The charger detects when the battery is fully charged through a process called "voltage regulation." Here's how it works:

1. Constant current charging: When you connect a battery to a charger, the charger initially supplies a constant current to the battery. This current flows through the battery and charges it.

2. Voltage rise: As the battery charges, its voltage gradually increases. The charger monitors the voltage of the battery.

3. Endpoint detection: The charger is designed to detect a specific voltage level that indicates the battery is fully charged. This voltage level is typically determined by the battery manufacturer and is based on the battery's chemistry and characteristics.

4. Switch to trickle charging or float charging: Once the charger detects that the battery voltage has reached the full charge level, it may switch to a different charging mode. This could be trickle charging, where a small current is maintained to keep the battery topped up, or float charging, where the charger maintains the battery at a constant voltage.

5. Monitoring and protection: Some chargers also include additional features to monitor the battery's condition and protect it from overcharging. They may use temperature sensors to detect if the battery is getting too hot and adjust the charging accordingly.

The specific charging algorithm and detection method can vary depending on the type of charger and the battery being charged. Some chargers use more advanced techniques, such as charge termination algorithms based on voltage, current, or time, to ensure accurate and safe charging.

It's important to use a battery charger that is compatible with the battery and follow the manufacturer's instructions for charging to ensure optimal battery performance and longevity. Additionally, modern devices often have built-in charging circuits that handle the charging process automatically, providing protection and indication when the battery is fully charged.