iPhone 4S Battery
The black ground contact needs still more "flagging." This is a simple metal strip that is attached ONLY by the 1.5mm (upper) screw. It will help if you look at it before you remove the upper screw. Notice the gold contact arm and point. That contact point must be in a plane above every other surface inside the device when you replace the battery connector, because it makes a ground contact with the metallic inner surface of the device back plate.
When I replaced it, I used the Pentalobe driver to line up battery connector and ground contact with the screw socket post. I then attached the 1.7mm (lower) screw loosely, removed the Pentalobe driver gently, and inserted the 1.5mm (upper) screw into the assembly. There is a tab on the ground contact that helps it align properly once pressure begins to applied. I completed driving the upper screw, pressing lightly with the spudger on the battery connector plate to make sure it was seated on the pressure contacts. Finally, I completed driving the lower screw.