1.  Field of the Invention
The present invention generally relates to an electric power unit.  More specifically, the present invention relates to a bicycle electric power unit for supplying electric power to electrical components that are capable of being installed on a bicycle. 
2.  Background Information
Recently, various electrical components other than headlights are being installed on bicycles.  For example, electrical components such as power assist motors and control devices for controlling such motors are being installed on bicycles, known as “assist bikes.”  Also, electrical components such as gear shifting motor units and gear shift control devices are being installed on bicycles having electrically-operated gear shift functions.  Primary batteries or secondary batteries are used as power sources for these electrical components.  Secondary batteries, capacitors or other such electrical storage elements are used as power sources for storing electricity generated by electric generator bicycle hubs. ( See, for example, Japanese Patent Application Laid-Open No.  2003-011879.) 
An electric power unit that uses a conventional electrical storage element as a power source has a power conservation circuit.  The power conservation circuit uses a microcomputer to block the supply of electric power to electrical components when a control system is not operating while the bicycle is moving.  This prevents unnecessary consumption of electric power from the power source.  Also, the microcomputer is configured to function in a power conservation mode.  In the power conservation mode, less power is consumed when a control operation has not been performed for a specific time.  Thus, consumption of the electric power from the power source is reduced by this power conservation mode.  In the power conservation mode, when an operating switch is actuated or a speed signal is entered, the system returns to regular control mode if it is determined that the bicycle is not stationary. 
Even with such measures for reducing power consumption, a control unit of the power conservation circuit consumes a certain amount of electric power in the power conservation mode.  Therefore, the electric power from the power source is consumed even when the control system is inactive and the bicycle is stationary.  When the electric power of the power source is consumed even when the control system is inactive, the power source may over discharge.  Thus, the functionality of the power source is reduced.  When a secondary battery is used as the power source, the functionality of the secondary battery is particularly likely to be severely reduced due to an over discharge. 
A known example of techniques for preventing such over discharge includes a technique of making multiple comparisons, including a comparison between an average value of the power source voltage over a specific period of time and a threshold value.  The supply of electric power to the motor is halted when results of any of the multiple comparisons exceeds the threshold value. ( See, for example, Japanese Patent Application Laid-Open No.  8-251713.)  In this conventional technique, the secondary battery, composed of a plurality of individual batteries, is used as the power source.  The supply of electric current to a motor is stopped when at least one value selected from the group consisting of the average value of the secondary battery over a specific time, an instantaneous voltage of the secondary battery and the instantaneous voltage of the individual batteries reaches a specific threshold value.  The supply of electric current to the motor is also stopped when at least one value selected from the group consisting of the amount of electric current discharged after the secondary battery is charged and the distance the bicycle has traveled after charging of the secondary battery is complete reaches a specific threshold value. 
In this conventional technique, information that the discharge has stopped is stored in the nonvolatile memory after the electric current of the motor has stopped.  When a rider turns a power source switch on after previously turning off the power source switch to stop the control unit, the control unit is restarted.  Pertinent information is then read out and the state of suspended discharge is maintained when the information that discharge has stopped is read out.  Thus, a discharge-ready state is enabled at all other times. 
Another known example of techniques for preventing such over discharge includes a technique of stopping a discharge of the secondary battery when a remaining capacity of the secondary battery falls below a specific capacity.  A measurement of the remaining capacity is stored in nonvolatile memory. ( See, for example, Japanese Patent Application Laid-Open No.  8-227731.) 
In either of these conventional techniques for preventing over discharge, a plurality of operations is controlled by software in the control unit having a CPU.  Therefore, if a state of over discharge is reached and the supply of electric current to the motor is stopped, the state of over discharge continues due to the electric power consumption of the control unit as long as the control unit is operating.  Therefore, it might not be possible to effectively prevent deterioration of the power source due to over discharge. 
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved bicycle electric power unit to effectively suppress deterioration of the power source due to over discharge.  This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.