1. Field of the Invention
The present invention relates to a technology for preventing an overcharge of a secondary battery, and more particularly, to a charging control of the secondary battery as a backup power source, incorporated into an apparatus which can shift to a power-saving mode.
2. Description of the Related Art
A secondary battery such as a lithium battery or nickel-hydrogen battery is used as a power source for temporary backup purpose, if an external power source supplied to an apparatus (e.g., commercial power source (AC power source) as a major power supply source) is cut off. It is supposed that a power source is supplied from the secondary battery so that, for example, image information of a facsimile or the like (facsimile-received image data) stored in a volatile memory such as a dynamic random-access memory (DRAM) incorporated into the apparatus, can continue to be retained even when the power source is unexpectedly cut off.
Since such the secondary battery for backup purposes is provided, in many cases, in preparation for a sporadic short power-off such as power outage, it is in a charged state for most of the time. However, in the event that the secondary battery falls into an “overcharged state”, more specifically, the secondary battery still continues to charge, even when it has reached a fully-charged state, this may cause considerable deterioration of the battery, and significantly affect a service life of the battery.
In such a case, conventionally, it has been proposed, when a fully-charged state of the battery is detected, to switch to a method for charging the battery after that using an electric current in an amount which compensates for only a self-discharge of the secondary battery (hereinafter, called a trickle charging).
Several full-charge detection techniques have been already discussed. Firstly, a full-charge detection method based on voltage is available. The full-charge detection method is a method for detecting the full-charge from the fact that a battery potential has reached a predetermined voltage, and then switching to a trickle charging.
However, the battery potential significantly varies in values depending on an ambient temperature, and thus the battery potential cannot be controlled in some cases by a simplified electric circuit (circuit without temperature sensor). As a control method which complements the above technique, a method is proposed that switches to the trickle charging, if an elapsed time from the start of charging reaches a predefined time. Generally, control of this method is performed such that normal charging is executed for a given length of time at the time of power-on, and thereafter, the normal charging shifts to trickle charging when a predetermined length of time has elapsed or full-charge is detected (charging is not executed depending on temperature). The length of time of the normal charging is generally about 12 to 16 hours.
On the other hand, since it is supposed that power is continuously supplied to the apparatus, an apparatus as described above may shift to the “power-saving mode” to reduce power consumption of the apparatus for a period of time during which a user is not using the apparatus.
“Power-saving mode” is a standby mode in which electric power supply except that for minimum required circuits is cut off, in many cases, and power consumption of the entire apparatus is suppressed. In a multifunction image forming apparatus provided with, for example, a facsimile function, even in the nighttime during which copying and printing are not performed, power needs to be continuously supplied in a case of a FAX reception. In recent years, in the power-saving mode, suppressing power consumption during standby has been implemented, in which power is supplied such that particularly only a signal reception portion, rather than the entire facsimile function, effectively operates, and power supply to a circuit other than that portion is cut off. The same holds for the print function.
In a case of an apparatus provided with the “power-saving mode” described above as the standby mode, even a power source of a central processing unit (CPU) is cut off which controls a timer or the like in order to suppress power consumption as much as possible, which has become a mainstream. In the power-saving mode, since even the power source of the CPU is cut off, the apparatus is generally set to a trickle-charging state rather than a normal-charging state, from a viewpoint of safety.
In the apparatus provided with the power-saving mode as described above, power source is often connected at all times. The secondary battery provided in such apparatus is subjected to the normal charging for a total of about 12 to 16 hours by a timer function, if the power is not turned off/on, and enters into the full-charge at a time point when the predetermined time is reached, and thereafter, the full-charge will be maintained by the trickle charging.
However, depending on way of use by the user, the power may be irregularly turned off/on, or the apparatus may be set at a place where power outages frequently occur, therefore, such cases should be sufficiently taken into consideration. Also, if a temperature condition is bad, the full-charge detection may fail. When the normal charging for 12 to 16 hours described above is executed to a battery, each time the power is turned off/on or initialization is executed due to power outage, under such a condition, the battery will be charged again before the battery is completely discharged. In other words, a charging amount of the battery will significantly go above a discharging amount of the battery, and extreme overcharge of the battery will be repeated, which might greatly affect a lifetime of the battery.
In order to solve this problem, a method for calculating a length of time from power shutoff to power resumption, by detecting a power-off signal and performing time stamp of the time-of-day, and for determining a next charging amount depending on its result is discussed in Japanese Patent Application Laid-Open No. 2009-201171.
However, many image forming apparatuses, after a power switch (hereinafter, a power SW) is turned off and then the time stamp is pressed, are not supposed to stop power feeding to the image forming apparatus. This is because a general power SW is used to directly connect/disconnect an AC line, and the power feeding to the CPU will be stopped concurrently with turn-off of the power SW.
Further, the CPU enters into a reset state, when the power source is lowered to a predetermined voltage, for the purpose of stabilization of the system, which makes it difficult to generate a time difference between turn-off of the power SW and stoppage of power feeding to the CPU.
Hence, there is a problem that in order to generate the time difference since the power SW is turned off until the power feeding to the CPU is stopped as described above, it will in turn invite complex apparatus configuration and increase in costs.
In a method in which the CPU operates only by detecting the power SW, the CPU does not operate at the time of plug-in or -off of a receptacle or turn-off/on of SW on a table tap, or when a power outage occurs, and the apparatus cannot respond to such cases.
If the power SW is turned off of or the power outage occurs during the power-saving mode in which electric power supply to the CPU is stopped, the above-described method cannot deal with such cases. Alternatively, it is also possible to provide a control circuit (e.g., CPU) to which power is supplied even in the power-saving mode to prevent secondary battery overcharge, but there is a problem that this configuration reduces efficiency of power-saving, and increase the costs of the entire apparatus.