18650 batteries are known for it's high energy density, but this comes at a price. In order to protect the battery from overcharging or over-discharging a number of ICs need to be used. Luckily these charge and protector circuit boards are readily available for sale in all shapes and sizes. In this blog post I am going to build a charger that uses such a circuit.
Not only the charger can use protection circuitry, but also the battery itself. When buying the batteries make sure that they are protected. I have bought mine in a Chinese online webshop.