What is an 18650 Battery?

The 18650 (pronounced “eighteen six-fifty”) battery is a type of rechargeable lithium-ion battery. The term 18650 refers to the battery’s dimensions – 18mm in diameter and 650 tenths of a mm in length. The zero at the end is often confused for indicating that the battery is cylindrical in shape, but that is not the case. Why is length is measured in tenths of a millimeter instead of millimeter? It just somehow became the convention for naming batteries.

So, what is a lithium-ion battery?  First, let’s define some terms. A cell is a single unit of device that converts chemical energy into electrical energy. A battery is a collection of such cells. A single-celled battery may often be called a cell and a battery interchangeably. A lithium-ion battery is a rechargeable battery that uses lithium ions to carry the current between the negative and positive electrodes. Li-ion batteries have become increasingly popular due to their high energy density and low self-discharge.

18650 batteries have risen in popularity in high-drain consumer devices because of their high capacity and ability to be recharged many times. They are often used in flashlights, laptops, electronic cigarettes, power tools, electric vehicles, and more. Multiple 18650 cells can be combined to form larger batteries to suit their intended use. For example, Tesla uses between 6831 and 7920 cells of 18650 in their batteries to power their cars.

When choosing an 18650 battery – or any type of battery for that matter – it is important to understand some key terms that you’ll find on their labels.

  • Capacity - Measured in milliampere-hours (mAh). The higher the mAh, the longer the battery will last.
  • Discharge Rate – Refers to the maximum continuous current that can be drawn from the battery. Measured in amperes (A). The higher the A, the faster the battery can be discharged.
  • Nominal Voltage – Measured in volts (V). It is the voltage level where the battery will spend most of its time during its discharge curve. It is usually around the midpoint between the minimum and maximum voltage.
  • Protected vs. Unprotected – Protected 18650 batteries have an extra layer of protection around the cells to protect the battery from overcharging, over-discharging, overheating, or short-circuiting. Unprotected 18650 batteries do not have this extra layer of protection and are therefore more susceptible to damage. Protected batteries are often slightly wider in diameter or slightly longer in length.
  • Self-Discharge Rate – All batteries will slowly discharge by themselves while in storage. A lower rate is better to ensure that the batteries will be ready to use when needed.
  • USB Charging Port – Some 18650 batteries have USB ports built into them so that they can be charged without the need for a dedicated charging device.

A single 18650 li-ion cell has a nominal voltage of 3.6V or 3.7V, with a minimum voltage as low as 2.5V and maximum voltage of 4.2V. The cell would typically reach up to around 4.2V when being charged then quickly drop back down to 4.0V after being removed from the charger. Capacity of 18650’s ranges from 1,800mAh to 3600mAh. Any battery claiming to have more than this is very likely misrepresenting its capacity. Discharge rate can vary, so it is important to take into consideration what device the battery will be used in when selecting an 18650 battery.