When it comes to batteries in your gun there are two main types of batteries. There are Nickel Metal Hydride (NiMH) and Lithium Polymer (LiPo). The NiMH is the standard type of battery that almost any Automatic Electric Gun (AEG) can run off of, whereas the LiPo usually requires a MOSFET to be installed to use.
First off we will touch on what each of the components of the batteries mean. The rate of fire of the gun is dependent of the voltage of the battery. The higher the voltage, the faster the gun will fire. The mAh, or Milliamps hour, determines the life of the battery between charges. A 1600 mAh will normally last all day at an 8am-5pm Airsoft Event, of course is all about how much you are firing.
The NiMH batteries commonly come in 7.2, 8.4, 9.6, and 10.8 volt whereas the common batteries for the LiPo are the 7.2, and 11.1 volt. Note that the 7.2v LiPo gives AEGs the same rate of fire as a 9.6v NiMH. They both come in wide variety of milliamp hours (mAh) as well. There are also 4 main style of battery. They consist of the stick, brick, nunchuck, and PEQ box. There is also a fourth call the Butterfly however this is non-compatible with a majority of guns. The stick style battery is used mostly in the AK platform, the brick in most fixed stock non AK platforms, the nunchuck in most front-wired, and crane stock non-AK platforms, and the PEQ box battery in front-wired guns with a PEQ box.
The 7.2v LiPo can be used in most guns without a MOSFET however it is not recommended. The 11.1 LiPo requires a MOSFET as the LiPo battery produces more energy than the NiMH and the MOSFET helps absorb some of the excess energy so you don’t risk over heating your internals. The MOSFET can either be attached to the wires in the gun or you can use a drop in MOSFET which just requires connecting the battery to the drop in MOSFET and connecting the MOSFET to the wires in the gun.
I hope this was useful in your future battery purchases. -DevilGuardian
