There is now a new jailbreak application available on Cydia which helps in extending battery life on iPhone 4, 3GS and 3G. Available by the name of Auto3G on BigBoss repository, 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.
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.
You will of course need to have a jailbroken iPhone to install it. Follow our step by step guide posted here to jailbreak your iPhone, iPad or iPod touch with JailbreakMe if you haven’t already.
Auto3G can be downloaded via BigBoss repo on Cydia for $5.99.