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21700 vs 18650 Battery are both rechargeable lithium-ion batteries used in various devices. A 21700 battery is larger, measuring 21mm in diameter and 70mm in length, while an 18650 battery is smaller, with dimensions of 18mm in diameter and 65mm in length. 21700 batteries offer higher energy density, making them suitable for electric vehicles and devices requiring a longer range. They both operate at 3.6-3.7 volts and can be charged with a standard lithium-ion charger.