Speed Controllers And Batteries
Electronic speed controllers (ESCs) are used in many R/C applications. They translate signal to electrical supply. On a multi-rotor, every motor gets its own ESC, each of which connects to the flight controller. After computing the inputs, the controller directs each ESC to adjust its speed in order for the craft to perform them.
ESC refresh rates vary. For multi-rotors, given the balance of multiple motors critical to the craft's ability to stay airborne, high refresh rates are more important than many other hobbies where ESCs are used.
In essence, we're talking about programmable microcontrollers, and they employ firmware to define and carry out their tasks. In the world of multi-rotors, SimonK is the supreme ruler of ESC firmware, creating revisions optimized for multi-rotor use, stripped of irrelevant features, and sporting refresh rates as high as 400 Hz or so. ESCs can be flashed or purchased with SimonK's optimizations pre-loaded.
The only other major factor to consider is an ESC's maximum current rating, which must exceed the current draw to each motor. Generally, 30 A for medium/large quads and 10 to 12 A for a small quad is plenty.
Clearly, those are high current draws. But such is the nature of multi-rotors. A medium-sized hex can easily pull 40 A on a steep ascent. As a result, hefty batteries are a necessity for decent flight times.
The industry standard is lithium-ion polymer (LiPo) batteries. Relatively lightweight, compact, and offering high discharge rates, LiPos are well-suited for multi-rotors.
Ready for another set of specifications? There are three to consider as you start perusing the cyber-aisles of LiPo batteries. The first is voltage. A single cell supplies a nominal voltage of 3.7 V (4.2 V at full charge). Each additional cell wired in series adds 3.7 V to the nominal voltage of that pack. Cell counts are denoted by the number of cells followed by "S". A 4S LiPo, therefore, is a battery of four 3.7 V cells at a summation of 14.8 V.
LiPo packs also have C ratings that indicate the maximum rate at which a pack can be discharged, with C standing for capacity. A 20C pack can be discharged at a rate 20 times its capacity.
Capacity, therefore, is the third important factor. It's measured in milliamp-hours (mAh). Let's say our 20C pack has a capacity of 4000 mAh. Given what we know about C ratings, we can do the math and determine its maximum discharge at up to 80,000 mA, or 80 A. Similar to ESCs, you need a discharge rate that's higher than the combined draw current of your motors.
LiPos connected in parallel add to capacity (rather than affecting voltage). In turn, the aforementioned S notation is modified. A 3S2P arrangement, for example, consists of two three-cell LiPos connected in parallel.
Batteries do not last forever. They vary in cost, and the pricier LiPos typically last for more cycles than the cheaper ones. A pack will “puff” in its plastic wrap as it gets to the end of its rope. Excessive heat after use is another bad sign.
The best way to prolong a LiPo’s life is to follow the 80% rule. You should try to avoid discharging more than 80% of the battery's listed capacity (a maximum of 4000 mAh from a 5000 mAh pack, for example). Also, monitor voltage when you're flying, and land before reaching 3.3 V per cell. Voltage falls more rapidly as charge is depleted, and at 3 V per cell, you might drop out of the sky. Some flight controllers have protection mechanisms to help prevent over-discharge.
