Power Struggle with Samsung Galaxy S3

Right after I got a Samsung Galaxy S3, I came to a realization that I'm practically tethered to a USB cable - a freshly-charged battery would last at most 10-12 hours. I bought an external battery, just for my own peace of mind and as well to convince myself that I actually do have a mobile phone. However, the external battery and cables turned out to be a lot bulkier than I thought and I decided to figure out what's wrong with the phone.