Why does my LiFePO4 pack disconnect at 80% SOC under load?

Usually an overcurrent or undervoltage trip, not a bad cell. A water pump or compressor surge can exceed the BMS discharge current limit for a few seconds; the BMS opens the MOSFET to protect itself. Raise the BMS current threshold to match the inverter surge, or size the BMS for peak load, not just continuous. A genuinely dead cell would show as one series group dropping far below the rest at rest.

What does it mean when the BMS shows a cell-imbalance alarm?

One or more cells have drifted beyond the alarm threshold (often 300mV). On passive balancing it can take hours to correct; on active balancing it's faster. A persistent imbalance that won't clear points to a weak cell or a failed balancing resistor, not a setting.

LiFePO4 BMS Faults Explained: Why Your Pack Keeps Disconnecting

Q: How do I tell a BMS fault from a failed cell?

Two checks: at rest (no load, fully charged and settled 2 hours), the cell voltages should all be within a few mV. And under a known load, watch which series group drops first. If one group collapses while others hold, that's a cell. If all cells are even but the pack still cuts out, the fault is in the BMS protection setting or the MOSFET.

A distributor called us about a pack that cut out at 80% state of charge every time the customer's well pump kicked on. He was sure the cells were dying. They weren't. The BMS discharge limit was set below the pump's startup surge, so it opened the contactor to protect itself. Two minutes of reconfiguration fixed what he thought was a dead battery.

A BMS trips for real reasons. Before you blame the cells, learn what the fault codes actually say.

The faults you'll actually see

Fault	What triggers it	Fix
Overcurrent / discharge cutoff	Load surge (pump, compressor, tool start) over the BMS limit	Raise current threshold to inverter peak; size BMS for surge
Cell imbalance alarm	One cell drifts >300mV from the rest	Let balancing run; if it won't clear, a cell or balance resistor has failed
Temperature cutoff	Charge below ~0°C or discharge above ~60°C	Warm the pack / shade it; this is protection, not a fault
Communication loss	CAN/RS485 drop between BMS and inverter	Check terminator resistor, cable, baud rate mismatch
Undervoltage cutoff	A cell hits its floor under load	Recharge; if it happens at high SOC, see the weak-cell check below

How to read the codes

Most BMS units signal through an app (Bluetooth) or a blinking pattern on the board. The app is the faster path: it shows per-cell voltage, which is the single most useful number you have. A healthy 16S 51.2V pack at rest shows all cells within a few millivolts. If one group sits 50-100mV low while charged and resting, that group is your problem, not the BMS logic.

BMS fault or dead cell? A two-step check

Step 1 — at rest. Fully charge, then let the pack sit two hours with no load. Read each series group. Even within a few mV = cells are fine, fault is in settings or MOSFET.

Step 2 — under load. Apply a known load and watch which group drops first. One group collapsing while the others hold = a weak cell. All groups even but the pack still cuts out = BMS protection setting or a failed MOSFET.

This distinction matters because replacing cells when the BMS was misconfigured wastes money, and vice versa. We publish the per-cell test data with every pack so this check takes five minutes, not a teardown.

When it really is the BMS

A failed MOSFET or a BMS that won't communicate after a reset is a hardware fault — swap the BMS board, not the cells. For how to spec a BMS in the first place (balancing type, protocols, current rating), see our BMS selection guide. Our 51.2V modules ship with the cutoff thresholds and communication protocol documented per batch.

Pack disconnects at 80% SOC under load?

Almost always overcurrent, not a dead cell. Match the BMS current limit to the inverter's surge, or size the BMS for peak load.

What does a cell-imbalance alarm mean?

A cell drifted past the alarm threshold. Passive balancing is slow; active is fast. If it never clears, suspect a weak cell or failed balance resistor.

How do I tell a BMS fault from a failed cell?

At rest all cells should be within a few mV. Under load, watch which group drops first — one collapsing group is a cell; even groups but still cutting out is the BMS.