2011 iPhone 4s First-Time Battery Replacement
The battery's lifespan has severely degraded after 7 years. The indicator gets stuck on 1% for a long time.
It went well with the exception of the removal of the 1.7 mm Phillips screw by the battery connector. The reason it took my battery replacement process 3 hours is that I stripped the aforementioned screw. The included iFixit screwdriver's Phillips screw attachment for reason didn't go well with the 1.7 mm screw. After I miraculously removed the stripped screw, everything else went without a hitch.
Don't strip any screws. If you think you are careful during the unscrewing process, you should probably be a little bit more careful.