Debra Kahn wrote:
Can you tell me what causes a swollen battery?
There are all sorts of things that cause swollen batteries, including manufacturing defects, usage conditions, or just random bad luck. Other than this one, I've seen a few other swollen batteries including a 2007 MacBook where the semi-external battery (kind of attaches) swelled out and didn't damage anything inside. Or a Mophie USB power bank that's clearly ballooned.
Debra Kahn wrote:
Will they diagnose it at the Apple store? And is it worth replacing the battery considering the age of the computer? How much will it cost? Thank you for your responses!
A diagnosis at an Apple Store should be free. I wanted to get my battery replaced for some time but was waiting for a message to have it serviced. I believe Apple doesn't perform battery replacements unless they're clearly malfunctioning or depleted below 80% battery health.
I still enjoy using my mid-2012 MBP. It's not the latest but it does what I need from it, and I was able to install my own memory and drive. I got it for $129 plus tax at my local Apple Store. The price isn't listed, but it's one of those things that are still done. One has to dig a bit for information that they may still do battery replacements up to 10 years after a particular Mac model was discontinued.