The battery of my old Macbook Pro was seriously compromised lately on (it had > 1000 cycles) and sometimes OSX was shutting down with ~40% battery health, so really strange behavior.
Quick and smoothly!
I have some experience in repairing hardwares, so for me was really easy, but the process could be done by anybody! With the tools provided by iFixit and their step-by-step guide it's a child play.
Be ready with a duster when you open the bottom case because there was a "little" dust on the air vents and the fun :)
I'll need to clean it often from now on...