You can use pretty much ANY battery that will fit the stock... BUT... if the gun is designed for a 8.4V battery :
- A slightly lower voltage battery (such as 7.2V) will reduce the rate of fire of the gun and may not run high power springs
- A much lower voltage (lower than 7.2V) will not give enough power to the motor to run the gun
- A slightly higher voltage (9.6V) battery will make the motor run faster, increasing the rate of fire and allow higher springs to be cycles. Motor life will be slightly reduced.
- A much higher voltage battery (10.8V, 12V and +), your gun will run incredibly fast, you'll probably strip your gears and/or piston and burn out your motor (and I talk by experience, an AEG set up with High-Speed internals and powered by a 12V Motorcycle battery is fun, but it doesn't last long...)
The MAH number is the capacity of the battery, higher is the number, longer the battery will last. Higher mAh will also provide a more 'stable' voltage under load.