Engorged Battery in Apple MacBook Air

A friend asked me recently to look at her 2011 MacBook Air which was swollen around the trackpad, allowing her to only use a tiny corner of it. We opened it up and found this engorged battery cell.

This turned out to be a pretty easy fix, only requiring a new battery. The batteries pop out after removing a couple screws and the whole panel of cells can be replaced which is how the batteries are sold.