A Look Inside Batteries: The Difference Between Lead-Acid And AGM

When it comes to high-performance engines and their capabilities, the battery isn’t often thought of as part of the system. However, when you expand the view a little, you start realizing that OEM batteries really can have an impact on your high-performance engine’s operating requirements. Higher compression requires more work from the starter motor to get the engine cranked, and in certain racing applications, not only do you have mounting concerns for the battery, but also performance issues, especially in high-draw applications (like nitrous) or in applications without an alternator.

Jason Fenske of Engineering Explained teamed up with Optima Batteries to work his usual magic in explaining the differences between the standard flooded-cell lead-acid and Optima’s Absorbed Glass Mat (AGM) style of battery.

As the name implies, a flooded-cell battery consists of six (in a 12-volt application) individual cells containing lead plates, which are submerged in battery acid (the electrolyte), and a permeable separator dividing the cell between the positively charged plates (cathode) and the negatively-charged plates (anode). If you remember basic science, when the anode and the electrolyte react, they want to shed electrons, and the cathode wants to take those electrons. That flow of electrons is electricity.

In the usual EE fashion, Fenske illustrates the makeup of a spiral-cell absorbed glass mat cell, along with how the fiberglass separator pulls double-duty as both the insulator and electrolyte “sponge”.

As Fenske explains, the drawback of the flooded-cell battery is that they require maintenance, since they aren’t sealed units. The fact that they aren’t a sealed design—and vent to the atmosphere—also brings up two other issues. First, if you were to relocate the battery into the passenger compartment, you would need to ensure that the battery is properly vented to the outside of the car. Two, is that you have to be make sure you are mounting the battery perfectly upright in the car.

Additionally, due to the overall design required by a flooded-cell battery, it is susceptible to internal parts breakage in harsh, high-vibration environments, much like you’d find in a competition vehicle subject to tire shake or high cornering forces, or even a street car which has switched over to all solid engine and driveline mounts.

Moving on to the Absorbed Glass Mat batteries—specifically the spiral-cell AGM design that Optima uses—the science behind how they work is very similar, with a cathode, anode, electrolyte, and permeable separator. However, the large difference is in their construction.

As the name suggests, each spiral-cell consists of a single sheet of 99.99-percent pure lead for that cathode, a fiberglass separator, and a single sheet of lead as the anode, which is then wound into a spiral. The compression of the winding allows for the softer pure lead to be used, which leads to increased performance over the stiffer, stronger lead alloy used in a flooded-cell battery.

As seen here in a front-wheel drive drag car, the Optima AGM battery’s lack of liquid electrolyte allows it to be mounted in a wide variety of non-traditional locations and orientations.

One of the key differences, and the reason for the “A” in “AGM” is that the fiberglass is not only a separator, but also a sponge for the electrolyte. That prevents gases from forming and escaping, the benefits of which are three-fold. One, you don’t ever need to worry about refilling the water in the battery that has vented out. Two, you don’t need to worry about the battery off-gassing into the passenger compartment in normal operation if you’ve relocated it inside for weight distribution reasons. Three, since there is no liquid electrolyte sloshing around inside the battery, you are able to mount the AGM battery in a wide variety of positions.

So what does all of that mean for you and your high-performance engine? Well, if you are taxing your starter more heavily, and demanding more from your electrical system with high-draw solenoids—or are physically beating up your battery in a rigid chassis—it only makes sense that you upgrade your battery to one that is designed to live and thrive in the environment you’ve created.

About the author

Greg Acosta

Greg has spent over a decade in automotive publishing as Senior Editor of Race Pages magazine. In his free time, he is a firearms instructor and volunteer in the police armory.
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