About 2 years ago, my iPhone started shutting off at about 25% power. I thought it was because of a nanosecond power outage that occurred while my iPhone was plugged into my laptop. Now, I think it was because the battery was on its last leg of life. I didn't want to send it to 1 of Apple's service centers because it's the only cell phone I had (& have). As time passed, I noticed that the phone shut off at higher and higher percentages (35%, 50%, 75%, 86%). I searched online for answers because I didn't know, at the time, that it was the battery that was causing the shut-offs. Numerous findings helped me realize that my iPhone was shutting off because the battery was dying. In a forum, I read a comment that directed me to the ifixit website for parts and repair guides. I ordered the iPhone 3GS battery replacement kit, plus the black spudger (a recommendation from a commenter). I was hesitant to replace the battery in such a small device as the iPhone. I waited about a week after the battery replacement kit arrived before taking on this task.
Because I imagined the worst scenarios but knew I had to replace the old battery in my iPhone 3GS with a new one, with trembling hands, I embarked on the journey to new iPhone life. First, I watched the battery replacement video here in ifixit and read all the steps (& looked at all the pictures); however, the video was not detailed enough for me to feel comfortable enough to perform a batteryectomy on my iPhone. I used a YouTube video, instead.
I had to stop and start the video several times along the process, so the total time it took for me to replace the battery was much longer than the 20-30 minutes ifixit stated on the replacement battery web page. Also, because I had never done this before, I knew that the time would be much longer for me.
If you are thinking about replacing your iPhone 3GS battery, keep in mind that the parts are small, and the screws are even smaller! Tiny, to be exact. I wear eyeglasses, so I'm fully aware of the tiny screw difficulties I encountered in the battery replacement process. I used a plastic lid with a piece of masking tape rolled up and stuck to it (that's where I placed the screws when I removed them from the iPhone). I'm very grateful for the magnetic Philips head screwdriver that came with the kit as that made screw-removal and screw-placement much easier. Taking out the screws was ten times easier than putting them back in, especially the camera's screw with bracket. That tiny screw (and equally tiny bracket) gave me some trouble (and caused me to spew several expletives) before I was able to successfully put both back in place.
Even though I also ordered the longer black spudger, I didn't really have to use it that much. Mostly, I used the smaller green spudger that came with the kit. The suction cup with the attached key ring (highly recommended, by the way) helped me open the iPhone after I removed the docking screws. Overall, the experience was not as unpleasant as I thought it would be, and the end result was a successful battery replacement. While my phone was booting up, I video-taped it and posted that short piece on my Facebook profile page. Toward the end of the video, I joyously exclaimed that the battery replacement was a success!
No rogue ravens flew in to snatch my iPhone, but a couple of my cats did try their best to botch the job. They were, however, banished to the living room floor for the duration of the process. Fortunately, for me, they found cozy napping spots far away from my desk.
I thought about having a pair of tweezers on hand during the battery replacement process, but I was in the middle of it and getting out of my chair and away from my desk was too tempting for any of my three cats, who would have relished lying on my desk (and hair-ifying the iPhone parts) while I was looking for the tweezers. I thought, too, that a pair of plastic tweezers would have been better; I don't have such a pair, so I pressed onward to replace the battery. Most of the problems I encountered had to do with putting the screws back in their little holes (e.g. The camera screw & bracket quickly come to mind). I did have a little difficulty keeping some of the cables up while I was putting the new battery in, particularly the cables toward the bottom of the phone (this is where a pair of plastic tweezers would've come in handy, as well as a third hand).
One piece of valuable advice is to keep the pets away at all costs. I replaced the battery in the afternoon because usually that's when my cats are trying their best to catch up on their 16-hour beauty sleep (btw, a cat-nap is not short). If you have pets, keep them away and, once you've removed the iPhone's innards, air-spray it to chase away the inevitable pet hairs you will no doubt see.