No. As you can see in the picture there's a cogwheel that is only half cogged. This pulls the piston back, and then when there's no more cogs, it lets go, and the spring pushes the piston forwards. So no matter how fast or slow it takes for the cogwheel to reach that point, the spring takes over there and piston travels with the same speed forward - so battery should not affect FPS. Thou hypothetically.. if the motor was damn fast and you was firing it on auto the cogwheel could grip onto the piston before it travelled the whole way and it would get lower fps.. not sure if this is possible thou, or if theres a function to prevent this from happening (I'm no gerabox expert - I fiddle with them as little as I can!!)