Troubleshooting a 21V Drill Battery Pack: Key Points for Diagnosing a Non-Charging BMS

Many users encounter situations where a 21V lithium-ion drill battery pack doesn't charge, despite the charger showing a green light. In such cases, the issue often lies not with the charger, but within the battery pack's BMS (Battery Management System). This post outlines how to determine whether the BMS or internal FETs are malfunctioning, and how to measure them using a multimeter.     Understanding FET Behavior Inside a BMS, there are typically two FETs (Field-Effect Transistors): one for charging and one for discharging. Each works like an electronic switch that turns on or off depending on the gate voltage supplied by the controller IC. Charging FET: Controls the return (negative) line from the charger to the battery pack. Discharging FET: Controls the negative line from the battery to the drill or load. When charging or discharging is allowed, the controller supplies around 10V to the gate of the respective FET. If the gate voltage is miss...