5 - 30 minuten
I've had my iPhone 4 for years and the battery life had deteriorated to the point that I had to charge it every night.
I considered my options: Buy a new phone ($$$$), or get the battery replaced at an Apple repair center ($$). Then I ran across iFixit with their replacement battery and detailed replacement instructions for so little cost that I figured I'd give it a try.
Following the instruction I opened the iPhone and removed the old battery. However when I put the new battery in I could not get the connector to fit properly so I gave up, put the old battery back in and closed it up. Several days later I figured I'd try it again. I finally got the connector to mate correctly after bending the metal tab on the connector very slightly to allow it to fit. It wasn't easy to get everything lined up but it finally snapped into place. After I got everything put together I had to put the charger on it for quite some time but eventually the phone came alive. A couple hours later the charge showed 100% and I'm just now going to see how long this new battery lasts. It's been about 12 hours now and the battery shows 98%, so I'm very encouraged.
The battery connector fits into a slot that is very tight. The tolerance on the supplied connector metal tab was such that it was just too wide to fit. I carefully bent the end tab a tiny bit and this along with re-shaping the flat cable allowed the connector to snap into place.
If you haven't worked with delicate electronic equipment before I would suggest getting some who has in order to perform this task. The installation process was very finicky and you could cause some damage if not done correctly. Would I do it again? Yes, now that I know what I'm doing!