS gives you that data
• Temperature — been a cold snap lately? Lithium capacity drops noticeably once you're below about 5°C, which catches people out every autumn without fail

My money's on a calibration issue rather than anything physically wrong with the cells — Fogstar and Victron lithiums don't usually just decide to misbehave randomly without a reason showing somewhere in the logs.

Has anyone else on here had the Victron SmartShunt suddenly lose the plot after a firmware update? Mine went a bit peculiar after the last one and I'm wondering if that's more common than Victron let on.

What firmware version are you running?

@VE_Electric yeah the BMV sync issue is real but also worth checking your parasitic drain before blaming the monitor. Had this on my static van — turned out the Fogstar 200ah was fine, something was pulling 2-3A all night that I'd missed. Inverter standby was the culprit.

Stick a clamp meter on the negative before bed and see what's actually drawing. If it's genuinely near zero then yeah, recalibration is probably your answer.

Also — what's your tail current set to in the BMV? Default is often wrong for lithium and it'll never properly sync to 100%.

@VE_Electric nailed it — BMV-712 SOC drift is almost always a "never seen a proper tail current" problem rather than actual battery grief; mine was reading 47% when the cells were sitting at resting voltage for 100% like smug little rectangles.

Set your charged voltage, tail current (typically 4% of capacity, so ~8A for a 200Ah bank), and charge detection time correctly in the BMV settings and let it complete one full absorption cycle to sync — Victron's own docs are surprisingly decent on this for once.

Also worth checking: if you're on a MultiPlus or SmartSolar, use VE.Smart Networking so the charger tells the BMV exactly when charge is complete rather than the BMV guessing. Eliminates the drift almost entirely.

Bit late to this thread but had almost identical panic on my motorhome last summer — convinced myself the battery was dying, nearly ordered a replacement Fogstar at 2am 😅

Quick question for the OP though: what have you got set as your tail current in the BMV settings? I've seen people leave it at the default and it never actually triggers

Really useful thread this. One thing nobody's mentioned yet — have you checked your charged voltage and tail current settings in the BMV-712 configuration? Out of the box the defaults are often too conservative for lithium, so it never actually registers a full charge even when the battery is genuinely topped up. I had mine set to a tail current of 4% which was fine for AGM but lithium needs something closer to 1-2% before it'll accept a sync. @ZFS_OffGrid makes a fair point about parasitic drain too — worth sticking a clamp meter on the negative while everything's supposedly off. Sometimes a faulty inverter standby or even a cheap 12v socket adaptor left plugged in will bleed more than you'd expect overnight.