How to keep rarely-used Li-Ion batteries functional?
Optimum temperature: 15 degrees Celsius and even lower
Optimum state-of-charge: 40 %
Avoid temperatures above 30 degrees Celsius (both during usage and storage)


Note that after 300 to 500 charge/discharge cycles, the battery may never be able to charge at its full capacity.

Here is an useful table:


By Isidor Buchmann. How to Prolong Lithium-based Batteries

Sources: batteryuniversity.com: How to Store Batteries and How to Prolong Lithium-based Batteries