What causes battery power loss?
Do you mean as far as everyday use or the longevity of the battery itself over time?
Hundreds of charging cycles and non-100%-compatible chargers can cause a battery to hold less charge and run down faster over time.
Screen brightness, display on time, number of processes and apps running, signal strength searching, WiFi activity, defective cells in a battery, and the type of computing hardware vs size of battery (mah) can all play havok on a battery's ability to last longer after a complete charging cycle.
