I found to get through 15 hours is to turn mobile off every time I am not using it.
I streamed lastfm at work for 3 hours and turned everything off so it would not
be hitting the internet. Checked email and messaged through out the day. But everytime I put it down for any reason its turned it off. It lasted from 6:30am till 7pm and still had 2o% left. It has got better but its pretty weak.