There are many types of airsoft batteries that you can use to power your electric airsoft guns. Just for the record, batteries are only used for electric airsoft guns, not spring or gas ones. That should be inherently obvious, but sometimes it is good to explain everything just so there is no confusion.
Depending on what kind of electric rifle you have, you may need a special type of battery, and not all airsoft guns come with batteries (although most of them do). Another reason that you might want another airsoft battery is just to have a replacement lying around, in case your battery runs out of energy and you need to replace it. As you can see, there are many instances in which it would be useful to have an airsoft battery on your side.
One popular type of battery is the Large Type 8.4V 3600mah. This battery is used mainly for large airsoft rifles that need a fair amount of power, but this is actually not the most powerful battery out there. Since it is still large, it costs over forty dollars, but that is not bad compared to some of the other batteries out there.
A very large battery is known as the Crane Stock Battery, which is a 9.6V that runs at 4200mah. This is clearly more powerful than the preceding battery that was described, and it is about twenty dollars more expensive. Some rifles can either use a large battery or a crane stock, and the crane stock simply delivers more power to the rifle. This interchangeability makes the rifle that has that capability a lot more valuable, so you should consider getting a product that is able to use both types.
There are a multitude of small batteries as well, that are meant to be used with pistols. These can cost as little as twenty dollars, and they are fully rechargeable and can be used for a long time.