My Lumia 920 runs out of battery in around 6 hours. I have full battery when I leave my home in the morning at around 08:50 and at 14:33 it's empty (Battery Widget). I have disabled all background programs at settings -> Programs -> background tasks. I have also disabled push notifications from mail and Facebook.
How can I increase the battery life? I need to have my phone working at least 12 hours a day with small tasks.