An electric battery is a device consisting of one or more electrochemical cells with external connections provided to power electrical devices such as flashlights, smartphones, electric cars, and in our case, e-cigarettes. When a battery is supplying electric power, its positive terminal is the cathode and its negative terminal is the anode. The terminal marked negative is the source of electrons that when connected to an external circuit will flow and deliver energy to an external device. When a battery is connected to an external circuit, electrolytes are able to move as ions within, allowing the chemical reactions to be completed at the separate terminals and so deliver energy to the external circuit. It is the movement of those ions within the battery which allows current to flow out of the battery to perform work. Historically the term “battery” specifically referred to a device composed of multiple cells, however the usage has evolved additionally to include devices composed of a single cell.
The most commonly used sizes of batteries in e-cigarettes include; 14500, 18350, 18490/18500, 26650, and by far the most common, the 18650. As a general rule, the first two numbers (18XXX) will designate the diameter of the battery and the last three numbers (XX650) will designate the battery’s length; so an 18650 should measure something close to 18mm wide by 650mm long. In reality, they vary slightly from manufacturer to manufacturer, and sometimes even within a particular manufacturer’s range. All of these sizes output (ideally) 4.2v when fully charged and (preferably) are not used below 3.2v. The main difference between the different sizes is in their mAh ratings and amp draw ratings.
An important consideration when choosing a battery is the continuous amp draw rating, you generally have to sacrifice battery life for the sake of gaining higher continuous draw. I personally am happy charging a little more often if it means I’m never at risk of overtaxing my batteries, so with that in mind, I always recommend the LG HB6 for mechanical mods/rebuildable toppers and the Samsung 25R for regulated mods. The LG has only a 1500mAh capacity, but can output 30A continuous, while the Samsung can output only 20A continuously, it has a more respectable 2500mAh capacity.