To recharge an 18650 battery safely, you need a dedicated lithium-ion battery charger designed to detect the battery type and chemistry and apply a safe charging current to the cell. These chargers are specifically made to charge lithium-ion batteries, which typically operate between a range of 2.5V to 4.2V, with 2.5V being fully depleted and 4.2V being fully charged.
It's important to note that 18650 li-ion batteries have a nominal voltage of around 3.7V, unlike standard AA and AAA alkaline batteries which have a voltage of 1.5V. Therefore, charging an 18650 battery requires a different protocol, and using the wrong type of charger can be dangerous and damaging to the battery.
One of the most critical features of a dedicated li-ion charger is the ability to stop the charge cycle when the battery reaches 4.2V. Overcharging an 18650 battery can cause internal damage and eventually lead to battery failure, which is why it's crucial to use the appropriate charger for safe and efficient charging.