These are two lithium batteries, but the specifications of lithium batteries are different and the voltage is the same. But the difference is in the size and capacity of the battery. The 21700 is significantly larger than the 18650. size and capacity.
These battery models are the appearance specifications of cylindrical battery. The first two digits are the diameter of the battery, the two digits 34 are the length of the battery in millimeters and the last one. The number is 0, whatever it is. This model only describes the appearance of the battery and does not include the battery type. For example, 18650 specifications include lithium-ion batteries, lithium-iron phosphate batteries, and nickel-metal hydride batteries. , which is the appearance specification of a common AA battery.
Early R&D
Lithium batteries were first used in cardiac pacemakersques. Lithium batteries have the advantages of extremely low self-discharge rate and gentle discharge voltage, allowing pacemakers implanted in the human body to work for a long time without recharging. Lithium batteries typically have a voltage rating above 3.0 volts and are best suited for use as an integrated circuit power supply. Manganese dioxide batteries are widely used in calculators, digital cameras and watches. In order to develop higher performing varieties, various materials are researched to create unprecedented products.
Reference for the above content: Baidu Encyclopedia-Lithium Battery