Anyone else had their Fogstar Drift 100Ah randomly drop to 0% SOC mid-voyage?

Mine did it twice last week on the boat — Victron SmartShunt showing 87% one minute, then the BMS just panics and cuts out like I've personally offended it.

Running a pretty standard 12V setup: Fogstar Drift 100Ah, Victron MPPT 100/30, and a Multiplus 12/500 for the odd luxury moment. Everything's wired per the manual, fuses on both terminals, nothing exotic.

Wondering if it's a dodgy cell or just the BMS being oversensitive to voltage sag under load — I've got a 70W fridge and a chartplotter both kicking in simultaneously, so maybe it's just seeing a momentary dip and throwing a wobbly?

Curly90 | 847 posts

@SophieHill Had almost identical with my Drift 100 last spring — turned out to be a loose cell group connection internally causing a momentary voltage sag under load. The BMS reads it as a undervoltage event and latches off in protection mode even though overall capacity is fine.

Worth checking what loads were running at the cutout moment. If you've got an inverter or windlass kicking in, that inrush current can spike enough to trigger it even at 87%.

Also — are your SmartShunt settings dialled in properly? The Peukert exponent and charge efficiency factor can cause the SOC reading to drift from reality, meaning it thinks 87% but cell-level it's lower than that.

Try logging via the Victron app and watch individual cell voltages if your BMS exposes them. That'll tell you quickly whether it's a rogue cell or a settings gremlin.

@SophieHill mine did exactly this in the shepherds hut last winter — turned out the BMS was throwing a wobbly over a loose negative terminal giving it phantom voltage drop readings, basically the battery equivalent of a hypochondriac with a WebMD subscription.

LisaKelly | 847 posts

@SophieHill Oh I feel your pain — had almost identical behaviour with mine last season! Turned out my cell group 3 was drifting significantly out of balance, so even though overall SOC looked healthy, one cell was hitting the low voltage cutoff and the BMS was doing exactly what it's designed to do — just felt completely random from the outside.

Worth connecting via Bluetooth and watching individual cell voltages under load rather than relying solely on the SmartShunt SOC figure. The two don't always tell the same story, especially if you've got high current draws from a bow thruster or inverter.

Also — have you checked your negative busbar connections? A high-resistance joint can cause a sudden voltage sag that triggers the BMS unnecessarily. Caught me out embarrassingly badly last summer! 😅

Classic BMS overcurrent trip — on a boat you've got surge loads that can spike hard and fast. Worth checking what's pulling current at the exact moment it cuts. My money's on an inverter startup or a bilge pump kicking in.

Fogstar's BMS is pretty sensitive to those short sharp spikes even if your average draw looks fine on the SmartShunt. The shunt won't catch a 10ms overcurrent event but the BMS absolutely will.

Try logging via the Victron app with high-frequency sampling and see if there's a load spike just before it trips. If you're running anything motorised — windlass, fridge compressor — that's your culprit.

@SophieHill Had a similar ghost-cutout with my Fogstar Drift in the garden office setup — not a boat, but the BMS behaviour was identical.

What sorted it for me was checking the cell voltage spread via the Victron app rather than just overall SOC. One cell was lagging slightly under load, dragging the whole pack into a protection trip even though the SmartShunt reckoned I had plenty left.

Worth pulling the Bluetooth BMS data during normal load if you can — the Fogstar app isn't brilliant but it'll show individual cell voltages in real time. If you've got one cell sitting noticeably lower than the others under any decent draw, that's your culprit rather than the BMS itself behaving randomly.

Also double-check your cable connections at the terminals — poor contact causes localised resistance spikes that look exactly like overcurrent events to the BMS.

Good shout from @CurrentAffairs96 on the surge loads — but before you go hunting individual appliances, worth checking whether your SmartShunt's shunt rating actually matches your peak draw. I had a weird one in the shepherd's hut where the Victron was reading fine but a loose connection on the shunt itself was causing momentary voltage drops that spooked the Fogstar BMS into thinking something catastrophic was happening.

Basically the BMS sees what looks like a dead short and pulls the plug before the SmartShunt even blinks.

On a boat I'd also look hard at any 12V bilge pump or windlass — those inrush currents are savage and the Drift's BMS isn't the most forgiving I've used.

Try logging with VictronConnect during a trip and watch for any sub-second voltage dips. That'll tell you whether it's a genuine overcurrent or something spookier upstream.

@SophieHill worth checking whether this is happening at the same time of day — mine did something similar in the shepherd's hut and it turned out to be a temperature thing. The Drift's BMS has a low-temp cutoff and on a boat with poor ventilation the cells can get surprisingly cold overnight, even in summer. The SmartShunt will still show 87% because SOC is calculated, not measured — the BMS just pulls the plug without warning the shunt first. Check your Victron app history for any voltage dips immediately before the cutout.

Had this exact thing in my van last winter — turned out my battery cable connections had developed a tiny bit of resistance. Under surge load, voltage would momentarily dip hard enough to spook the BMS into thinking it was dying.

Worth checking every terminal with a multimeter under load rather than just eyeballing the connections. Even connections that look fine can have enough oxidisation to cause grief.

Also double-check your BMS low voltage cutoff setting if it's adjustable — mine was set a bit conservatively from factory and one decent inverter spike was enough to trigger it.