The Internet is full of "experts" on nearly every subject, but sadly, there are an increasing number of websites that publish pages not with the intent to educate or inform, but with the hope that they can earn an affiliate commission on a sale. As such, what they post is often tied directly to what they think will result in a sale (and profit for them). When someone asks what the main disadvantage is of an AGM battery, there is only one answer. If there were more than one answer, the question asked would be "What are the main disadvantages of an AGM battery?"
The main disadvantage of an AGM battery is the up-front cost, when compared to a traditional flooded lead-acid battery. That's it. There is no second place or runner-up position, when someone asks for the (singular) main disadvantage is for an AGM battery.
AGM batteries tend to be heavier than identical group sizes in flooded lead-acid batteries, but if someone is really concerned about the weight of their battery, like a race car driver, they'll also be concerned about the risk of acid spilling out of a flooded battery. In every other application, more weight in a battery generally equals more power and that's a good thing, especially in modern vehicles with high electrical demands.
AGM batteries tend to cost more than flooded lead-acid batteries up front, but they can last far longer than a flooded lead-acid battery on the back end. OPTIMA batteries can last up to three times as long as a flooded lead-acid battery, so until they cost more than three times as much as a flooded lead-acid battery, their main disadvantage will continue to become an advantage as you move through the ownership life cycle.
AGM batteries are better-suited to handle extreme temperatures, both hot and cold, than flooded lead-acid batteries, they are more tolerant of deep-discharge cycles (discharge them more deeply & more often) than flooded lead-acid batteries and they are far more resistant to damage from vibration than flooded lead-acid batteries (up to 15x for OPTIMA).
Anyone who tries to tell you lead-acid batteries are better than AGM batteries in any regard simply does not understand basic battery technology. AGM batteries ARE lead-acid batteries. The same goes for anyone who confuses AGM batteries with gel batteries. Those are two very different technologies and in nearly every consumer application we see, the lead-acid batteries being used are either flooded or AGM and not gel.
We've been roaming car shows for years and often hear people mistakenly refer to an AGM battery as a "gel" battery, but the next time we actually see a gel battery in a car at a car show will be the first. If you're ready to make a battery purchase, please check out our offerings and know that we offer free shipping, factory-direct to your doorstep, anywhere in the continental United States.
