5.1 Definition

The electrical power system has three jobs: generate power, store energy, and distribute power. The electrical power system supports spacecraft bus components that must consume energy at a specified power. Of these spacecraft bus systems, the most notable power consumers are the payload, onboard computer, communications system, attitude control system, propulsion system, and ECLSS systems when relevant. The electronics that drive these subsystems have associated properties that must be managed by the electrical power system, like peak power draw, average power usage, and lifetime energy consumption. These high-level characteristics may be broken down into voltage regulation, current limits, and resistance.

Complex electronics are composed of many simple circuits interacting with each other to form a more complex circuit. The key terms we must review are voltage, current, resistance, energy, and power that you may have seen from a circuit analysis course.

• Current is the “rate of flow of electric charge past a point”, or literally imagine electrons moving along a copper wire. The higher the current, the more electrons move through the wire. The unit for current is Ampere or Coulomb/second.
  • Electrical charge is a fundamental property of mass, consisting of positive, negative, or neutral quantities. The smallest units of charge are held by protons, neutrons, and electrons. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. As the protons and neutrons reside in the nucleus, the current refers to the movement of electrons. The unit of charge is Coulomb.

• Voltage is “the difference in electric potential between two points” or “the work needed per unit of charge to move a test charge between the two points”. The electrical potential may be converted to electrical work, which drives circuits. An analogy may be made to mechanical potential energy, like a compressed spring, that is converted to mechanical work, pushing a block. The higher the voltage, the more electrical energy is pent up to be transferred. The unit for voltage is Volt or Joule/Coulomb.
  • Electrical energy is the potential energy, kinetic energy, or work in the electrical domain. Electrical energy is important when characterizing the storage of energy in batteries. The unit of energy is Joule.
  • The relationship between energy and voltage is that electrical potential energy is the storage of energy per charge, Joules/Coulomb or Volts.
    

    

• Resistance is the “measure of its opposition to the flow of electric current”. Everything that the current runs through has resistance, the question is how much resistance. The resistance is mainly a function of the material; insulators, like rubber, have high resistance, and conductors, like copper, have low resistance. Conductors are useful for efficient circuitry and insulators help contain a charge. The unit for resistance is Ohm.

• Power is the amount of electrical “energy transferred or converted per unit time”. Power converts static potential energy to electrical or mechanical movement. For system-level design, we pay attention to power as the high-level characteristic of a component or subsystem to form a power budget and profile for the mission. The summation of power consumption from the various subsystems is the overall strain on the battery. The unit of power is Watt or Joule/second or Volt*Ampere.
  • Power is the time derivative of energy: Watt = Joule/second.
  • Power is also the product of voltage and current: Watt = Volt*Ampere. Power may be calculated instantaneously by measuring voltage and current draw.