Drone batteries are the most abused part of the hobby. A little care adds years to a pack.
Storage
Store LiPo batteries at roughly half charge (storage voltage), in a cool dry place, in a LiPo-safe bag. Never store them fully charged or fully empty for weeks.
Charging
Always balance-charge. Never leave charging batteries unattended. Let packs cool to room temperature before charging after a flight.
When to replace
Retire a pack if it puffs, if one cell reads far lower than the others, if flight times drop sharply, or if it gets hot during normal charging. A bad pack is not worth the risk to a $1,000+ drone. I stock genuine Yuneec batteries, and a LiPo-safe storage bag.