Auto3G is a new jailbreak application that helps for extending battery life on iPhone 4, 3GS and 3G.This app basically turns off the 3G radio and switches to 2G (EDGE) when you lock your phone,and turns it back on as soon as you slide-to-unlock your phone again.
Requires iPhone with 3G
Auto3G extends your battery life by automatically turning off the 3G radio (leaving on the 2G radio) when you lock your iPhone and automatically turning on the 3G radio when you unlock your iPhone. This way you always have the speed of 3G when you need it without the battery loss.
As explained by Apple, this results in up to 100% greater battery life: “7 hours of talk time on 3G, 14 hours of talk time on 2G.” (apple.com/batteries/iphone.html)
Use the Auto3G icon to change settings
This App is particularly very useful for the iPhone users and can increase the battery life for 4 to 5 hours which is remarkable indeed. Only downside of this app is the higher price tag US $5.99.
You will of course need to have a jailbroken iPhone to install it.Follow the step by step guide posted here to jailbreak your iPhone.
Thanks to Redmond Pie