An enlarged laptop, tablet or smartphone battery caused by gas building up when the electrolyte that separates the layers within the battery starts to degrade. Evident when the back of the phone or the laptop keyboard swell, batteries can be replaced with a bit of effort because they are no longer plug-and-go like earlier models. Battery removal in laptops often requires just a screwdriver; however, phones and tablets are not designed to come apart and either require special screwdrivers or tediously prying off the back that is attached to the case by adhesive. See non-removable battery
A Bulging Battery
When a phone's battery contains a gas buildup, the phone case expands, and the battery must be replaced.