I have noticed that if my drone sits without use for 3 weeks or so, the batteries will drain to about 2/3 capacity. It only took once for me to now put all batteries on a charge a few days before I fly...
Yes, Lipo battery will self discharge when you don't use it, when you storage your battery, you'd better keep your battery's voltage at 3.8V, there are some tips about lipo battery storage. Lipo Battery Storage:
Do not directly connect the terminals with metal objects. This will short-circuit Batteries, resulting in heat and electrical discharge.
Never store loose Batteries together, the Batteries’ terminals may contact one another causing a short circuit.
Never storage Batteries to extreme temperatures or direct sunlight. The battery should be stored within -10℃ ~ 45℃ range environmental condition. If the battery has to be stored for a long time (Over 3 months), the environmental condition should be:
The voltage for a long time storage should be 3.6V~3.9V range per cell.
Always disconnect Batteries when not in use and store Batteries in a non-conductive and fireproof container.
Never alter, puncture or impact Batteries or related components.