The 18650 is a rechargeable cylindrical lithium-ion battery with a diameter of 18mm and a length of 65mm, offering a nominal voltage of 3.6V-3.7V and a capacity of typically 2000mAh-3500mAh. Compared to AA batteries, it offers much higher energy density and a superior cost/performance ratio with thousands of charge cycles. It is mandatory to use with a charge management circuit such as the TP4056 and over-discharge protection before connecting directly to Arduino or ESP circuits; unprotected cells are particularly susceptible to short circuit and over-discharge. The portable ESP32/ESP8266 is the standard power supply choice for IoT projects, DIY power bank construction and wireless sensor stations that need to operate for long periods of time.
