The battery does not charge to 100 %, this is not a defect in the phone but rather a sort of internal protection so as to not damage the battery. Indeed, it happens many times to leave the phone connected to the charger much more time necezzario.
The battery by doing so you may receive a surplus of energy than necessary, and in the long run, deteriorate more quickly. Samsung has inserted inside the Galaxy S3 a limiter that operates in the following way: while the phone is charging, you download slightly of 2-3% for then recharge automatically up to 100 %. This process is repeated automatically from your phone until it is removed from the charger.
Use this app.for monitoring battery ;
https://play.google.com/store/apps/...ch_result#?t=W251bGwsMSwxLDEsImNjYzcxLmJtdyJd
If necessary make a battery calibration uses these applications ;
https://play.google.com/store/search?q=calibration&c=apps