[sd_boxster]

Allowing a "normal" car battery to discharge completely and remain discharged for a significant period of time causes sulfation - a chemical deposit on the plates of the battery - that reduces the capacity or renders it completely useless.

One or two seasons - especially if you have some parasitic drain - is enough to destroy a new battery.

That said, you might have a charging system fault - but if you haul the battery to Autozone and have it tested, I bet they'll tell you the battery is shot. Order a maintainer now, while you're thinking about it, so the battery can be properly conditioned the next time you store it.