Li-Po batteries are categorized by several markings, most important of them being
- mAh, milliamper-hour
- S, cell count
- C, discharge rate
mAh directly defines the battery's capacity to store power. More mAh means more power, a good analogy would be the difference between a glass and a bucket. Bigger mAh is a bigger bucket, so 1000mAh is four times less than 4000mAh and therefore would theoretically provide four times shorter flight time. With bigger capacity comes more weight of course, and there comes a point where adding a bigger battery has a diminishing effect so the gains aren't linear. Choosing a properly sized battery is a "make it or break it" component of each drone.
S indicates the cell count connected in series. Most batteries are multi-cell, so in example a four cell battery would be indicated as 4s. Cell count also directly indicates the total voltage output of the battery. Since each cell is about 4.2 volts, four cells would make a total of 16.8 volts. This is an important factor since the propulsive components of each aircraft is rated for a certain voltage. Adding a 4s battery to an aircraft that is rated for 3s will likely result in permanent damage.
Finally, C indicates the discharge rate, meaning how much amperes the battery can output continuously. More C is generally better, but also more expensive. This of all factors is the most important one though because if the aircraft attempts to pull energy which the battery is simply incapable of providing, the results can be catastrophic, causing the battery to most likely instantly vent gas thus permanently damaging it but also to combust or explode.
Lipo batteries should be handled as if they are a hand grenade. They are extremely volatile and by far the most dangerous components of every aircraft. They WILL explode given any excuse to do so, including but not limited to puncturing, overcharging, overdischarging, storing at full capacity for prolonged times or simply due to manufacturer's error. These are high energy devices, often capable of outputting in excess of 100 amperes, which is significantly more than what most common welding machines.