Is iPhone 3G and 3GS battery the same?
To answer the OP as long as the mAh is the same it will not be a problem. The batteries are not officially replaceable by the user.
How do I get the battery percentage on my iPhone 3GS?
Open the Settings app. Select the “General” menu. Select the “Usage” option. Toggle on the “Battery Percentage ” switch.
How long does iPhone 3 battery last?
When did iPhone 3GS release?
Hold the sleep/wake button at the top right and the home button at the same time. Hold them until the screen goes black. You may even need to keep holding them after the red power off slider appears. If it does, just keep holding the buttons.