battery level suddenly drops and shuts down iPhone 4S

I have a 4s phone that is alittle over 3 months old.  It has just started a problem. It shows say 60% battery life on the phone and suddenly drops to say 10% and then shortly after that it shuts down completely. Then when the phone is plugged into a charger, the battery life shows 60% again.  I have closed out all apps running and have shut off wi fi. I have also done several other "tips and tricks" that I have found on here to try.  Nothing helps. I have taken it to 2 different verizon stores and they have done a cold shut down and restarted it.  The other said that it needed to be updated, I have since done that and it is still doing it.  Very frustrating, I run around with chargers everwhere, because I never know when it's going to die!

Any suggestions???

Jodi