My four year old Iphone 4S was struggling to hold a charge for more than 4 hours. I decided to order a replacement battery and recommended tools from Ifixit site, and followed their replacement repair guide exactly. I suggest that anyone planning to do this assemble some extra items prior to starting the replacement- latex or vinyl surgical gloves (to keep your skin oils off delicate parts), fine twissors to pick up the small screws, and plenty of light on your stable work table. Also consider spreading out a large white paper or paper towels to provide a high visibility surface on which to work. Finally, have an alcohol swab or alcohol with cotton ball handy to wipe down the battery connection plate- removing any fingerprints or skin oils you may have left during dis-assembly and re-assembly.
I had no problem removing the case screws and removing the back. Removing the small phillips screws holding the battery connection was a bit hairy but the plastic wedge tools helped lift the part up without damaging the four connection grid below. Using a tray to hold all screws, pressure pin, and other parts during disassembly is a must as the screws can be easily confused.
Patience and delicate movements are called for throughout the replacement, and you should have the online replacement pictorial guide open on your desktop or Ipad to give you a clear sense of what you should touch and not touch. Remember- this is a delicate device- not a carburetor!
I removed the battery using the wedge tool and the plastic tab inside the phone- the glue slowly yielded. It was then a matter of setting the old battery aside, and positioning the new battery inside the phone, angling the top of the battery in first. Remember to tuck the battery connection strip flat and beside the battery in the narrow space, pushing the connection strip into the motherboard.
Next, I positioned the battery pressure clip underneath the retaining plate, and inserted/tightened the upper Phillips screw. Then I inserted and tightened the lower phillips screw, checking to make sure all connections were made, screws were tight (but not overly tight to the point of stripping the screw or its head!).
Finally, I slid the case back onto the phone making sure that it was 2 mm higher than the case and then pushing it flat to align with the metal case. It was then easy to push the case back toward the bottom of the phone, closing the 2 mm distance, and making it an easy matter to re-insert and tighten the pentalobe case screws. Although it was recommended to buy replacement phillips screws to use in place of these easily stripped original screws, I found that the original screws tightened without damaging them. I doubt they would hold up to mutiple removals over time, but for now, my phone repair was complete.
I flipped the phone over, and saw that it had automatically rebooted. Within two minutes, I had complete wireless and phone system access. I will follow the new battery directions card advising installers to let the battery drain to below 10% and then charge up to 100% to callibrate the battery.
All said and done, I was very impressed with how straightforward Ifixit made this battery replacement. Rock on Consumers- fight against upgrade, and make those classic phones work harder and longer!
See my narrative above for the additional items (gloves, paper towels