LiPo battery storage: the 3.8V rule
Store LiPo packs at about 3.8V per cell -- not full, not empty -- in a cool, dry, fireproof place, and check cell balance and cycle unused packs monthly.
A LiPo cell is happiest sitting at roughly 3.8V (some chargers target 3.80-3.85V). Leaving a pack at a full 4.2V/cell, or draining it flat, stresses the chemistry and is what makes packs puff and lose punch over time -- full charge accelerates electrolyte breakdown, while going too low risks lithium plating, both of which permanently shrink capacity.
How to do it: most modern chargers have a Storage mode that will either charge up or discharge down to about 3.8V per cell automatically. Use it whenever a pack wont be run within a day or two. If a pack sits unused for more than about 4 weeks, run it down to storage voltage and back up once to keep the cells balanced.
Where to keep them: cool and dry, out of direct sun, and in a LiPo-safe bag or a metal ammo can. Never store or charge a pack on carpet or near anything flammable, and never leave one in a hot car.
Before you put one away: check that all cells are within about 0.05V of each other -- an imbalanced pack is a sign it is wearing unevenly.
The hard limits: dont charge a cell above 4.2V, and dont pull it below about 3.0V under load -- most ESCs have a low-voltage cutoff to protect the pack, so leave it switched on. A puffed, hot, or damaged pack should be retired, not run.