What causes a Samsung Galaxy S3 mini's battery to swell?

[AC Question]

One year ago I bought my Samsung S3 mini,it worked perfectly.Then I noticed that the back of my phone had a little bump but I didn't bother too much about it.The phone began to shut itself down when i tried to watch videos on YouTube and when i was playing games.I bought another battery and changed the swollen one.After 2 months i was beginning to experience the same problem.I charged my phone mostly at night time when I go to bed.

What did I do? or didn't do?

 

[B. Diddy]

Welcome to Android Central! A swollen battery is a failing battery. It's probably nothing you're doing--the replacement battery you got might have been defective. Did the replacement come with any kind of warranty? If so, take advantage of it. Otherwise, get another battery, and let us know if this third one swells up as well. (Twice is coincidence, but three times would be a pattern.)