Some airsoft batteries do it in the rifle’s butt stock.
Airsoft Batteries come in several different voltages.
They also come in different levels of power storage, measured in milliampere-hours (mAh). The mAh is one-thousandth of an ampere-hour (i.e., 3.6 coulombs), and is a technical term for how much electrical charge a particular battery will hold.
More to come…