I still think its just a coincidence,5 years is a long long time to have a battery,and by now a lot of sulphation of the plates has occured,and at least one cell must be giving problems....
A normal-functioning car that is driven every day can be assured that the battery lasts a considerable period because the alternator keeps the battery at full charge. However, if the battery is left for days or weeks in a semi-discharged condition, crystals are formed causing adverse chemical reactions which will damage the battery cells, and the battery would no longer be able to keep full charge.
Nonetheless, batteries have an average life span of between 18 and 24 months. The life-span is dependent on the rate at which the lead plates are sulphated. Sulphating occurs more rapidly if the battery is allowed to remain fully or partially discharged, or if the battery is left low on water.
In most cases, the life-span of a battery is not dependent on the amount of work it has done. Since the battery is a chemical device, its capacity will decrease as the battery grows older and loses some of the active material from its plates. Therefore, the common belief of economising on the use of electrical equipment, such as headlights and the radio, to extend the life of a battery is usually false.