A quadcopter is a machine for converting stored chemical energy into moving air, and the store is a lithium polymer battery. Its properties decide the voltage the whole craft runs at, the current the wiring must survive, and the few minutes of flight the pilot gets. The power system is unglamorous and it is where a large share of builds fail, through a battery that cannot deliver the current, a wire too thin for it, or a connector that melts. The physics is the ohm's law and the loss of any electrical course, applied to a craft that pulls a hundred amps from a pack the size of a paperback.
Cells, voltage and the S number
A lithium polymer cell has a nominal voltage of 3.7 V, sitting at 4.2 V when fully charged and considered empty at about 3.5 V under load. One cell alone is too little to run a motor usefully, so cells are wired in series to add their voltages, and the count is quoted as an S number: a 4S pack is four cells in series, nominally V and 16.8 V fresh off the charger. The S number is the first choice in a build because it sets the voltage every other component must match. A given motor's speed is its Kv times this voltage, so moving from 4S to 6S raises the speed of the same motor by half, which is why the hobby drifted from 4S toward 6S: higher voltage reaches the same power at lower current, and lower current is easier on everything downstream.
Capacity, C rating and the current a pack can give
Two more numbers finish the battery. The capacity, in milliamp hours, is how much charge it holds: a 1500 mAh pack can in principle give 1500 mA for an hour, or 1.5 A, or proportionally more current for less time. The C rating is how fast it may be discharged safely, as a multiple of the capacity. A 1500 mAh pack rated at 100C can give A, and a pack asked for more than its C rating sags hard, heats, and ages fast. The two multiply into the maximum safe current, and matching that to the craft's real draw is the point of the specification. A racing quad that pulls 120 A in a punch needs a pack whose capacity times C rating clears 120 A with margin, or the pack becomes the weak link.
Under load the voltage droops below its resting value, an effect called sag, because the pack has its own small internal resistance and the current through it drops voltage inside the cell. A pack at 16.8 V resting might sag to 14 V during a hard climb, and since motor speed follows voltage, the craft has less power exactly when it is asking for most. A tired or under-rated pack sags more, and heavy sag that pulls a cell below about 3.3 V under load is what damages lithium cells and starts fires. Respecting the C rating and landing before the pack is flat are the two habits that keep it safe.
Example. A quadcopter hovers drawing about 18 A from a 4S pack and can pull 90 A at full throttle. It uses a 1300 mAh pack. What C rating does it need for the full-throttle draw, and roughly how long can it hover?
For 90 A from 1.3 Ah the pack must be rated at least C, so a 75C or 100C pack. For hover, a rough flight time is the usable capacity over the current: taking 80 per cent of 1.3 Ah as usable, h, about 3.5 minutes. Racers fly short; the endurance rules of the propeller lesson are how camera craft reach twenty.
Now you. A camera drone hovers at 9 A on a 6S 5000 mAh pack. Using 80 per cent of the capacity, roughly how long can it hover?
Answer
h, about 27 minutes. Large capacity and gentle current is the endurance recipe, at the cost of carrying a heavy pack.
Power, current and why voltage is chosen high
Electrical power is voltage times current, . A craft that needs 300 W to hover can take it as 20 A at 15 V or as 12 A at 25 V, and the same power at higher voltage means lower current. This is the entire reason to prefer a higher S number: the losses that plague a power system nearly all grow with current, not with power, so delivering a given power at higher voltage and lower current makes the wiring, the connectors and the ESCs all run cooler. The motor does the same work either way; it is the delivery that gets cheaper.
Wiring, connectors and the square law again
Current has to travel from the pack to the four ESCs, and every centimetre of wire and every connector has a small resistance that turns some of the power into heat at the rate . That square is why the main leads and the battery connector are the fattest wires on the craft: at 90 A even a few milliohms of resistance wastes several watts and warms the wire noticeably. A wire too thin for the current sags the voltage further, heats, and in the worst case melts its insulation and shorts. Wire is sized by gauge to the current it will carry, thicker for the main battery leads and the arms of a high current build, and the battery connector, commonly an XT60 or XT30 by current, is chosen the same way. Many builds route the pack through a power distribution board that fans the one battery input out to the four ESCs and the flight controller, often with a capacitor across the input to absorb the voltage spikes the switching ESCs throw back down the leads. Those spikes are real and they damage electronics; the low-ESR capacitor near the battery is cheap insurance that a surprising number of builds skip and regret.
Reading the system as one loop
The power system is a chain and it is only as strong as its weakest current-carrying link. The pack must be able to give the peak current without sagging past the cell's safe floor, the connector and wire must carry that current without overheating, the ESC must be rated above it, and the motor and propeller set what that current is. Trace the worst case current from propeller back to pack and check every link against it, and the craft flies for its whole flight instead of failing on the first hard throttle. Get one link wrong, a pack under-rated for the draw or a wire too thin for it, and the failure is not subtle: it is voltage sag and heat, and at these currents heat is measured in melted plastic. The sensing and control that make the craft flyable, which the rest of the course covers, all assume this loop is sound.