how can i get my riding mower's battery to last?

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we had the riding mower for 1 year before the battery was dead! we do store it outside (too big to fit in shed and no garage) but would that affect the battery life? am i suppose to unplug the battery during the winter months i dont use it? or am i going to have to buy a new battery every year? im about to buy an replacement so im hoping i can get some tips on how to make this one last!!