Three summers ago the removable battery in my 17" MacBook Pro swelled. Although it is hard to be sure, the cause may have been that it was left for a couple days in a vehicle in temperatures at the very top end of the scale for summertime temperatures in the American southwest. Finding myself in New York City soon thereafter, I went to the iconic Apple Store there with my battery to demand a new one. The first store employee who spoke with me turned me down outright. When I pressed the issue, she sent me up the famous glass stairs to the Genius Bar, where a genius turned me down. Undeterred, I finally dropped the magic word: lawsuit. No, I did not threaten to sue, nor would I have done so, I merely stated "Although this individual battery is not a part of any of your recalls, I know there have been many lawsuits over this batter..." The Genius briefly conferred with the store manager, and I was given a brand new battery.
Lo these thirty months later, that battery also swelled up! The symptom of impending explosion was familiar: the mouse button became weak, soft, and shallow. Unlike last time, this battery had never been subjected to extreme heat, either hot or cold -- and that makes me question whether the heat was the cause of the previous battery's swelling.
In any case, today I took the battery to the Apple Store in Madison, WI. Upon entering the store, the employees near the door asked how they could help, and I showed them the swollen battery. He called it "bulge", which is apparently their preferred nomenclature. Also, the key words are "safety issue", as in "My Apple-branded battery has swollen, which I feel is a safety issue." He used those words, which I dutifully repeated back to him. Without me even having to ask, he immediately took me to a shelf with new batteries on it and gave me one. He kept the old battery, and the new packaging (as had the employee three years ago). This employee gave me a boilerplate admonishment about buying AppleCare, but I have done quite well without AppleCare on my current Mac. It is those low-quality iPods which need to be insured.
So if you experience swelling in your battery (lasting more than four hours), go to an Apple Store and politely demand a new one.