Brocktune

I've read some similar threads but I haven't been able to piece together what is happening with my phone.

This started last Thursday. I've had the S3 for several months and have gotten into a routine of putting it on charge overnight (while still on.) Thursday morning when I got to work I noticed the battery indicator was already down to 60%. Normally this early in the morning it is still in the high 90s. I brushed it off and thought maybe I forgot to charge it the night before.

Friday I noticed that as soon as I disconnected the charger, on which the battery level was showing 100% charged, it instantly dropped to 69%.

Saturday, Sunday and Monday the phone charged normally overnight.

Tuesday morning - battery level is showing 100% while connected to charger, dropped instantly to 58% after disconnected. I quickly went into the settings to what the battery stats indicated. When I went back to the home screen the battery showed 100% again. However by the time I made it to work 1 hour later, the indicator was already down to 51%.

Do I have a bad battery, does it need calibrated? Any ideas?

Thanks.