Battery performance in iphone 5 started nose-diving badly after 4 & 1/2 good years.
The toolkit w/ the new iphone 5 battery was excellent. The only unforseen glitch was that the battery had expanded to about 3 times normal volume so it was tightly crammed in its compartment ( like an airbag going off inside the front seat of a compact car) . It was wedged so firmly I went a bunch overtime on spudging it loose.
Another interesting thing was the battery 'expansion' had actually caused the phone to split slightly on one side... a phenom I never noticed before since the Otter Box case had effectively concealed the symptoms ! Actually made it very easy to separate the screen/display assembly from the main body of the phone.
When you get the phone initially separated ...pause for a second & take a peek before beginning the battery replacement steps . I noticed a loose screw bouncing around inside & another half sticking out of the 'camera' assembly. I went over the thing & tightened up any other screws I could easily.reach.