One of the positive aspects of dealing with battery storage is if you have done it in the past, you understand what to avoid and how to store them all. The key thing you want to avoid when you are storing the battery in an electric mower is to leave it in an area that experiences extreme temperatures one way or the other. Instead, store them in a cool and dry environment. In addition, before the end of the mowing season ensure the battery is fully charged before storing.
Now, it is important to note that batteries will self-discharge over time. Ironically, this is the primary reason why you need to avoid storing them in warmer areas as this will speed up the discharging process. On the flipside, a cool and dry location will slow the reaction down. Also, never allow the battery to freeze as that is a big no-no.