Why does a lithium battery have three wires? What is the function of each wire?

Lithium batteries with three wires are commonly found in various devices, especially in single-cell applications. The three wires generally have these functions:

1. **Positive Wire (usually red):** This is the main positive terminal of the battery, carrying the positive voltage.

2. **Negative Wire (usually black or blue):** This is the main ground or negative terminal of the battery.

3. **Thermistor or Temperature Sensor Wire (usually white or yellow):** This wire is often connected to a thermistor or a temperature sensor inside the battery. Its purpose is to monitor the battery temperature to help protect against overheating, which is crucial for safety in lithium-ion batteries.

The temperature sensor is typically used by the device’s battery management system (BMS) to monitor and control the charging and discharging process. This design improves safety by reducing risks like overheating, which could lead to fire or explosion in lithium-ion batteries.