Why Does the Battery Swell? Why Does the Battery Swell? Generally, a swollen battery happens when its cells become overcharged and overheated. This reaction produces heat, which heats the gases inside the battery and causes them to expand. Without any means of ventilation, the battery's casing expands with gases and the battery swells. Mistakes That Cause Battery Swelling: Power outages while the device is charging, aftermarket battery or suitable [...]