Undervoltage BMS shutdown with 72% showing on the BMV

Had something similar happen last month with my static caravan setup. Running a 200Ah Fogstar Drift LiFePO4 with a Victron BMV-712 and a separate Daly BMS. The BMS cut out under load — had the EV charger pulling hard — yet the BMV was confidently showing 68% SOC. Took the battery offline for a few minutes, everything reset, came back fine.

After digging into it, I'm fairly convinced the issue is voltage sag under high current draw rather than actual capacity. The BMS is reacting to instantaneous cell voltage dropping below its protection threshold, while the BMV is calculating SOC based on coulomb counting — so it's showing what should be in the bank, not what the cells are actually doing under stress.

A few things worth checking if you're seeing this:

• Internal resistance — older or cold cells sag much harder under load
• BMS low-voltage cutoff setting — some are quite aggressive out of the box
• Cable sizing and connections — voltage drop before it even hits the BMS terminals

I've since tweaked the Daly's undervoltage threshold slightly and made sure my ring terminals were properly torqued. Sag events are rarer now, though the EV charging load is still brutal on the system if the solar isn't keeping up.

Curious whether anyone else has had this with Fogstar cells specifically, or if it's more of a BMS calibration issue. Also wondering whether a Victron SmartShunt with proper battery profile would do a better job of flagging when sag is the real culprit rather than SOC. Anyone integrated that kind of early warning into their setup?

@VivaroDream classic symptom of voltage sag under load fooling the BMV's SoC reading. The BMV calculates SoC via coulomb counting, so if it wasn't properly synchronised from a full charge it'll confidently display 72% whilst the actual resting voltage tells a different story.

The Daly BMS however is reacting to real-time cell voltage — if one cell is sagging below its undervoltage threshold under load, it'll pull the plug regardless of what the BMV thinks.

Few things to check:

• Cell balance — have you got a way to see individual cell voltages? A weak cell will sag hard under load
• Peukert settings on the BMV — worth reviewing
• BMS low-voltage cutoff — what's the Daly set to?

Had nearly identical on my narrowboat with a Fogstar pack. Turned out one cell was noticeably weaker than the rest.

@MrBodge65 is right about the sag, but also worth checking your BMS low voltage cutoff setting — Daly units often ship with the threshold set quite high from factory (around 2.8v per cell). Under a chunky load like an EV charger that sag can briefly kiss that threshold even when your overall SoC looks healthy.

Had almost identical on my cabin setup. Turned out my Daly was cutting at 2.85v/cell. Dropped it to 2.75v and the nuisance trips stopped completely.

Also — is your BMV shunt sized correctly for that charger load? An undersized shunt reading inaccurately will skew the coulomb counting and make the 72% figure unreliable anyway.

@VivaroDream the Daly BMS is seeing the cell voltage tanking under that EV charger load, not the pack average your BMV-712 is cheerfully reporting — 72% SoC means nothing if one cell's hitting the cutoff threshold while the others are fine, classic sign your pack needs a proper top-balance.

Stick a cell-level monitor on it (a cheap JK BMS or even a Bluetooth cell checker) and you'll immediately see which cell is the runt of the litter dragging everything down.

Also worth noting: Fogstar Drift cells are decent but fresh packs sometimes ship with uneven cell voltages — run a few full charge/discharge cycles and let it settle before diagnosing anything more serious.

@SussexSolar that's a really good point about cell vs pack voltage — had me wondering, does the Daly actually report individual cell voltages anywhere? I know you can pull data from it via Bluetooth if you've got the right dongle, but I've never tried it myself on my narrowboat setup.

Also @VivaroDream — what cable sizing are you running between the battery and that EV charger? Even a slight resistance in thick cables can cause nasty voltage drop under a big sustained load. I learned that the hard way when I was pulling around 30A consistently and my BMV was reading fine but things kept tripping. Turned out my lugs weren't crimped properly. Worth checking before you start tweaking BMS thresholds?