u/UnicornFlow3

I’m troubleshooting a 4S lithium battery pack (model: P1904-4S1P-MM, likely from a vacuum cleaner) using a BM3451-T20B BMS, and I’m stuck on a strange issue.

After reassembling the battery pack, the system behaves like this:

Cell voltages are balanced and normal (~4.12–4.15V each, total ~16.5V)

Charging works normally (charge MOSFET is enabled)

Discharge is completely blocked

Output voltage at P− is low (~2–8V depending on measurement point)

Gate of discharge MOSFET (Q1, 4N2R6A) is held at 0V

Charge MOSFET gate (Q5) shows ~10V and appears to be working correctly

BM3451 DO pin is at 0V relative to B− (and ~16.4V relative to B+)

One NTC sensor is present and reads ~8kΩ (seems normal); second thermistor goes to the main device and is not used by the BMS

No buttons or reset circuit on the pack

It looks like the controller is intentionally keeping the discharge MOSFET off, not a hardware failure of the MOSFET itself.

I already checked:

Cell balance: OK

MOSFETs: likely OK (charge side works)

NTC: OK

Wiring between DO and gate: OK

My suspicion is that the BM3451 is stuck in a discharge protection latch state after battery reassembly / reconnect sequence.

Is there a known latch/reset sequence for BM3451 BMS boards? Can discharge protection be cleared without programming tools? Could this be caused by wrong power-up sequence after reassembly? Has anyone seen BM3451 permanently disable discharge while allowing charge?

Any ideas or recovery tricks would be appreciated.

At what point does it stop making sense to keep upgrading and just build a new PC?

BM3451 4S BMS discharge MOSFET not turning on (charge works, discharge locked) – P1904-4S1P-MM battery pack