The latest gizmo for the household shows this in the leaflet:
If the battery pack is to be stored separately, place it in a dry environment
of a temperature between 10-35 °C and keep it away from direct sunlight.
To extend battery life, we recommend you charge the battery to
approximately 30 to 50% capacity and recharge it every three months to
prevent the battery from over-discharging.
IIRC it also says NOT to keep battery above 90% for any extended period of time, however I cannot find the paragraph now - so [grey matter dislaimer here].
cheers,
FD