If you use a battery tender make sure it is a float charger, quick charging with the wrong kind of charger does more harm than good. A deep discharge (leaving the lights on for example) will also considerably shorten the life of the battery.

Sulfation from water loss, undercharging, electrolyte stratification, excessive temperatures or prolonged periods of non-use account for most of the deep cycle and starting battery failures that are not used weekly, vehicle charging systems cannot by themselves cope with certain driving habits (short trips with high loads leaving the battery constantly undercharged).

Average battery life has become shorter as energy requirements have increased. In general car batteries last three to five years on average. Life span depends on usage; I've read that only 30% of all batteries actually reach the 48-month mark.

A battery is like a piggy bank. If you keep taking out and putting nothing back you soon will have nothing.

Since Porsche cannot control the owner's care of the battery I can sort of see why they treat them like brake pads but when something happens on the extreme short end of the expected life like Howi's you have to think the call is awfully close.