Below you will find a few useful details about the official advise for storing your battery.
From the Bosch battery guide
Charging – The batteries should be charged at room temperature in a dry location where a smoke detector is installed.
Storage during winter – Store the batteries in a dry location at room temperature.
Fully charging or fully discharging results in higher loading of the battery.
The ideal charge status for storage is approx. 30 to 60 % or two to three LEDs on the battery indicator.
From the Yamaha guide
Store the system in a place that is:
• Flat and stable
• Well ventilated and free from moisture
• Sheltered from the elements and from direct sunlight
Long storage period (1 month or longer) and using it again after a long storage period;
• When storing the bicycle for a long period (1 month or longer), remove the battery pack and store it using the following procedure.
• Decrease the residual battery capacity to where one or two battery capacity indicator lamps are lit, and store it indoors in a cool 15–25 °C, dry place.
• Check the residual battery capacity once a month, and if only one battery capacity indicator lamp is flashing, charge the battery pack for about 10 minutes. Do not let the residual battery capacity become too low.
• If you leave the battery pack at “full charge” or “empty”, it will deteriorate quicker.