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

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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 missing, the FET stays off, preventing current flow.

 

 

How to Measure FET Gate Voltage

Using a multimeter in DC voltage mode:

  • Place the black probe on the battery negative terminal (B−).
  • Use the red probe to measure the gate pin of each FET.

If the gate voltage is near 10V, the FET is turned on. If it's 0V, the FET is turned off. Compare measurements between a working battery pack and a faulty one to identify abnormal readings.

A 2D digital diagram illustrating troubleshooting steps for BMS circuits, including labeled measurement points for multimeter testing, MOSFETs, and controller IC components.

Measuring Source and Drain

With the FET's source usually connected to B− (ground), the drain should match the total battery voltage (21V for a 5S pack) when the FET is on. If it's 0V, the FET is off or damaged.

 

 

Common Issues and Checks

  • If gate voltage is present but no voltage at the drain, the FET might be faulty.
  • If the controller does not output gate voltage at all, suspect controller IC failure.
  • Always verify that all cell voltages (B1~B5) are above 2.5V.

Summary

To troubleshoot a battery pack that appears dead, use a multimeter to inspect gate, drain, and source voltages. By comparing these values against a known-good pack, you can narrow down the fault to either a FET, a control IC, or cell imbalance.