Unfortunately LiPo batteries will still degrade even if not in use. The electrolytic solution in the LiPo battery begins to decompose after a long period of inactivity. This is called calendar aging, where the battery degrades as a function of time. Calendar aging is unavoidable because the degradation occurs even when there is zero battery usage.
We do not recommend purchasing extra batteries to keep on hand for future use. If you have more than one battery, be sure to use them all, rotating through them, to get the most out of your batteries. Follow the instructions for charging, use, and storage of your particular battery. Always keep them at the storage voltage* until ready to use, then charge to full before EACH use.
*Storage voltage is the voltage printed on the battery.