Help! My Phone Won’t Hold Charge

Ok, so it's not enough that I have ZERO signal at work and cannot receive or send text messages or phone calls whenever I want to but now THIS is happening! Geez! I sure paid enough for the phone and the same for the service. I walked in this morning at 6:00 a.m. with 100% battery and a few short hours later, when I get off the air I'm at 20. What the heck?

Anyone have a Samsung like mine and if so what causes the battery to run out so quickly? Can ya help a girl out, this only started happening this week! Stupid phone. lol The struggle is real with celly's and I know a lot of people suffer from the same problem.