Understanding the Role of Vent Plugs in Battery Safety and Performance

All valve-regulated sealed lead-acid batteries are equipped with a one-way valve. Its functions are as follows:

1. To maintain a certain internal pressure within the battery, thereby improving the efficiency of the sealed electrochemical reaction;

2. During overcharging or under abnormal conditions such as high charging voltage, excessive oxygen—and even hydrogen—may be generated, causing the internal pressure to rise. This could lead to casing deformation or even battery explosion. The valve opens to release the gas, preventing such hazards;

3. Under normal internal pressure conditions, the valve prevents outside air from entering the battery;

4. It helps prevent the evaporation of electrolyte, avoiding battery drying.

The opening pressure of the one-way valve must take into account the casing material. For example, when using a polypropylene case, the valve pressure limit is typically between 5 kPa and 10 kPa. Most standards for sealed lead-acid batteries only give general requirements for pressure relief valves, without specifying exact values or test methods—only Japanese standards provide specific regulations.

Requirements for Valves in VRLA Batteries:

1. High sensitivity to pressure response, meaning the valve must reliably and accurately open and close at the specified pressure;

2. Durability in harsh working environments: The valve material must be resistant to acid, strong oxidizing agents, and aging. The service life for small sealed battery valves is 3 to 5 years, while for stationary sealed battery valves, it is 15 to 20 years, and the pressure sensitivity must remain stable throughout the service life;

3. The valve must function reliably within a temperature range of approximately -30°C to 80°C.

As a pressure-sensitive component, the valve's relative displacement under a certain pressure is critical to controlling internal battery pressure. This performance depends on the valve's structure, the properties of the valve materials, and the structure and materials of the battery itself.

Common Valve Structures

Both domestic and international manufacturers offer a variety of valve structures for sealed lead-acid batteries. The most common types include:

• Cap-shaped

• Column-shaped

• Plunger-type

• Membrane-type

For example, the cap-shaped valve is widely used in China. This type opens to release gas under higher pressure, but the opening pressure tends to vary greatly, making it difficult to ensure repeatable performance. This variation is mainly due to the inconsistency in the resealing fit between the valve and the cover. Adhesion between the valve and the cover may also occur. Therefore, a retaining clip is required on top of the valve to ensure that the safety cap is not fully ejected during operation.