18650 BATTERY PACKS

18650 battery packs are rechargeable, lithium-based battery packs which use a component cell commonly known in the industry as an 18650 (“eighteen six-fifty”).

This nomenclature is actually a description of the cell itself, based on its dimensions. 18 denotes the approximate diameter of the cell in millimetres. 65 is the length, also in millimetres. The final number is a reference to the fact that for a cylindrical design, there is no third dimension necessary – so it is a zero.

18mm x 65mm x 0mm = 18650!

As hinted at in the title, there are many cell sizes available for a rechargeable battery pack and the most common cylindrical ones are 18650, 21700 and 26650 but there are more. Again, the name of these cells comes from the dimensions – i.e. 21mm diameter, 70mm length and “0” height for a 21700 and 26mm diameter, 65mm length and “0” height for a 26650.
Within these physical cell formats, a variety of chemistries and chemistry variants are found. That chemical mix, together with the details of the internal physical construction determines key performance factors such as:

Usually, there is some trade off between a cell’s various performance features. For example, a cell designed for power tools will typically have the ability to deliver very high currents for a high-power requirement, but the energy density is likely to be lower than a cell optimised for lifetime.

The most common cell chemistries found are lithium nickel manganese cobalt oxide (NMC) or variants thereof and lithium iron phosphate (LFP or LiFePO4).

NMC type cells will be commonly referred to as “lithium ion” an is extremely popular, due to a great balance of energy density and lifetime and cost.
LFP or LiFePO4 (known as lithium iron phosphate) may deliver 10x the cycle life as well as much longer calendar life than NMC, but at the cost of energy density – meaning a bigger and often more expensive battery at the point of purchase. It may be a more cost-effective solution over the product life, however, and is considered a more robust and marginally safe product.