A brief introduction to electricity.
Electricity is the flow of electrons within a conductor. The flow is caused by a potential difference measured in Volts. Depending on the resistance of the conductor, the flow of electrons can be fast or slow, and the rate of this flow is measured in Amperes (amps for short).
Power is a measure of the rate at which work is done, and is measured in Watts. The power dissipated by a circuit is calculated by:
Watts = Volts * Amps
Energy is the measure of the amount of work done, and is measured in a variety of units (in the same way that distance can be measured in both feet, or meters). Common units are joules and calories, but for electrical work Watt-hours are the most common unit. One Watt-hour is the amount of work done by using one Watt of power for an hour. (One Watt-second is equal to one joule)
The energy capacity of batteries is often measured in Ampere-hours (or milliAmpere-hours, where one Ampere-hour is equal to 1000 milliAmpere-hours). This can be converted to Watt-hours by multiplying by the battery's normal Voltage.
Resistive losses in wires and electronics increase as the current increases, so for high power applications it is best to favor high Voltage sources. As the Voltage rises above 50V however, the body's resistance drops rapidly, increasing the possibility of electric shock.
Many components will have a rating, often given in terms of Volts or Amps. A servo or electronic speed controller may specify that they can handle up to 6V (for a servo) or 14V (for a speed controller). This is a limit on the maximum voltage you can supply without damaging the equipment (so, in this example, the speed controller can take up to a 14 Volt battery, and the speed controller will likely contain a so-called Battery eliminator circuit to drop the battery voltage down from 14V to around 5V).
Current ratings are also maximum ratings, but typically only the current needed will be delivered. Electric motors are to a large degree self-limiting in the amount of current they can draw (as long as they are not stalled), so a 15A motor can safely be driven by a 30A speed controller, supplied with electricity with anything above a 15A battery (although for better battery life you would be better off with something more capable).
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