My iphone 3GS was turning off and rebooting by itself repeatedly in about 3-5 minute cycles. Also was not charging properly. I looked up the problem and the consensus was that the battery should be replaced. Ifixit was strongly recommended as a reputable parts and information source on several sites I researched.
I purchased the battery replacement kit from ifixit and studied the instructions. The instructions are detailed in the exact process required to do the repair. I followed the instructions, replaced the battery and now the phone works as it should. Sounds simple and straightforward, and it was. Took less than an hour.
Well, of course it helps to have some experience with removing and replacing delicate ribbon cables. Those things are like paper...be very gentle. If you get frustrated, walk away and come back when you chill out.
TEENY TINY SCREWS- These screws are small as dust particles... (lol). Don't drop any and keep them organized. Know where each screw belongs. Maybe work over a cloth or something so if you do drop it, it will not bounce off into infinity. The ifixit screwdriver seems to be magnetized, so that helps, but the problem is more in handling them. There are 3 or 4 different sizes of screws and the difference is not visually apparent (to the naked eye), so keep track of them with little plastic bags or some sort of trays or an egg carton or something.
REMOVING THE DISPLAY- (Step 3,4) When you remove the display with the suction cup, be sure to put a piece of wide strong tape (duct/masking/gaff) across the display and case end with the switch and headphone. That is the side opposite from where you are pulling with the suction cup. This will make a hinge for the display bezel. All the display connections are just under the display on that end, and you do not want to have them get broken when you pull the display away from the case. I heard some horror stories of that happening. Don't let that happen to you. That would kill the phone and render your purchase irrelevant.
Also hold the phone with one hand in a manner where your thumb and another finger can act as "stoppers" to prevent the display from yanking out too far. You only need the display to come up a little bit. You can then spudge it up further.
THE CONNECTOR 3 RIBBON CABLE- (Step 7) My phone has the slide-out ribbon cable. Removal is simple enough- just slide it out. However, reconnection is a bit tricky, and I ended up using a small needle-nosed pliers to grasp the ribbon and push it back into place. Of course, you must first position the ribbon partially (half way) into the socket. It is challenging to hold the phone, eyeball the socket and place the ribbon and then hold the phone so the ribbon does not slide back out…be patient!
STEP 12 re-assembly. When reinstalling the motherboard, be certain that the screw holes line up. They will line up when the motherboard is correctly in place. This can be a bit tricky to accomplish. The main thing that may get in the way of this is if the SIM card holder is not lined up in its bracket on the phone case. If you have a problem getting the motherboard into position, check the SIM card holder position.
And don’t forget that camera holding bracket!
OVERVIEW- Experience helps, but if you are patient and gentle, you will get your phone back to it’s proper condition.