Hertz - The standard unit of frequency of one cycle per second.
Hibernation state - A state in which the the status of the various functions of a circuit has been saved in memory and the circuit has been switched off save energy. When power is reapplied, data taken from the memory is used to restore the circuit to the status it had before switch off. (See also “Standby state” below)
High rate discharge - Discharge at a current of 2C or more.
Horse Power (Hp) - The rate of doing work. 1 Hp = 746 Watts or 550 foot pounds per second.
Hydrometer - A device used for measuring the specific gravity of a fluid. In the case of lead acid batteries the specific gravity provides a measure of the state of charge of the cell.
Hygrometer - An instrument for measuring humidity. Often confused with a hydrometer.