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Electricity

This section focuses on electricity.

Units are:

  • Voltage: volts (V)
  • Current: amperes (A)
  • Resistance: ohms (Ω)
  • Power: watts (W), volts * amperes
  • Energy: joules (J), volts * amperes * seconds

Ohm's Law:​

Voltage = Current * Resistance

Power Law:​

Power = Voltage * Current

Energy Law:​

Energy = Voltage * Current * Time = Power * Time

mAH is milliampere-hour, tells how many milliamperes a battery can provide for one hour. For example, a 2000 mAH battery can provide 2000 milliamperes for one hour, or 1000 milliamperes for two hours, etc.

With information on voltage and mAH, we can calculate the energy in joules a battery can provide. For example, a 3.7V 2000 mAH battery can provide 3.7 * 2 = 7.4 watt-hours of energy, or 7.4 * 3600 = 26640 joules of energy.

Discussion about 2 * 18650 Lithium ion battery which delivers 7.4V:​

These are usually rated like 2 * 18650 3.7V 3300mAh rechargeable lithium ion batteries. What this means is that the battery pack is made up of two 18650 lithium ion batteries connected in series, which gives a total voltage of 7.4V (3.7V + 3.7V). The capacity of the battery pack is still 3300mAh, which means it can provide 3300 milliamperes for one hour, or 1650 milliamperes for two hours, etc.

Total Energy = Voltage * Capacity = 7.4V * 3.3Ah = 24.42 watt-hours of energy, or 24.42 * 3600 = 87,912 joules of energy. 1 watt hour is equal to 3600 joules of energy.