It is gravity. As a sky diver falls (ignoring air resistance for now) he increases in acceleration due to gravity, or 9.81m/s^2 (Earth's gravitational acceleration). Inertia is, by definition, <span>a
property of matter by which it continues in its existing state of rest
or uniform motion in a straight line, unless that state is changed by an
external force (credit to Google). However, if the sky diver has some kind of webbing on his suit, a drag force opposite to gravitational weight cancels it, and it moves at a constant drag velocity.
The state of charge of the battery will determines the amount of current which will flow through a battery while it is being charged by a constant voltage source. A battery supplies electric power , battery is characterized by an equation with voltage and current variables.