What is 21700 cylindrical battery cell

The diameter of the 21700 battery is 21mm. To be more precise, it has an approximate length of 70mm and an approximate diameter is 21mm but technically 21700 cell size is allowed with some tolerance in length and diameter. Thus you could find specifications written as (say) 21 ± 0.41mm ✖ 70 ± 0.25mm on the datasheet and features of the li-ion cell.

21700 cell, as the name suggests, stands for a cylindrical cell with 21mm width and 70mm height. It was first introduced in 2017 by a Tesla and Panasonic collaboration. 21700 was introduced as an alternative to the long-running 18650 model, which was introduced by Sony in 1991.

The 21700 cell increased the working volume over the 18650 by a factor of >1.4x 21700 => ~21mm in diameter and ~70.0mm long These dimensions vary between manufacturers. Using data from the Cell Database we can see that 70g is a good nominal figure for the mass of a 21700 cell. The 21700 cell by definition should be 21mm in diameter and 70mm high.

However, most 21700 cells are based on lithium-ion (Li-ion) technology, which is widely used across many types of rechargeable batteries due to its excellent energy density and long lifespan. There are several types of lithium-ion chemistries that could be used within the 21700 format:

The most significant difference between the 21700 and 18650 batteries is their size and capacity. The 21700 is larger (21mm x 70mm) compared to the 18650 (18mm x 65mm), and this size difference allows the 21700 to store more energy. Capacity: The 21700 typically holds 5000mAh or more, while the 18650 generally maxes out around 3500mAh.

One of the key advantages of 21700 batteries is their energy density. Typically, 21700 batteries have an energy density ranging from 250 Wh/kg to 300 Wh/kg, depending on the chemistry used. This is a notable improvement compared to 18650 batteries, which usually offer around 180 Wh/kg to 250 Wh/kg.