In addition to the WeMos battery powered temperature server, it would be nice to know what the remaining level of the battery is, so we can recharge it in time. This is easily done using a voltage divider circuit and the analog input on the WeMos D1 mini module.
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.