There hasn’t been a clear tutorial for this, so I’m writing one:
- Jailbreak your iPhone.
- Install OpenSSH through Cydia.
- Copy the file /System/Library/CoreServices/SpringBoard.app/N82AP.plist to your Mac.
- Using Property List Editor add a new boolean child to the capabilities property list: gas-gauge-battery. Save this file.
- Hop over to Terminal and run a bit of code to convert the XML to binary: plutil -convert binary1 /path/to/N82AP.plist
- Replace the iPhone’s property list with the one you just created.
More details about how to use command line tools (SSH, SCP) to make this happen are left up to the reader. After doing this you should be able to access the toggle switch as is available to the 3GS by default.