Absorbent Glass Mat Batteries [All To Know About]
What is an AGM (Absorbent Glass Mat battery)?
AGM (Absorbent Glass Mat battery) is a type of sealed lead-acid battery that uses fiberglass matting to contain the electrolyte. It has a very low internal resistance and high depth of discharge (DoD) rate which makes it perfect for deep cycle applications.
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One of the main benefits of AGM batteries is that they are spillproof, so you can mount them on their side or even upside down. They are perfect for many RV applications, and they can be charged by both alternator and solar panels.
They are also more resistant to vibration and perfect for motorcycles, ATVs, or any other application where there is a lot of vibration.
AGM batteries have a low self-discharge rate which means that they can be stored for longer periods of time without losing their charge. Their low internal resistance also allows for faster charging times. When using the same power source, they can charge up to 5 times faster than a flooded lead-acid battery.
However, everything comes with a drawback. AGM batteries have a higher price tag than flooded lead-acid batteries with similar power output and specifications.
Although AGM batteries look very identical to flooded batteries, they are actually very different.
AGM batteries include glass mats that absorb all of the battery’s acid, keeping the conductive electrolyte close to the power plates and allowing them to provide quicker starts and greater cold-cranking power over traditional lead-acid batteries.
Due to the fact that the electrolyte is kept within the mat rather than sloshing about in liquid form, the absorbed glass mat cell provides rugged durability, enhanced vibration resistance, spill-proof, and leak-proof construction.
AGM batteries are not only more efficient but also safer and maintenance-free as well making them a clear choice in many cases over traditional lead-acid batteries. They don’t need to be watered and are much less susceptible to corrosion compared to wet cells.
How does an AGM battery work?
AGM batteries work by the same principle as traditional lead-acid batteries, but with a few key differences. The electrolyte is stored in glass mats and there are no free liquid pools of acid inside the battery.
Also, these batteries are made of a sponge-like glass mat that is soaked in electrolytes. This design allows for the electrolytes to be completely absorbed by the mat.
Because the electrolytes are kept within the mat rather than sloshing about in liquid form, there is no chance of electrolyte leakage. This design also makes the battery spill-proof and safe to use in any position, including while it is turned upside down.
AGM battery cases are usually made of reinforced polypropene that is resistant to shock and vibration, making them particularly durable. They have two lead plates, one on each side of the mat.
Lead plates have a paste of lead oxide that is pressed between two grids (one positive and one negative). However, the negative plate also has a fine mesh of the powdered sulfates to make the chemical reaction happen.
Almost all AGM batteries have built-in safety valves to release excess gas buildup without letting out any harmful chemicals into the environment.
Once the pressure reaches a certain level, a safety valve opens and releases the gas into the air. Following this, the valve closes tightly to ensure the battery is completely sealed from the environment.
AGM Battery Applications
AGM batteries are used in a wide range of applications, from vehicles to aircraft and even aerospace applications.
Their ability to hold a charge longer than other batteries makes them superior for deep cycle use. They are also used in applications where the battery is constantly being cycled, such as solar PV systems and electric vehicles (EVs).
Although AGM batteries don’t perform as well as lithium-ion batteries, they are still a very good option for many deep cycle applications.
How to choose an AGM Battery?
Choosing an AGM battery is not very different from choosing a lead-acid battery. The first thing you want to do is identify your application, then choose what size AGM battery you need.
Things you want to pay attention are voltage requirements, cold-cranking amps (CCA), depth of discharge (DOD), and amp-hour capacity, weight, and a few other factors we will cover.
Cold-cranking amps (CCA)
Cold-cranking amps (CCA) is a rating that represents a battery’s ability to start an engine in cold weather. For instance, if your car requires 750 CCA and you choose a 450 CCA-rated battery, the battery is not going to provide enough power to crank your car engine at 0 degrees Fahrenheit.
In applications where the batteries are regularly discharged below 50 percent of their capacity, it is best to choose a battery with an amp-hour rating greater than the cranking amps required.
Depth of discharge (DOD)
Depth of discharge (DOD) is another rating that should be considered when choosing your AGM battery. The DOD rating tells you how deeply the battery can be discharged before it is damaged. Choosing a battery with a greater DOD rating is better because it allows you to use more of the battery’s capacity.
Normally, the depth of discharge is measured as a percentage. If a 100 Ah battery is discharged for 30 minutes at a current of 50 A, the depth of discharge is 25% (50 * 30 / 60 / 100). The state of charge and the depth of discharge are inversely related: as one rises, the other falls.
Weight
AGM batteries are lighter than traditional lead-acid batteries. A typical 12 Volt 100Ah AGM battery weighs 64 pounds. The weight for a 200Ah model will double as it holds twice the amount of active material.
However, not all AGM batteries are created equal. The weight of the same battery can vary by up to 20 pounds depending on the manufacturer and technology used.
As the manufacturing and material technologies are improved, the weight is becoming less of an issue.
Size
Sizes for two of best selling 12V 100Ah AGM batteries are: (12.99 x 6.81 x 8.35 inches) and (12.99 x 6.81 x 8.35 inches). Also, the average size of a 12V 200Ah AGM is (20.6 x 9.4 x 8.8 inches)
This is how big an AGM battery is on average. These numbers are just to give you a general idea of how much room you will need to install an AGM battery without problems.
Self-discharge rate
The self-discharge rate is an important factor. The lower the self-discharge rate, the more battery will maintain its charge. The self-discharge rate for AGM batteries is lower compared to flooded lead-acid batteries.
An average self-discharge rate for AGM batteries is around three percent per month. On the other hand, flooded lead-acid batteries have an average self-discharge rate of five to ten percent per month.
Some manufacturers tend to report their products’ discharge rates in weeks rather than months. Don’t let this fool you. Always ask for monthly discharge rates when it comes to AGM batteries.
Life expectancy & Warranty
All manufacturers want you to believe that their products last much longer than other brands. However, they are less willing to offer you a longer period of warranty for their products than their competitors.
By looking at the warranty periods offered by different products, you can understand which batteries are more durable than others. You should also check what components of the battery are covered by the warranty.
If the battery casing is poorly built no warranty on other components can save it from being a useless purchase. Make sure your warranty covers every part of your battery including:
- The battery connectors
- Battery case & Terminals
- Electronic parts including the cell log and protection circuits.
Finally, make sure the serial number on your battery is also included in the warranty. This is something that’s often missed when people are checking the warranty.
Advantages of using AGM Batteries
Here are the most important advantages of using AGM batteries:
Spill-proof
AGM batteries are completely sealed and therefore cannot be spilled, which is why they are often referred to as ‘maintenance free’. Even if the position of the battery is turned upside down, the acid will not leak out. This makes them a great solution for the applications that use electric winches or cranes in the mining industry where vibration is an issue.
Low internal resistance
The internal resistance of an AGM battery is among the lowest of any lead-acid battery. A new flooded lead-acid battery may have an internal resistance of 10-15%, but a new AGM battery may have an internal resistance as low as 2%.
Internal resistance is the resistance that batteries experience within their cells, which can reduce overall performance. The lower internal resistance of AGM batteries makes sure that electric current flows more easily and with less voltage drop when starting a larger engine or motor.
Also, internal resistance is the reason why your battery gets warm while charging or discharging, and it has a significant impact on the efficiency and lifespan of your battery.
Better depth of discharge
AGM batteries have a depth of discharge (DoD) of 80%, which is higher than the 50% DoD of a flooded cell battery. As a result, AGM batteries are perfect for deep cycle applications.
Low self-discharge rate
AGM batteries have a low self-discharge rate, which means that they can sit unused for long periods of time without discharging below the 80% mark (per manufacturer specifications). If you leave your boat or RV stored in the winter with an ordinary lead-acid battery it will discharge 10 to 15% in just a month.
But with AGM batteries, the self-discharge rate is much lower because they don’t have liquid to evaporate and electrolytes can’t seep out of their cases since these are sealed.
Faster charging
AGM batteries’ charging time will depend on the output of the charger and the state of charge of the battery. The time to fully charge the AGM battery changes from several hours to 12 hours.
Temperature Resistance
AGM batteries work optimally in a temperature range of 32 degrees Fahrenheit to 104 F. However, they have an excellent thermal resistance that allows them to work even in extreme temperature ranges like -40 F to 130 F.
Longer Lifespan
A typical AGM battery lasts up to 10 years depending on its manufacturing technology, operating temperature, and the number of cycles it has undergone.
They can reach below cycle numbers at given battery capacities:
| State of battery charge | Cycles |
| 100% | 200 |
| 80% | 250 |
| 50% | 500 |
| 25% | 1200 |
| 10% | 3200 |
Disadvantages of using AGM Batteries
High production cost
AGM batteries, as opposed to flooded batteries, have a higher production and manufacturing cost. However, they are still less expensive to manufacture than gel-type batteries.
Even though the cost of manufacturing these batteries has decreased since their debut into the market in the 1980s, they are still generally more expensive than other battery types on the market. As a result, they are slightly more expensive for usage in some devices.
Sensitive to overcharging
AGM batteries can be damaged by overcharging and high voltages, and they need to be fully charged between uses, making them unsuitable for certain applications where returning to full charge is difficult.
Frequently Asked Questions
Can AGM batteries be recycled?
The recycling rate of AGM batteries is over 90% so there is little concern about the environment being polluted by these batteries. Once an AGM battery completes its service life, the lead and acid are 100% recyclable.
The remaining parts of an AGM battery can be recycled too. The plastic case, the fiberglass separators, and other non-metallic materials are all recyclable with no risk to human health or harm to the environment.
Can AGM Batteries withstand extreme temperatures?
Can AGM batteries be used in solar PV systems?
Is sulphation a major problem with AGM batteries?
How are AGM batteries different from flooded lead-acid?
Is the recovery from a deep discharge rapid?
What is the life cycle of AGM batteries?
They can handle about 2000 deep cycles and still maintain 80% performance. Most flooded lead-acid batteries only last for 400 – 500 deep cycles, which means replacement every two to three years.
How long will an AGM battery last?
How to store AGM batteries?
If you plan on storing batteries for a long time, we recommend that the battery is stored in an environment below 80 degrees Fahrenheit and with humidity control to keep moisture levels down.
During its storage never expose the battery to a source or spark, such as cigarettes and power tools. Do not leave the battery in an enclosed space without ventilation for long periods of time.
This can cause damage to the case from high heat exposure which may lead to leaks and corrosion between cells within your batteries that will reduce their overall ability to hold a charge over time.
Keep the battery in the original manufacturer box with padding around it. This will provide extra protection to your battery while not in use. The room temperature ideally should be between 16-20 degrees Celsius.
Although AGM batteries can operate at temperatures as low as -20 degrees Celsius and as high as 45 degrees Celsius, these extremes can still harm your battery’s cells enough to decrease their overall ability to retain a charge over time.
Can you jump-start an AGM battery?
Attach the positive cable on your jumper cables to the positive terminal of both batteries, and then connect the negative cable from one car’s live battery to the other car’s negative terminal.
Always follow safety precautions when performing this step as jumpstarting the battery because you are handling live current.
Can I add water to an AGM battery?
Can I trickle charge an AGM battery?
If you store your battery for an extended period of time, trickle charging will keep it fully charged and ready for use when you need it. Otherwise, there is no harm in trickle charging an AGM battery.
Do I need a special charger for an AGM battery?
These chargers will also provide a slow charge for long-term storage needs, as well as the ability to recharge quickly if necessary.
Are AGM batteries better in cold weather?
The chemical reaction slows down in colder temperatures, reducing the battery’s ability to store and provide energy when needed.
How much does an AGM battery cost?
At what voltage is an AGM battery flat?
A flat battery is generally not covered under warranty. So if the battery is drained to this level or lower, you’ll likely have to pay for a replacement.
How to maintain AGM Batteries?
AGM batteries are deep-cycle batteries that are built for deep cycling. They are designed to be depleted of power over time and then recharged again.
Conventional lead-acid batteries are cranking batteries that you use to start your car. They are meant to offer a lot of power upfront, but they are not designed to have a lot of power pulled out of them over a long period of time. Therefore they are not suitable to power portable fridges and lights.
If any power source draws energy out of the battery faster than it can be replenished, the battery will be depleted and eventually die. This is the worst thing you can do to your battery. In fact, batteries are flat at 11.8V which means that they are already out of juice.
If you have depleted your battery to 10.8V or less it will not only kill your battery but also void the warranty because most manufacturers will not repair batteries that much have been depleted.
For a typical AGM battery the below table shows the state of charge:
| State of Battery charge | Voltage |
| 100% | 12.7 – 13.2V |
| 75% | 12.4V |
| 50% | 12.2V |
| 25% | 12.0V |
| Discharged/Flat | 0 -11.8V |
Yes, you don’t need to fill AGM batteries with water. However, it doesn’t mean you can leave them completely empty either.
You have to monitor the battery voltage. To do this you need to have a multimeter or you have to know what the voltage of your battery is. Because without it, you have no idea if you’re charged or discharged your battery.
Obviously, you cannot sit all day in front of your battery to make sure it doesn’t go too much discharged. That’s why the best thing is to have a smart charge regulator that will take care of this for you.
Conclusion
AGM batteries can be a suitable choice for the applications that need to be installed in a given position. They are not sensitive to vibrations and have no liquid that can spill out or evaporate, making them maintenance-free batteries.
They are suited for use in boats, solar energy plants, or caravans. Their major downside is that they are more expensive than traditional flooded lead-acid batteries. All in all, they are a good alternative to regular lead-acid batteries and can be used in both deep cycle applications such as the ones listed above.