The battery percentage display is only applicable for iPhone 3GS and it is not turned on by default. Navigate to Settings, General, Usage and set Battery Percentage to On and the indicator will immediately be displayed beside the battery bar.
But, this does not help to optimize your iPhone battery life. You can check how heavy is your iPhone battery life being used on your daily usage. So that, you can notice what are the features consume more battery life.
I’ve noticed that surfing the web by enabling 3G on my iPhone 3GS, the battery consumption is more compare to surfing the web through Wi-Fi connection.
