Battery are the power source for electric airsoft guns (an AEG)
Batteries in airsoft are used for powering AEG's. They range in voltage from 7.2 to 12 volts and vary in capicity from 200 MaH to 480 MaH. Increasing the voltage of a battery in a gun increases the rate of fire of the gun. Increasing the MaH of a battery increases battery life.
AEG's use batteries to power the gearbox which fires BB's. They use batteries ranging from 7.2 to 12 volts and range in capcity of usally 1000 to 4800 MaH. The most common voltage is 8.4 volts and most guns come with a 8.4 battery stock. A common upgrade is a 9.6v battery which increases rate of fire slightly and usally has a longer battey life (due to higher quality cells with more capacity.
In airsofting 3 main types of batteries are used:
Commonly called Nimh, This is the most common airsoft battery available. They are know for reliablilty, power, and are relatively inexpensive. This battery consists of cells which allows them to be arranged into a varity of shapes which makes them very versatile.
Nickel-Cadmium also knowns as nicd batteries were used before NiMH became widely available. They can store more energy per cell than nimh which means better battery life. However, they are most susceptible to the memory effect (see below). They are also more dangerous to the environment that nimh batteries. This battery also consists of cells like the Nimh.
Comonly called LiPo, Lithium-Polymer batteries are some-what new to the airsofting community. They have more energy capacity than nimh and NiCD which allows batteries to be smaller than nimh and nicd batteries with the same power and capacity. However, they are currently the most expensive batteries on the market and most guns can't take them out of the box because they are too powerful. Also, they are limited mostly to brick shaped which limits the guns which have different cell arragnments but due to their small size this is rarely a issue.
These are most often used in cars but in minigun's, they use them as a power souce to spin the barrels due to the heavy weight, it takes a incredibly robust and powerful battery to spin them but they large and very heavy which make them useless for most airsoft guns.
The memory effect
With all rechargeable batteries, as they age and recharge and are discharge, an effect called the memory effect takes hold. It decrease the storage capcity of the battery slightly each time and after enough cycles, the battery can barely hold a charge and becomes useless. NiCD batteries are the most supceptable to this. Lipo are the best at resisting this effect and some tests have shown they can survive 10,000 cycles. Nimh are inbetween nicd and lipo.