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

Picked up a Fogstar Drift 12V 100Ah a few months back specifically as emergency backup — the whole point was to have something reliable when the grid goes down. Last week we had a proper outage, about 6 hours, and roughly 3 hours in the BMS tripped and disconnected everything. Battery was showing around 40% SOC on the display beforehand, so it shouldn't have been a low-voltage cutoff.

Couldn't get it to reset without disconnecting and reconnecting the terminals, which obviously isn't ideal when you're fumbling around in the dark. Wondering if it was a temperature issue — the cupboard it's stored in can get a bit cold overnight, maybe dropped to 5–6°C. Does the BMS on these cells get twitchy at lower temps even on discharge?

Has anyone dealt with similar with Fogstar or other LiFePO4 setups used purely for backup? Would a battery heater pad be worth adding, or is there something else I should be checking first? Also curious whether a different BMS (separate unit rather than built-in) would give me more control over the protection thresholds.

Ella1994 | 847 posts

@MandyMorris oh no, that's such awful timing! The Drift is a decent unit generally but I've seen a few people mention the low voltage cutoff catching them out unexpectedly. Quick question — had the battery been sitting unused for a while before the outage? Lithium cells self-discharge slowly but if it dropped below the BMS protection threshold whilst in storage, it won't wake up properly under load without being "tickle charged" back above the cutoff voltage first.

Try connecting a small trickle charger briefly even if the display shows some charge remaining. Sometimes the BMS just needs that nudge to reset itself.

Worth keeping a cheap battery maintainer connected permanently if it's purely for emergency backup — keeps it topped up and ready without you having to think about it. Lesson learned the hard way unfortunately! 😔

SunnyTinker | 312 posts

@MandyMorris worth checking whether it's actually the BMS cutting out or the inverter/load dropping below the low-voltage threshold. Had something similar on my shepherd's hut setup — turned out the battery was sitting at a low state of charge from not being topped up regularly, and the first decent load spike caused the BMS to trip on undervoltage protection.

Fogstar's app (if you've got the Bluetooth version) should show you the cell-level data after the fact — worth pulling the logs.

Also — is this battery on a trickle charge between outages, or just sitting idle? A 100Ah sitting uncharged for weeks will drift low enough that the BMS gets twitchy under load. A cheap Victron IP65 charger on a maintenance cycle would sort that.

ExmoorCamper | 1,204 posts

@MandyMorris Same thing happened to me in the motorhome three winters back — Victron BMS, stone cold night on Exmoor, needed the heating and the whole lot shut down. Turns out low temperature protection had kicked in. Battery was sitting at about 4°C and the BMS wouldn't allow discharge below a set threshold.

Worth checking your Fogstar's low-temp cutoff spec. Most LiFePO4 cells genuinely don't like discharging in the cold — the BMS is doing its job, annoyingly.

If emergency backup is the whole point of the thing, you need either:

• A self-heating battery (Fogstar do variants)
• The battery kept somewhere that won't get cold
• A small trickle heater wrapped round it

Lesson I learned the hard way. Keep it warm, keep it honest.

WillWebb | 203 posts

@MandyMorris had a similar scare with my shepherd's hut setup last winter. Turned out my battery was sitting at a low state of charge going into the outage — BMS tripped on undervoltage almost immediately under load. Classic mistake, I'd been lazy about topping it up.

Worth checking your resting voltage before the next outage. I now keep mine on a small Renogy trickle panel year-round just to maintain charge. Also double-check your low-voltage cutoff settings if your BMS is configurable — some default thresholds are surprisingly conservative.

Basically don't assume a backup battery is ready just because it's sitting there. Learned that the hard way.

Nessa55 | 423 posts

@MandyMorris That's really frustrating, especially when you've specifically bought it for backup purposes! One thing worth checking that nobody's mentioned yet — what were the temperatures like during the outage? LiFePO4 BMS units will often cut out in low temperature protection mode if the cells drop below a certain threshold, especially if the battery is stored somewhere unheated like a garage or outhouse. During a cold snap with no grid heating running, that can happen surprisingly quickly. If that's the culprit, a simple insulated battery box or even wrapping it makes a big difference. Also worth downloading the Fogstar app if you haven't already and checking the event log — it should tell you exactly what triggered the cutoff rather than leaving you guessing.