Automotive battery chemistry boils down to two main options: flooded and AGM. Flooded batteries are available in two varieties, but both have acid sloshing inside. One version has removable battery caps to check the electrolyte level and add water when needed. These are said to have the highest rate of self-discharge. The other version of the flooded battery does not have removable caps and often is labeled maintenance-free. However, that doesn't mean it is sealed, as you'll discover if you tip it over. Flooded batteries do have the advantage of lower initial cost, about half that of an AGM battery.
Corvette owners should know the disadvantages of flooded batteries. Flooded batteries will mist, expelling droplets of acid under certain conditions. This can lead to corrosion of the battery terminals, battery cable ends, and surrounding parts. Flooded batteries can also expel hydrogen, which is explosive in concentration. When mounted in the passenger compartment (in C3s, for example), use care when charging them. Flooded batteries also self-discharge faster than AGM batteries. Therefore in seldom-used vehicles, flooded batteries need more maintenance.
The AGM battery uses an absorbed glass mat to hold the electrolyte between the lead plates. An AGM battery doesn't slosh like a flooded battery because the mat is designed to absorb all the electrolyte. In fact, it can be mounted on its side, and it won't leak acid even if its case is broken. This also means there's no need to add water. Positioning the mat solidly between the plates makes the battery much more resilient to vibration, too.
If unused, AGM batteries can last eight to twelve months (when disconnected) and still start the car. They have more starting power than similar flooded batteries. AGM batteries maintain higher voltage during cranking, which is critical to some aftermarket electronic ignition systems. They are more likely to recover after being discharged. There is no acid misting-so no corrosion-and there is no gassing. AGMs can have a considerably longer free-replacement warranty. And AGMs can last much longer. Optima states that its batteries typically last more than twice as long as flooded batteries.
Once you've settled on a battery type, the next choice is capacity. Choose a battery with cold cranking amps (CCA) equal to or more than what is specified for your car. However, if you have increased compression, more cubes, or need to start in very cold temps, consider a battery with a higher CCA rating.
The third choice is the brand, and there are many. Most are private-labeled; there are only a few major U.S. automotive battery manufacturers, with Johnson Controls said to be the largest. When shopping, find out how old that "new" battery really is. Look at the manufacture date code stamped on the battery case. Avoid a flooded battery that has been sitting on a shelf self-discharging for over four months. This is not as much of an issue with AGM batteries because of their significantly lower self-discharge rate.
With a little knowledge and maintenance, your automotive battery can provide reliable service for many years. And then every time you turn the key, you'll be happy to hear that rewarding sound of a big V-8 coming to life.