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.
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.
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 won't be run within a day or two.
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.
The hard limits: don't charge a cell above 4.2V, and don't 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.
Source: Established LiPo safety practice (charger Storage mode; manufacturer guidance)