IPhone 4s charging problems - won't charge past 83%?

My iPhone 4S won't charge past 83%, but when I plug it in it shows the little plug symbol instead of the lightning bolt. The plug symbol means it is fully charged… Something is wrong. What can I do?

I've tried doing the reset by holding both the menu and power button down.

Thanks!

Added (1). PS, this has only occurred as of today. Never had a single problem with the battery before, it still lasts very well.

You need a new battery replacement.

I don't have a i phone I have a android phone
Mine was doing the same thing.
it did this for about 5 months then one day the phone just broke.
Turns out it was some charging thing with the phone.
maybe take it to apple and see if they replace the battery if that will help. Best of luck