ATTENTION: Also read HowTo: Get Battery Percentage on iPhone 2G/3G Running OS 3.0
- SSH into your iphone
- Go to /System/Library/CoreServices/Springboard.app/
- Download the file M68AP.plist
- Open the file using a text editor or a plist editor
- Add a Boolean value under the capabilities dictionary
- Boolean Key = gas-gauge-battery / Boolean Value = true
- Save and upload the file to the same directory that it was originally in
- Go onto the iPhone and respring
- Now go to the application Settings -> General -> Usage and there will be a Battery Percentage toggle and you can use it to enable battery percentage next to battery icon in top-right corner.
props to wgm214
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