LiPo fundamentals
Most drones run lithium polymer (LiPo) packs because they deliver high current for their weight. A cell is nominally 3.7 V, fully charged at 4.2 V, and should not be pulled below roughly 3.0 to 3.3 V under load. Pack labels use S for cells in series: 4S is four cells, about 14.8 V nominal.
Capacity is given in milliamp-hours. The C rating estimates safe continuous discharge: capacity in amp-hours times C. A 1500 mAh 100C pack is theoretically good for about 150 A, though marketing ratings are often optimistic.
Charging safely
Always balance charge, which equalizes cell voltages while filling the pack. Charge attended, on a non-flammable surface, away from anything that burns, with a fire-safe container or bag. Never leave LiPos charging overnight or unsupervised.
Retire any pack that is puffed, physically damaged, or that will not hold balance. Dispose of it according to local regulations. A damaged lithium pack is a fire waiting for a trigger.
Storage and field use
For anything longer than a day or two, store packs at roughly 3.8 V per cell in a cool, dry place. Storing packs full or empty permanently reduces capacity. Cold weather cuts usable capacity and increases voltage sag, so warm packs before flying in the cold.
Watch voltage sag in flight: a healthy pack recovers when you ease off the throttle. A pack that sags hard and stays low is worn out or undersized for the aircraft.
Connectors and protection
XT30, XT60, and XT90 connectors are the common standard, rated for increasing current. An anti-spark connector or a 'smoke stopper' limits inrush current when plugging in and can save an entire build with a wiring mistake. A multimeter and a smoke stopper are the two cheapest insurance policies in this hobby.
When traveling by air, LiPos generally must be carried in the cabin at storage charge with terminals protected. Check the current airline and aviation authority requirements before every trip.