MBA was almost three years old with heavy daily use - battery was so dead it was failing unpredictably.
Extremely well, 5-10 mins start to finish.
go slow with the pentalobe screws, the screwdriver is good but the design doesn't allow for much torque.
T5 Torx Screwdriver
P5 Pentalobe Screwdriver Retina MacBook Pro and Air
MacBook Air 13" (Mid 2011/Mid 2012) Battery