Batteries are the predominant cost in the price of a cordless tool. The batteries control the tool's runtime, and (more or less) the power that the tool outputs. There are a few main types of batteries for power tools, and each has different maintenance and usability requirements.
It is important to remember though that a cordless tool is only as good as it's battery. You can also get 18650 lithium ion battery online.
Voltage is a measure of the power of the power source (for a certain current level). Rechargeable batteries for power tools typically contain cells with a voltage of 1.2V, 1.5V, or 3.6V. Manufacturers use these cells to make batteries that can have voltages between 3.6V and 48V.
The battery voltage does not necessarily imply the specific power output of the instrument. This is determined more by the quality and design of the internal components of the tool, especially the gearbox. Thus, it is very common to find a high quality 14.4V instrument that is more powerful than a low quality 18V instrument.
Batteries with voltages between 3.6 V and 12 V are typically developed in light instruments, while batteries with voltages between 14.4 V and 36 V are made for use with heavier instruments. The most commonly used voltage among manufacturers (due to weight, price and potential output power) is 18V.
Once fully charged, the maximum operating time of a wireless instrument is largely determined by the battery boost value (Ah). Basically, this value represents the instrument's fuel tank. The bigger the fuel tank, the longer it will last.