I have an android kit kat 4 and the battery runs down fast why?

[AC Question]

I plug into my charger overnight and the battery does not seem to be fully charged.

 

[reactorh]

U need to change to battery to a new one immediately!

 

[TenshiNo]

reactorh is correct. Lithium Ion batteries degrade over time, losing their ability to charge, or hold a charge. This occurs normally, over time, but can be accelerated by things like heat or letting the battery's charge get too low. This is why your phone starts telling you to charge, once your battery gets below 15% or so. It's not because Google is worried that your phone might die, it's because you are "damaging" the battery by continuing to use it when the charge is that low. Even charging the battery causes it to age. Lithium Ion batteries very much prefer many short charges, rather than charging all the way back up from a very low charge. This called a "deep cycle charge" and is the most common reason people see their device's battery capacity diminish over time.

This article from Battery University explains in much greater detail, if you're interested. Short version: it's time to replace the battery.

How to Prolong Lithium-based Batteries - Battery University
Charging Lithium-Ion Batteries – Battery University