Lithium Battery Failure Diagnosis -One Page Decision Matrix

Created by Scott Chen, Modified on Tue, 14 Oct at 4:27 PM by Scott Chen

How to classify “Improper maintenance” vs “Defective cell” vs “BMS failure” clearly for warranty review

Observation	Likely Cause	Classification	Notes / Action
Total voltage 0–9V, stored long time	Delayed charging / deep discharge	Improper maintenance	Cells degraded due to long-term low voltage storage; recharge immediately after purchase or after each use.
1–2 cells <2.0V, others normal	Storage imbalance / no maintenance charge	Improper maintenance	Top balance and monitor; if no recovery after balancing, irreversible damage likely.
All cells <2.0V and balanced	Long-term storage without recharge	Improper maintenance	SEI breakdown and lithium plating; replacement required.
All cells healthy (≈3.2V), but pack no output	BMS MOSFET or fuse failure	Defective BMS	Replace BMS board; check communication and MOSFET integrity.
Cells balanced but 1 drops fast under load	Cell defect / high internal resistance	Defective cell	Replace the weak cell; record IR reading and update QC records.
Swelling, burn marks, or melted terminals	Overcurrent / short / overheating	User damage	Non-warranty case. Scrap immediately for safety.
BMS shows undervoltage lock (UV lock)	Extended undervoltage event	Improper maintenance	Caused by long idle time; unlock BMS or replace if permanently protected.

* Note: Improper maintenance typically presents as low voltage (<2V/cell), no swelling, and long idle time. Defective units usually hold normal voltage but fail under load or due to BMS malfunction.