AGM batteries are a type of lead-acid battery technology popular in various applications, from vehicles to backup power systems. They are frequently referred to as “sealed,” which is accurate for the average user. The sealed nature eliminates the need for maintenance and prevents acid from leaking, offering a distinct advantage over traditional lead-acid designs. However, the term “sealed” is not absolute, and understanding the construction nuances is important.
How AGM Batteries Are Constructed
The construction of an AGM battery makes it non-spillable and gives it the characteristics of a sealed unit. Instead of free-flowing liquid, the electrolyte is absorbed and held in place by fine fiberglass mats. These mats are woven into a sponge-like material positioned between the battery’s positive and negative lead plates.
This design immobilizes the sulfuric acid electrolyte, meaning no free liquid can spill out even if the casing is damaged. The battery is manufactured to be “acid-starved,” containing only enough electrolyte to keep the mat saturated. This construction is why AGM batteries are classified as Valve Regulated Lead Acid (VRLA) and are often called Sealed Lead Acid (SLA) batteries.
What Non-Spillable Means for Users
The sealed, non-spillable design provides practical advantages for the consumer. Since the electrolyte is contained within the glass mats, the battery is maintenance-free; there is no need to check or add water throughout its service life. This eliminates the periodic maintenance required by traditional flooded batteries.
The immobilization of the electrolyte also allows for greater flexibility in installation. AGM batteries can be mounted in almost any orientation, such as on their side, without the risk of acid spillage (installation completely upside down is not recommended). The tight packing of components provides resistance to vibration and shock, making them durable and suitable for demanding applications like marine, off-road, and high-performance vehicles.
Venting and Safety Considerations
While AGM batteries are sealed under normal operating conditions, they are not hermetically sealed or completely gas-proof. They are VRLA batteries, meaning each cell is equipped with a pressure relief valve. This valve is a safety mechanism that only activates if the internal pressure becomes excessive, which typically happens during severe overcharging.
When the battery is overcharged, the internal recombination reaction is overwhelmed, causing a buildup of hydrogen and oxygen gas. This reaction normally involves oxygen gas produced at the positive plate reacting with the negative plate to form water. To prevent the casing from swelling or rupturing, the one-way valve opens to release this excess gas into the atmosphere.
Because flammable gases are released under these fault conditions, AGM batteries must never be charged or used in a completely airtight container. They still require some ventilation in their installation environment.