A lithium-ion battery requires a specific dedicated charger to safely recharge the battery. Many of these dedicated chargers are designed to automatically detect the battery type and chemistry and apply a safe charging current to the cell. Most lithium ion batteries operate within a range of 2.5V to 4.2V where 2.5V is fully depleted and 4.2V is fully charged.
Unlike standard AA and AAA alkaline batteries which have a voltage of 1.5V, 18650 li-ion batteries have a nominal voltage of ~3.7V and as such require a different protocol for safe charging. Another very important feature of a dedicated li-ion charger is to end the charge cycle when the battery reaches 4.2V as over-charging an 18650 battery can cause internal damage and eventually cause the battery to fail.
