The formation of inner pressure: because the battery is sealed, when the battery is under the process of charge and discharge, the moisture and organic solvents inside the battery will resolve. If the gas produced by resolved haven’t been released or consumed, they will collect in the internal of the battery, and these gathered gas will produce pressure.
The strong and weak of the inner pressure are related with many factors. For example, battery materials, manufacturing technique, battery structures, use status, etc. Especially the use status, when the over-charging with high rate, the battery temperature will rise rapidly, and increase the inner pressure. If the inner pressure is too large, it will causes battery leakage, out of shape, and battery performance destroyed, etc.