Since VRLA uses a tight assembly method and the battery has an internal pressure, the battery case will have a slight degree of bulging on the battery case. The reason is that the safety valve must reach a certain pressure to open the safety valve. At this time, the battery is not at a high temperature. After the battery releases gas, the deformation of the battery will recover, but user must perform normal maintenance on the battery to avoid overcharge and thermal runaway. High temperature thermal runaway will cause irrecoverable deformation of the battery.