Apple - Macbook Pro mid-2015 15" swollen battery
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.
Update: It may be worthwhile to note here, that certain 2015 MBPs are now eligible to a battery recall: https://support.apple.com/15-inch-macbook-pro-battery-recall
Although I really like @Allan's compact answer (first version), I think the danger of a swollen battery should be stressed here.
- "It happens", however, very rarely.
- It is potentially dangerous as the battery may catch fire or explode.
- The device should be disconnected from the power source and not be used anymore as of now.
- The battery should be replaced immediately by a professional.
- Store the computer in a fire-safe environment until you get it fixed.
- In case it should catch fire, emit smoke or start smelling, do not use water when trying to extinguish it as Lithium reacts with water.
You find more info here: https://www.ifixit.com/Wiki/What_to_do_with_a_swollen_battery
A SWOLLEN LITHIUM-ION BATTERY MAY CATCH FIRE OR EXPLODE. PROCEED WITH EXTREME CAUTION AND AT YOUR OWN RISK WHEN REMOVING A SWOLLEN BATTERY FROM AN ELECTRONIC DEVICE. IF YOU HAVE ANY DOUBTS ABOUT YOUR ABILITY TO DO SO SAFELY, POWER DOWN AND ISOLATE THE DEVICE, AND CONSULT A PROFESSIONAL REPAIR TECHNICIAN IMMEDIATELY.
Removing a swollen battery can be hazardous, but leaving a swollen battery inside a device also poses risks. To prevent potential device and bodily harm, a device should not operate with a swollen battery. These guidelines offer best practices for removing swollen batteries, but cannot guarantee a safe repair. If you have doubts, power the device down, place it in a fireproof container, take to a repair professional and ask them to remove the faulty battery. Do not delay battery removal. If your swollen battery was purchased from iFixit, take photos of the battery before and after removing it, then contact our customer service team for warranty claims or replacement.
I purchased my mid-2015 15" MacBook Pro (top end model with dGPU) in 2016 with AppleCare. I had zero hardware issues with it in my 3 years of use. In August 2019 I read the news about the battery recall, so I visited Apple's website and typed in my Serial No. which said my machine was not covered in the recall. Since my AppleCare had just expired in July, and with nothing to lose, I decided to open my bottom lid to repaste the CPU and GPU (after seeing videos on the subject) and to clean out dust. To my surprise, I found my battery swollen. I was very surprised because my computer reported a 92% battery health and only 183 charge cycles over the 3 years of use (which is a very low cycle count).
When I telephoned Apple about my swollen battery, I asked if my leaving the MBP plugged into the official Apple power adapter most of the time could have led to a swollen battery. They told me, "No." They then told me, "it would be an inherent defect in the battery itself that would have led to it becoming swollen." Naturally I was not pleased with myself for waiting so long to open the bottom case and check. Had I done so during my AppleCare coverage (assuming the problem existed then), I could likely have avoided paying $200 to Apple for the replacement. But such also indicates that Apple's battery recall program is perhaps a bit limited in that it only covers a small number of 2015 15" models, whereas my case indicates there could be many more inherent battery defects which fall outside the coverage of the program.
I made a video about this topic today, showing my before and after battery condition:
https://youtu.be/Om1c-FCc5u0