http://www.mynrma.com.au/services/what-affects-battery-life.htm
Cycling
Repeated discharging and charging causes expansion and contraction of positive plates active material which leads to the active material separating from the plates.
The depth and frequency of discharge influences the amount of deterioration and over time this will reduce the usable life of your car battery.
Sulphation
Sulphation occurs when a battery is stored over a long period of time at a low state of charge. The crystalline structure of the discharged active material, (lead sulphate), is gradually transformed into a substance which resists charging, causing permanent damage.