The batteries age from the time the cells are manufactured. Use them eavenly. Most outings I will fly 3 packs, out of 5 i have in current service. Sometimes I fly 5. Any packs not flown are the first used on the next outing. The packs are numbered (no fancy labels required, magic marker works fine) and I upload all flights to healthy drones. Healthybdrones is a good way to keep a track on pack health, I will favour the strongest packs (highest mah, least V variations) for more risky flights (higher winds, over water, longer distance/time in the air etc).