I work on all Apple computers for an AASP. Apple insist we have a fire bucket, fire proof gloves, tongs etc incase a battery catches fire. I've been working on and off on Apple kit since 1990, and never seen a battery catch fire. All swollen batteries I've removed were cool. The first sign of a swollen battery is usually the trackpad clicker doesn't function, and on older MacBook series the trackpad movement slowly reduces. If removed at this stage the trackpad isn't damaged. Its not effected by leaving it with the power adapter connected. I've had them come into the workshop passing MRI battery test, then start swelling unpowered while waiting for a customer to make a decision re a quote. After the battery is removed it often continues to expand. I've seen MacBook Air batteries, once removed, expand to a bow tie shape, normal thickness in the centre and around 2" at each end with no heat. The trackpad often cracks as its made of glass. Although spectacular looking and creating comments on forums, the amount of MacBook's that fail like this are minute compared to those in use. I've also seen batteries in non computer related kit swell. As long as your MBP isn't as Apple class, vintage, an ARS or AASP can replace a trackpad economically. To remove a battery from a MBP 2010 onwards, excluding Retina models, a tri wing screwdriver is required.
