I had written a post before iOS 4 was officially released talking about how iOS 4 was a battery drain and whether or not you should upgrade to it. However, I did upgrade to iOS 4 and found it to be good feature wise. I have now been using the device for a couple of weeks and was disappointed with the battery performance after the switch.
Though iOS 4 has brought in a lot of nice changes like Folders and Multitasking, it also uses up battery pretty quickly even when the device is not being used. In my tests so far, the device holds up strictly OK and has a battery life of 7-8 hours in idle mode and approximately 3-4 hours with moderate use on an iPod Touch 3G. If you decide to play games or use heavy apps, the battery gets drained pretty quickly.
Now here is the problem, during the above tests, most of the apps I was using were not multitasking capable. With several apps already making use of the new multitasking feature, those numbers will go down even further.
Another problem I saw with my tests was that, if I left my device with 75%-80% battery overnight, the battery would run out in the morning. This is truly a pain since before iOS4 the iPod Touch would run without charge for 4-5 days. Now, I have to always remember to plug the device into the charging unit before I sleep. Also with heavy usage I have to usually keep the iPod Touch plugged into the charging unit.
This is definitely disappointing and like I had said it was something I had expected. You might see the same problem on an older iPhone. So if you want to use iOS4 you are better off doing it on an iPhone 4 which has better battery life and was created specifically for iOS 4 features.