Solved: Poor battery life on iPhone 3GS using Push / Exchange
Recently I upgraded to the iPhone 3GS and noticed almost immediately a significant drop in battery life. The first day of usage I was seeing the %age meter drop at an alarming rate ending up with about 5 hours of standby in the first day.

After some initial testing I realised that my usage time was increasing (albeit at a slower rate than the standby time) but even when the phone was idle.

To quickly explain "usage" and "standby", Usage is how long you've used the phone including background tasks like push, fetching e-mail, receiving SMS messages etc. Standby is how long the phone has been switched on and includes usage time. So, if you have 10 hours standby and 5 hours usage this doesn't mean you've had 15 hours in total. It means the phone has been switched on for 10 hours and of that, you've had 5 hours usage.

Anyway, what I noticed quite quickly was that when my usage time was say, 1 hour 30 minutes and the phone was left on standby for 10 minutes, it would end up showing 1 hour and 36 minutes of usage. This didn't add up.

The first thing was to turn off Push and immediately there was an improvement. This isolated the issue and so I tried removing the account, re-adding the account etc. I even tried switching to another test, Exchange server and running that with Push - nothing improved the situation unless I turned push OFF or used a different Exchange server and account.

Having determined that it was a combination of the 3GS, Push AND my Exchange server I went into Exchange, created a new folder and MOVED all my Inbox e-mail to a new folder "Old Inbox" leaving the default Inbox blank.

Problem solved.

The issue was *something* to do with e-mails in my Inbox which were causing the 3GS to almost continually be connected to the account. This was confirmed when I studied the log files on the Exchange server and could see in the days leading up to the 3GS switchover, file sizes of a few meg in size. Immediately after the switchover the file sizes quadrupled in size and once I applied the fix, returned to normal again.

What's interesting is I didn't get this on my 3G, only the 3GS.

Anyway I now have the battery life I'd expect on the 3GS and it's significantly better than that of the 3G even with Push e-mail and Push notifications switched on in OS 3.0

If you're using Exchange, the 3G/3GS and Push e-mail and experiencing battery life issues, I hope this helps!

Source: http://jasonkneen.blogspot.com/2009/08/solution-poor-battery-life-on-iphone.html