1. Field of the Invention
The present invention relates to an abnormal voltage detection apparatus for use in an assembled battery, and in particular, to an abnormal voltage detection apparatus for detecting voltage abnormality in the assembled battery.
2. Description of the Related Art
A sealed nickel-metal hydride battery (hereinafter referred to as a “nickel-hydrogen battery”) is excellent in basic characteristics, such as energy density, output density and cycle life. Accordingly, in recent years, attention has been given to such a nickel-hydrogen battery as a power source for motors and as a drive source for various loads in electric vehicles, such as pure electric vehicles (PEVs) and hybrid electric vehicles (HEVs). Development has thus been advancing to make such a nickel-hydrogen battery practical.
When nickel-hydrogen battery is used as a power source for electric vehicles, a total voltage of approximately 100 V to 350 V is required to obtain a predetermined drive output. The output voltage of a cell, which is the minimum unit constituting the nickel-hydrogen battery, is approximately 1.2 V. Therefore, an assembled battery including a plurality of battery blocks, each battery block including at least one cell, is used to obtain a desired total voltage.
The temperatures of the cells constituting the assembled battery are not uniform. In particular, in such an environment that the assembled battery is used in a vehicle, temperature differences may occur among the cells. Furthermore, the remaining capacity and the charging efficiency (the ratio of the charged electric quantity to the supplied electric quantity) of respective cells are different from each other depending on production process and usage conditions after the production. For these rcasons, the cells constituting the assembled battery have variations in the actual remaining capacity (SOC: state of charge), and the range of the capacity usable as the capacity of the assembled battery is narrowed. In other words, the service life of the assembled battery is apparently shortened significantly. In the assembled battery, it is important to detect the voltage of each cell or battery block constituting the assembled battery, and to judge whether or not the voltage is abnormal to carry out charging or discharging control.
The Japanese patent laid-open publication No. JP-9-159701-A disclosed an overvoltage detection apparatus for use in an assembled battery according to a conventional example. The overvoltage detection apparatus according to the conventional example can detect overvoltage for each cell (battery block) constituting the assembled battery and can judge whether its overvoltage detection function is normal or abnormal.
In an electric vehicle, a high-voltage circuit including an assembled battery is electrically insulated from the chassis from the point of view of safety. On the other hand, a low-voltage circuit including a controlling devices, such as a microcomputer for controlling the charging and discharging of the assembled battery, uses the electric potential of the chassis as a reference electric potential. Therefore, it is necessary to provide such a circuit configuration that the high voltage generated by the battery blocks or cells constituting the assembled battery is not applied directly to the low-voltage circuit including controlling devices, such as the microcomputer. Furthermore, in an assembled battery for electric vehicles, the difference between the electric potential of a cell having the highest electric potential and the electric potential of a cell having the lowest electric potential reaches 200 V or more. Therefore, the voltage levels of the input signal, output signal and the control signals of overvoltage detection circuits provided in each cell cannot be handled altogether. In the overvoltage detection apparatus according to the conventional example, an input transmission device and an output transmission device, each having an input terminal pair and an output terminal pair being electrically insulated from each other, are provided for each cell. Input signals, such as control signals for each cell, are inputted from the outside via the transmission devices, and then, an overvoltage detection signal for each cell is outputted via the corresponding transmission device.
However, generally speaking, such a transmission device, such as a photo-coupler, having an input terminal pair and an output terminal pair being electrically insulated from each other is expensive and large in size. The overvoltage detection apparatus according to the conventional example includes transmission devices, each having an input terminal pair and an output terminal pair being electrically insulated from each other, and the number of the transmission devices is an integral multiple of the number of cells. For this reason, the overvoltage detection apparatus has such a problem as being expensive and large in size.