Just like in a car, batteries wear out. Mine was three years old, so I replaced the battery as normal maintenance.
I was careful to follow the instructions and an additional YouTube video. It went well without any problems. I chose not to detach screen, used a rubber work pad, and a can of soup with a rubber band to hold screen in place vertically with body of phone horizontal on table.
The pentalope screw driver gripped screw really well. Neither Phillips head screw driver seemed to fit their screws. Seemed either too small or too big. But, being careful to seat the tool into the screw head, they were removed and replaced successfully.
Instructions are a little vague on how to pull old battery adhesive out. If body of phone is flat on table, pull glue strip towards bottom of phone parallel with table top. This is made easier if vibration motor below battery is removed first by unscrewing two Phillips head screws.
By leaving screen on, only six screws need be removed in total. Two at bottom of case, two securing battery clip, and two to remove vibration motor.
Battery case is slightly larger than battery. By placing new battery towards top and opposite side of power clip, it is easier to align clip with battery posts.