Anyone else had their BMS trip during a power cut when they needed it most?

Had a frustrating experience last week during the storm. We lost grid power for about 14 hours and my 200Ah Fogstar Drift LiFePO4 pack (with the built-in BMS) shut itself down around hour six. Voltage was sitting at a perfectly reasonable 51.2V on a 48V system, temperature was fine — no obvious reason for it to trip. The whole point of that battery is emergency backup, so this was pretty gutting.

After a lot of digging I suspect it was a high-current inrush event when my Victron Multiplus II briefly tried to reconnect to grid and then dropped back out — essentially a load spike that nudged the BMS into overcurrent protection. I've since read that some BMS units have quite aggressive OCP thresholds, sometimes as low as 1C for a fraction of a second, which on a 200Ah pack means anything over 200A instantaneous can trigger a shutdown. The Multiplus II can absolutely pull that during a transfer event.

Has anyone else seen this with Fogstar packs specifically, or with LiFePO4 in general on backup setups? I'm wondering whether a separate, more configurable external BMS (something like the Daly Smart BMS or even a JK BMS) would give me better control over those thresholds — or whether the fix is on the Victron side, maybe tweaking the transfer settings. Would love to hear how others have handled this before I start pulling the pack apart.

@CamperDan had almost the exact same thing happen in my static caravan last winter. Turned out my BMS was tripping on low temperature protection — not low voltage like I assumed. The cells were fine on charge level but it was a cold night and they dropped below the BMS cutoff threshold.

Worth checking a few things:

• What was the actual cell voltage when it tripped? (Not just the pack voltage)
• What temperature were your cells at?
• Was there any load spike that could've hit the BMS overcurrent limit?

My fix was adding a bit of insulation around the battery box and that sorted it. Fogstar's built-in BMS settings aren't adjustable from what I can tell, which is frustrating. Anyone know if there's a way to at least read the BMS fault log on the Drift series?

Really sorry to hear that @CamperDan - nothing worse than your backup power failing when you actually need it.

One thing worth checking that nobody's mentioned yet: low temperature cutoff. During that storm we had some pretty bitter nights, and if your battery compartment dropped below around 0°C the BMS may have triggered the low-temp protection rather than a voltage issue. LiFePO4 cells genuinely can't safely accept charge below freezing, so the BMS is doing its job - but it's cold comfort at 2am with no heating!

If that's the culprit, a small self-regulating heat mat on a thermostat inside the battery enclosure sorts it fairly cheaply. Worth logging your battery compartment temperature during normal operation first just to confirm that's actually the issue before spending anything.

What were conditions like where your pack is housed during the outage?

Hey @CamperDan, gutting timing with the storm! One thing worth checking that hasn't been mentioned yet - low temperature cutoff. If your battery is in an unheated space and the temperature dropped overnight, the BMS might have tripped on that rather than voltage. LiFePO4 really doesn't like being discharged below about 0°C and most BMS units will protect against it.

Worth having a look in your BMS settings or app (if it has one) for the exact fault code - that'll tell you straight away what triggered the shutdown rather than guessing. The Fogstar Drift should log this somewhere.

If it was temperature related, a simple insulated enclosure makes a massive difference and is a cheap fix before next winter. 🙂

@CamperDan what was the load at the time of shutdown? Six hours into a 200Ah pack suggests you were pulling decent current, and if ambient temps were dropping during the storm you could be hitting both an overcurrent and low-temp cutoff simultaneously — the BMS doesn't always log which threshold killed it first.

On my motorhome setup (also Fogstar, same Drift cells) I added a Victron BMV-712 specifically so I can see exactly what was happening at point of shutdown — current, voltage, temp readings. Worth the £80-odd investment before your next outage.

Also worth checking: are your cable connections properly torqued? Resistance under sustained load causes voltage sag that can fool the BMS into thinking the cells are lower than they actually are. Classic gotcha that catches people out.

@CamperDan this happened to me with my first LiFePO4 setup — turned out the BMS was nuking itself on a protection loop rather than a genuine cell issue.

Worth connecting a laptop to it (if yours supports comms) and pulling the fault log after you restore power. Mine showed a cascade: a brief voltage sag triggered undervoltage protection, which caused a reconnect attempt, which spiked the load, which tripped it again. Round and round.

What's between the battery and your loads? If you've got no pre-charge resistor or soft-start on any inverter, that inrush current on reconnect can look absolutely catastrophic to a BMS.

Since rebuilding around a proper Victron SmartShunt I can actually see what's happening in real time rather than guessing after the fact. Visibility changes everything with these systems.