Battery Description: Sulfation
WARNING: This page is about a different car, the 2010 Suzuki Kizashi. However, it is still accessible from the selected car via links, so may be relevant.
If the battery is allowed to sit for a long period in discharged condition, the lead sulfate changes to a hard, crystalline substance that will not easily turn back to active material during subsequent recharging.
"Sulfation" refers to the result of this reaction as well as the process. Batteries in which this has occurred can be slowly charged and restored to a usable condition, but their capacity will be lower than the original capacity.