These airsoft guns are powered by battery’s that connect to the motor of the gun and or normally stored inside of the gun and sometimes depending what gun it can be stored on the outside of the gun. There are two main types of batteries that an AEG uses, These are lipo (lithium polymer) and NiMH (Nickel Metal Hydrate).
Advantages of an AEG
First of all these guns are very quite. Since it is a battery all you hear is a small sound coming from the gun. The weight of an AEG is normally quite light compared to a GBBR. To run the gun and maintain it is very simple, plug your battery in the night before what a few hours and done your ready for a full day of airsoft. The maintenance is also very simple. There is none. The only time it will need to be oiled is if the gun is taken apart.
Disadvantages of an AEG
There really are not alot of disadvantages to and AEG the only significant one would be if the battery runs flat then you will need another one just incase. That is a minor problem.
So in turn the advantages heavily over-weigh the disadvantages.