I found the following two articles on the Net when looking for some concise information to give to my customers and came up with these two articles:

Quoted content from a blog on batteryuniversity.com

Sulfation is an unavoidable effect of battery cycling. Lead-Sulfate crystals are chemically formed by the discharge process as electrons are depleted from the negative plate. Where do they come from? from the electrolyte (sulfuric acid) that as the battery discharges the acid concentration decreases. IF the battery is sitting too long at either a discharge stage or a partial recharge stage, then those crystals can become well fixed on the surface of the negative plate and can never be brought back into solution. This is the reason we recommend a correct recharge and depending on the type of battery, a boost or a certain amount of overcharge to assure not just fully charged plates but crystal dissolution back into the electrolyte. Is this all your battery needs? no. Sulfation is one of the failure modes that is a direct consequence of user ignorance on battery processes. it will naturally occur in the battery and it is engineered to be a determinant cause of failure typically at a point around the warranty time.

More quoted content:

Dan Hornby, Battery Management Developer, 30 years experience (1989-present)