Mid 2011 11" Macbook Air Swelling Battery
A swelling battery in my Macbook Air was distorting the outer case (bottom) and the keyboard into a pronounced convex shape. The mousepad no longer worked properly. The local Apple dealer quoted approximately $200 and a few days to replace the battery. I ordered the battery for $99 plus the T5 Torx and Pentalobe screwdrivers. The battery swap took approximately half an hour (from first case screw removed to last case screw reinstalled). It's always good to be able to repair your equipment.
The repair couldn't have gone more smoothly. I did lose one of the 2.5 mm Pentalobe screws while attempting to install it, it bounced off the table and could not be located. Perhaps one solution would have been to put the computer in a plastic bin or such to better contain errant screws.
I have one remaining issue, the Macbook appears not to be charging because it cannot sense the data on pin #3 (the charge control pin). So, more troubleshooting to determine if the problem is on the power supply side or the computer side of the connector....
Be careful with the screws.
MacBook Air 11" (Mid 2011-Early 2015) Replacement Battery
T5 Torx Screwdriver
P5 Pentalobe Screwdriver Retina MacBook Pro and Air