18650 vs 21700 Lithium Ion Battery

There are many sizes of cylindrical lithium-ion (Li-ion) cells, and the number of sizes continues to grow. Some are optimized for use in simple devices such as toys and flashlights; others are mainly found powering portable electronics and electric vehicles. 18650, 21700 and others simply refer to the physical size of the lithium-ion cell. For 18650, it’s an 18 mm diameter x 65 mm length. 21700 is 21 mm x 70 mm.

18650 – are longer and wider in diameter compared with an AA battery. While the 18650 measures 18mm in diameter and 65mm long, there can be minor dimensional variations between manufacturers. 18650 batteries are generally 3.6/3.7 volts and have capacity ratings from 2,300 to 3,600 mAh.21700 – were designed to be a larger and higher capacity replacement for 18650 batteries. Like the 18650, the 21700 has a nominal voltage of 3.6/3.7V. The 21700 was designed to replace the 18650 in EV battery packs. The capacity of these batteries ranges from about 4,000 to 5,000 mAh.

The most obvious benefit of going with 21700 vs 18650 is that the extra density means there’s more available energy in them to run your tools and gear longer. Batteries that we see using 18650 cells for power tools range from 1.5Ah (1500mAh) to 3.0Ah (3000mAh) in each cell. 18650 cells on the higher end of that scale have reported issues and 2.5Ah (2500 mAh) cells seem to be where most manufacturers settle at the top. On the other hand, 21700 cells start around 3.0Ah (3000mAh) and go up to 4.0Ah (4000mAh) for power tool batteries. It’s why we see compact (1P) packs that have 3Ah or 4Ah designations all the way up to 12Ah (3P) big boys.

From the perspective of comparing 21700 vs 18650 in power tools, you’re looking at a 50%–100% runtime gain over today’s standard 18650 packs. Outside the power tool industry, some of these cells reach 5.0 Ah (5000mAh).