The 18650 battery is a lithium-ion battery with a diameter of 18mm and a height of 65mm. Its height and diameter are both greater than the AA size. They are not compatible with AA or AAA size batteries. Because of its high-level capabilities, such as 250+ charge cycles and increased energy density, the 18650-battery. The standard size of a 18650 battery is 18x65mm. 1. The 18650 battery is 65mm long 2. The 18650 battery has an 18mm diameter More specifically, it measures 65mm in length and 18mm in diameter; however, technically, the. A battery management system (BMS) monitors a battery pack, a collection of cells electrically grouped in a row x column matrix to supply a specific range of voltage and current for a set period response to projected load scenarios. Every 18650 cell can be charged up to 4.2V; we need three cells in series to make a 12.6V battery pack. In the figure above, the connections are indicated. The BMS is to be mounted as indicated above. To balance charge the.