1. Technical Field
The present disclosure relates to a discharge controller for controlling a discharge from a plurality of storage cells in a multiple cell battery to equalize remaining electricity storages in the storage cells.
2. Related Art
In the vehicle such as Electric Vehicle (EV), Hybrid Electrical Vehicle (HEV), or the like, the multiple cell battery for feeding an electric power to a motor is installed, and a plurality of storage cells connected in series with each other are provided in the multiple cell battery.
FIG. 23 is a block diagram showing a relationship between a multiple cell battery installed in the vehicle, a part of an electric drive system, and accessories. The vehicle shown in FIG. 23 is equipped with a multiple cell battery 10, an inverter 11, a motor 13, a DC-DC converter 15, a battery 17, and an electric motor 19 for a cooling fan (referred to as an “electric motor” hereinafter). An output voltage of the multiple cell battery 10 is a high voltage (e.g., 100 to 200 V), and an output voltage of the battery 17 is a voltage for the accessories (e.g., low voltage of 12V). The output voltage of the multiple cell battery 10 is converted from DC to AC by the inverter 11, and then is fed to the motor 13. Also, the output voltage of the multiple cell battery 10 is stepped down by the DC-DC converter 15, and then charges the battery 17. An electric power is supplied from the battery 17 to the electric motor 19. An airflow is generated by the cooling fan driven by the electric motor 19, and cools the multiple cell battery 10.
The multiple cell battery 10 shown in FIG. 23 includes a plurality of storage cells C1 to Cm (m is an integer of 2 or more) connected in series with each other (simply referred to as “cells” hereinafter), discharge switch units Cc1 to Ccm connected in parallel with the cells respectively, voltage detection units S1 to Sm connected in parallel with the cells respectively, and a control unit 21 for controlling respective discharge switch units. In this case, the discharge switch units Cc1 to Ccm, the voltage detection units S1 to Sm, and the control unit 21 are integrated and formed on an IC chip.
Each discharge switch unit has a discharge resistor R and a switch Sw connected in series. The voltage detection units S1 to Sm detect a voltage across the cell (cell voltage) among the series-connected cells respectively. The control unit 21 controls ON/OFF of the switches in respective discharge switch units, in response to the result decided by the voltage detection units S1 to Sm.
Also, the multiple cell battery 10 has a charge control unit (not shown). The charge control unit performs the control to prevent an overcharge of each cell (overcharge preventing control), in charging the multiple cell battery 10. There is a variation in cell voltages of the cells depending on application conditions of the multiple cell battery 10, qualities of the cells, and the like. Therefore, the charge control unit executes the overcharge preventing control to meet the cell having the highest cell voltage. As the cells of the multiple cell battery 10, a lithium-ion battery, a nickel-hydrogen battery are employed, for example. In this case, this overcharge preventing control is particularly needed in employing the lithium-ion battery.
In this manner, the overcharge preventing control of the multiple cell battery 10 is applied based on the cell having the highest cell voltage. Therefore, as shown in FIG. 24, when a variation in cell voltages is increased due to repetition of the charge/discharge operation, a reduction in capacity of the multiple cell battery 10 is brought about. That is, the remaining electricity storage of the overall multiple cell battery 10 is restricted by the overcharge preventing control applied in charging the multiple cell battery 10. As a result, as indicated with a dotted line in FIG. 24, a capacity of the multiple cell battery 10 is lowered. When a sufficient electric power cannot be fed to the motor 13 because a capacity of the multiple cell battery 10 is lowered, either the cells of the multiple cell battery 10 must be equipped much more or the multiple cell battery 10 must be replaced with another multiple cell battery having a larger capacity.
For this reason, in the multiple cell battery 10 shown in FIG. 23, the control unit 21 controls individually the switches of respective discharge switch units such that cell voltages of the cells are kept at the same level. For example, as shown in FIG. 25, when the cell voltage of the cell C1 is higher than the cell voltages of the other cells C2 to Cm, the control unit 21 turns ON the switch Sw1 of the discharge switch unit Cc1 corresponding to the cell C1, and closes the circuit consisting of the cell C1 and a discharge resistor R1. At this time, a current flows to the discharge resistor R1 from the cell C1, and this current is converted into a heat by the discharge resistor R1. As a result, the cell voltage of the cell C1 is lowered, and the control unit 21 turns OFF a switch Sw1 when the cell voltage of the cell C1 is set at the same level as the cell voltages of the cells C2 to Cm. In this manner, a variation in cell voltages is reduced by equalizing the cell voltages of the cells, and therefore a reduction in capacity of the multiple cell battery 10 can be prevented (see e.g., JP-A-8-19188 and JP-A-2003-164069).
In the multiple cell battery 10 explained above, a heat is generated along with the equalization of the cell voltages of the cells. That is, in the multiple cell battery 10, an electric power accumulated in the cells is consumed wastefully by the equalization of the cell voltages of the cells. Also, as described above, the integrated circuit consisting of the discharge switch units Cc1 to Ccm, the voltage detection units S1 to Sm, and the control unit 21 is provided in the multiple cell battery 10. Since the constituent elements whose characteristics are varied depending on the ambient temperature are contained in the integrated circuit, it is advantageous that an amount of heat generated in the discharge resistors R1 to Rm should be made small.
According to the Joule's law, an amount of heat generated in the resistor is proportional to “current2×resistance value”. Therefore, the resistor having a large resistance value is employed as the discharge resistors R1 to Rm. When a resistance value the discharge resistor is large, a discharge current becomes small, so that a quantity of heat generated by the discharge resistor is reduced. However, when the discharge current is small, a time required for the equalization of the cell voltages of the cells is prolonged. As a result, the resistance value of the discharge resistors R1 to Rm is decided in consideration of the balance between a quantity of heat generated by the discharge resistor and an equalizing time for the cell voltages. In any event, a heat is generated from the discharge resistors R1 to Rm along with the equalization of the cell voltages of the cells. Therefore, high-level heat measures such as thermal diffusion, heat resistance, etc. must be applied to the multiple cell battery 10 such that the integrated circuit is not affected by such heat.