Fogstar 16.kWh battery trips out but I don't know why. How to troubleshoot?

Similar story here, actually. Been running a Victron MultiPlus-II 48/5000 with a Fogstar Drift 16kWh in my cabin setup for about eight months now, and around month four I started getting these mysterious shutdowns — no obvious pattern, no screaming alarms, just... off.

What made it maddening was the timing. Three trips in a fortnight, then nothing for weeks.

Here's what eventually helped me piece it together:

VRM Portal logs — if you've got a Cerbo GX, dig into the detailed logs around the exact minute of each trip. Mine revealed brief voltage sag events I'd completely missed on the dashboard glance.

Check your BMS communication — the Fogstar units talk over CAN bus to the Cerbo. I had a dodgy cable seating that was causing the Cerbo to occasionally lose the BMS signal, which triggered a protective shutdown. Reseated the RJ45 and the problem vanished.

Load spikes — are you running anything that kicks in hard? My EV charger on a timer was occasionally starting up during a period of low solar and high background load. The MultiPlus didn't like that combination one bit.

A few questions worth asking yourself:

• Has anything changed in your setup or routine in the last month?
• What does the Fogstar BMS app show around the trip times?
• Any new loads added recently?

The Enphase microinverter angle is interesting too — had anyone considered whether a grid-forming conflict could be involved if you're in any kind of hybrid setup?

Would love to know what others have found. These intermittent faults are the worst to chase down.

@Brummie classic case of your BMS and Victron having a falling-out like two blokes at a pub quiz — check your DVCC settings in VictronConnect, because if the charge voltage ceiling doesn't match what the Fogstar BMS expects (usually 56.8V for a 16S LiFePO4), the BMS will just pull the plug mid-argument. Also worth grabbing the VRM portal logs around the exact trip timestamps — nine times out of ten you'll spot a cell imbalance spike just before it goes dark. Had the exact same drama on my narrowboat until I dropped charge current to 0.2C and let the cells breathe a bit. Fogstar's own support are actually decent on the phone too, not just a chatbot reading from a script.

@Brummie worth pulling the Fogstar's BMS data logs if you haven't already — the Drift units log fault codes internally. Connect via the Bluetooth app and look at the historical data, not just live readings. Nine times out of ten with the MultiPlus-II combo I've seen it's either a cell group hitting the high voltage cutoff during absorption (try dropping your absorption voltage to 55.2V rather than letting Victron push it to 56V+), or the BMS is tripping on a current spike during heavy loads.

What's your DC cable sizing and busbar setup like? Undersized cables create enough voltage drop to confuse both systems simultaneously. On my narrowboat I had identical symptoms until I uprated to 70mm² throughout.

@Brummie what's your DC cable run length between the battery and the MultiPlus, and what gauge are you using? Had something weirdly similar with my cabin setup — turned out my cable was undersized for the peak draw, voltage would sag just enough under load to spook the BMS into tripping. Fogstar's BMS is quite aggressive about undervoltage protection compared to some others.

Also — are the trips happening at a particular time of day, or under specific loads? Kettle, inverter AC, something kicking in simultaneously? Patterns matter here. Chuck us the details and we can probably narrow it down without you having to wade through Victron's documentation, which is approximately 4,000 pages of fun.

@Brummie one thing nobody's mentioned — check your battery temperature sensor is actually seated properly. Fogstar Drift units can throw a protection trip if the BMS is reading ambient wrong and thinks it's hitting thermal limits when it isn't.

Also, what's your charge current limit set to in VictronConnect? If you're pushing the full 5000W inverter capacity on discharge and haven't capped the charge side, the BMS occasionally doesn't negotiate fast enough and just shuts itself off as a precaution.

Had similar grief with mine around the same timeframe. Turned out to be a combination of DVCC not being configured properly and my charge voltage being set slightly high — 54.4V instead of the 54V Fogstar actually recommend.

Pull the Venus OS logs from your GX device if you've got one — timestamp the trips and cross-reference. Usually tells you exactly what triggered it.