It is worth noting that under normal operating conditions a battery cannot become discharged on its own (unless left for a long period of self-discharge). A battery is considered to be 100% discharged when showing a resting voltage of 10.5v and is 100% charged when showing a voltage of 12.73v +.

Sulphation occurs when a battery is allowed to stand in a discharged state for an excessive amount of time. When a battery voltage drops below a certain level, a chemical reaction known as sulphation takes place. In most cases this voltage is at or below 12.06v. Sulphation is normally irreversible, and can permanently impair performance of the battery, causing reduced performance and premature failure. Sulphation can be seen as a fine white/grey coating on the plates.

A battery showing a resting voltage of less than 10.5v will have become discharged due to a lack of charging - this is not a manufacturing fault.