Why is my battery bloated?

@nicobailey modified how? What did you "modify"? A bloated battery needs to be replaced. It will continue to expand and destroy at least the display. Since you did ask why it is bloated, here we go. Cellphone batteries expand because they have suffered internal chemical degradation, leading to a buildup of gas. This process is called gassing or venting and the worst case is venting with flame. That is when your battery actually explodes.

The main reasons for this decomposition is overcharging, exposure to excessive heat, deep discharge, or simply natural aging over years of use. A swollen (bloated) battery is a safety hazard and should be replaced ASAP.

To do that, and since iFixit does not yet have a specific guide for this phone, use something like this guide It is written in Spanish so use a browser that does the translating for you, if needed.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!