I bought the MBP around 2016 April. Few months ago, around 2018 Nov, bottom of the device appears a bit uneven, with slight gap with my desk. Over past few months, it wobbles more and more serious. Today I brought it to Apple Store Singapore for diagnostic, specialist at Genius Bar confirmed that it's caused by swollen battery. Consider it was just after about 2.5 years use, I find it hard to believe. According to the staff, it depends on usage. If under heavy usage or often charge it overnight, it's normal that MacBook Pro's battery can become swollen pretty soon.
My battery charge cycle is under 300, battery life drops a bit but still strong (9 hours instead of 10). I rarely charge overnight, normally turn AC power off and leave my MBP in sleep mode. I do use my MBP heavily though, say 10 to 13 hours a day. However, I still find it hard to accept the explanation given by Apple Singapore. I wonder if other MBP 15" users experience the same? Or… am I the isolated case?
— edit
To make the question more specific, is it generally expected that, Macbookpro battery will start swelling after 2-3 years use if laptop usage is heavy, e.g. 10-13 hours a day? The concern is not about the slightly reduced battery life (the battery life is still strong), but the physical expansion. I'm asking this because I've used few other laptops before, among which I never experience battery swelling except one (after near 5 years use). If the experience with my MBP is aligned with most other users, which means it's just a matter that I should adjust my expectation, and include the near SGD300 battery cost per 2-3 years into my next purchaser's consideration; Otherwise, I will try to ask them to consider replacing without me paying the full cost.
— update 2019/6/21
Apple has just announced a battery recall for Macbook Pro 15" mid-2015 model. They confirmed that these batteries may pose fire safety risk:
https://support.apple.com/en-sg/15-inch-macbook-pro-battery-recall
Best Answer
No, it is not “generally expected” that batteries will swell after a certain amount of time.
What you have is a defective battery - it happens. It needs to be replaced.
The swelling is caused by trapped off gasses from the chemical reaction not venting properly. It could swell and not cause any issues or it could go into a condition known as thermal runaway where it will burst and catch fire igniting the off gasses in an uncontrolled manner.
This is not typical, but it does happen. Batteries, like everything else produced by humans, aren’t “faultless.” Trying to pin down how common it is, is like trying to determine your odds of getting a “lemon” Toyota. It’s rare, but it happens.
MacBooks from 2013 on started using batteries that were glued into place requiring you to replace the entire top case.