Independent energy sources for portable devices have always been a matter of research and discussion. It all depends on the particular application the source will be used in. Requirements will not be the same for motors, lighting – whether stationary or mobile – low current drain devices, appliances permanently ON or frequently switched ON-OFF, etc. For use in flashlights, it appears that the most desirable features are the energy density that can be stored, the capacity of outputting high current, the size and weight.
Today there is a wide range of battery cells available.
Arguments for selecting the most adequate type of battery are beyond the scope of this paper. We shall focus on the particular 18650 battery which is of the Li-Ion rechargeable cell type.
A word on terminology. Strictly speaking, a cell is a single element which outputs its natural voltage (e.g. 1.5V for an alkaline primary cell). A battery is an assembly of several cells allowing for a higher voltage (series connection) or higher current and capacity (parallel connection) or a combination of both. However, in common language, it is usual that a cell used as a finished element be referred to as a single-cell battery or just a battery for short.
Why 18650 lithium battery?
Before flashlights adopted them, 18650 battery cells were industrial parts mostly used as components making up larger battery assemblies (e.g. laptop batteries). Then flashlight manufacturers adopted them in view of their characteristics which make them very suitable for that particular use.
These batteries are called 18650 because of their (approximate) dimensions: 18.6 mm in diameter × 65.2mm in length. This style of denomination has become a sort of standard in the industry. For example, we can find 14500 cells (14×50 mm), the size of an AA alkaline battery.
- But watch out: some brands are slightly bigger than others with the same denomination so you’d better check for compatibility with your flashlight before investing your money.
Most of the flashlights that take two CR123s will take one 18650. The difference in voltages is taken care of by the electronic circuitry of the flashlight.
18650 batteries are lithium so they pack extremely high energy density. Output voltages vary from 3.7V to 4.2V and the power to weight and size ratio is better than with other technologies. They usually have a very low self-discharge rate and no memory effect. 18650s usually can be recharged up to 600-1000 times, depending on the brand.
Due to their chemical content, batteries in general are potentially dangerous parts. Usually, any 18650 battery has a protection against over temperature (PTC) and over pressure (CID).
There was a time when 18650 cells were reputed to be prone to explode but this was changed with the addition of a Protective Circuit Board (PCB). Such batteries are called Protected 18650s. The PCB is inserted at the bottom of the cell. The protection works against over current (short circuit), over charge and over discharge. A protected 18650 is slightly longer and bigger than an unprotected one. They should be named by the proper designation: 19670 (19mm x 67mm) although for historic reasons, most people including manufacturers keep using the incorrect designation: protected 18650.
Best batteries to choose from
We present two brands of 18650 batteries. Both are PCB protected. They are top quality and very durable. Both get top review marks from multiple reviewers.
- High capacity: Panasonic NCR18650A 3400 mAh but also high proced:
- More affordable price: Tenergy 18650 2600mAh Protected Battery (If a little bit lower quality is not an issue get two of these instead of one of the above).