I just replaced the old battery. The soldered-battery is indeed a silly design.

I got a bunch of JST-XH connectors to ease the pain.

I asked because the main post said you must have polycomm before replacing the battery. If just replacing the battery worked for you, then great! I'm ordering a battery right now.

Can someone explain why exactly we need that polycomm software? The Technicians Instruction Manual ******didn't specify the reason either ¯\_(ツ)_/¯

As Randy mentioned, just use a tweezer or a pointy thing to push the lock pin to remove the spade connectors. Do NOT use pliers or your fingers to pull with force, you'd loosen the solder or even bend the PCB board.

i don’t know where ifixit got the replacement screens but the touch id ribbon cables might be defective in some batch.

i experienced the same issue. after doing every steps correctly, the touch id DID NOT work

I had to uninstall the original ribbon cable from my broken screen and used it to replace the faulty cable on the replacement screen.

then my touch id worked