Therefore, once the first battery is fully charged, it automatically switches to the second battery and starts the charging process over. The LEDs indicate what level your battery is at, and the charger protects them from short-circuiting and instantly shuts off if the batteries are overheating. Because of the switching power between 100 to 240 VAC, you are able to use the charger anywhere in the world. The DUO Charger is compatible with Nikon EN-EL18, EN-EL18a, EN-EL18b, and EN-EL18c type batteries.