Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells – With discharge of cells or batteries
Reexamination Certificate
2000-03-29
2001-02-20
Tso, Edward H. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
With discharge of cells or batteries
Reexamination Certificate
active
06191555
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a capacity leveling circuit which is used for a battery group configured by connecting plural cells in series, in order to level the capacities of the cells.
2. Description of the Related Art
For example, a traction battery for an electric vehicle is configured by a battery group in which a large number of cells are connected in series to provide a desired high voltage. In such a battery system, there is a problem that, when the cells are varied in capacity, voltage, temperature, and the like, the reliability of the battery system is lowered. Therefore, the states of each of the cells, such as the capacity, the voltage, the temperature, and the like are detected to monitor whether the cell is in a given condition or not.
In order to detect the voltage of each of such cells, usually, the configuration shown in
FIG. 7
is used. In the figure, for the sake of simplicity, only four cells are illustrated. Voltage dividing circuits P
1
to P
4
each of which is configured by connecting resistors RA and RB in series are connected between the output terminals of cells C
1
to C
4
and a ground line serving as one of common output lines, respectively. According to this configuration, the voltages of the cells C
1
to C
4
can be measured from voltages V
1
to V
4
appearing at the common junctions between the resistors RA and RB of the voltage dividing circuits P
1
to P
4
, respectively.
When the voltage dividing circuits P
1
to P
4
are connected to the cells C
1
to C
4
as described above, there arises a problem in that the capacities of the cells are varied and hence the reliability of the system is lowered. This problem is caused by discharging currents i
1
to i
4
respectively flowing through the voltage dividing circuits P
1
to P
4
. Namely, the discharging currents have relationships that, as shown in
FIG. 7
, the discharging current i
1
flows through the cell C
1
only, the discharging current i
2
flows through the cells C
1
and C
2
, the discharging current i
3
flows through the cells C
1
, C
2
, and C
3
, and so forth. Consequently, a larger current always flows through the cells C
1
, C
2
, . . . which are nearer to the ground line. As a result, the capacity of each of the cells is reduced in a larger degree as the cell is nearer to the ground line.
In the case where the battery is of the type in which overcharge does not produce a serious problem, the capacities of the cells C
1
to C
4
can be leveled by charging the cell C
1
the capacity of which is lowered by the largest degree, so as to be fully charged, and the other cells C
2
, C
3
, and C
4
to be overcharged. In a battery of the type in which overcharge should be avoided, such as a lithium-ion battery, however, the capacities of cells cannot be leveled by the above-mentioned technique.
SUMMARY OF THE INVENTION
The invention has been conducted under these circumstances. It is an object of the invention to provide a capacity leveling circuit for a battery group in which capacities of cells of the battery group can be leveled to enhance the reliability of the system.
<First aspect of the Invention>
According to a first aspect of the invention, in a capacity leveling circuit for a battery group configured by connecting plural cells in series, voltage dividing circuits for measuring voltages of the cells being connected between one of output terminals of the battery group and output terminals on one side of the cells, respectively, a correcting discharging circuit for discharging a cell(s) is connected between another output terminal of the battery group and the one output terminal of each of the cells, one of the voltage dividing circuits being connected to the one output terminal, the cell(s) being not connected to the one voltage dividing circuit in series. The leveling of the capacities of the cells can be realized by setting the amounts of discharges of the cells due to the correcting discharging circuits, and those of each of the cells due to the voltage dividing circuits to be equal to one another.
<Second aspect of the Invention>
According to a second aspect of the invention, in the capacity leveling circuit for a battery group according to the first aspect of the invention, each of the correcting discharging circuits and the voltage dividing circuits is configured by connecting a photo FET and a resistor in series. In other words, each of the voltage dividing circuits is configured by connecting a photo FET
1
and a resistor in series, and each of the correcting discharging circuits is configured by connecting a photo FET
2
and a resistor in series.
Each of the photo FETs is configured by disposing light emitting and receiving diodes in the gate portion, and packaging the transistor and the diodes. Therefore, the gate terminal of the photo FET is insulated from the drain terminal, and the gate voltage is not affected by the drain voltage. In other words, even when the photo FETs have different drain voltages, all the gate voltages required for respectively driving the FETs are equal to one another, so that the on/off states of the FETs can be easily controlled.
<Third aspect of the Invention>
According to a third aspect of the invention, in the capacity leveling circuit for a battery group according to the second aspect of the invention, terminal portions of all the correcting discharging circuits which are commonly connected to the other output terminal of the battery group are formed as a single common line, a single photo FET
2
which is commonly used by all the correcting discharging circuits is disposed in the common line, and a diode is disposed in a portion of each of the correcting discharging circuits except the common line, the diode blocking a current in a direction opposite to a current direction in a case of a turn-on of the photo FET
2
, from flowing through the resistor of the correcting discharging circuit.
According to this configuration, the single photo FET
2
can be commonly used by all the correcting discharging circuits, and a reverse current can be prevented from flowing through the correcting discharging circuits by diodes which are more economical than a photo FET. Therefore, the production cost of the circuit can be made lower than that of a circuit in which plural photo FETs are used correspondingly with the correcting discharging circuits.
<Fourth aspect of the Invention>
According to a fourth aspect of the invention, in the capacity leveling circuit for a battery group according to the third aspect of the invention, a ratio of a resistance of each of the voltage dividing circuits to a resistance of each of the correcting discharging circuits is equal to a ratio of a number of cells of the voltage dividing circuit to a number of cells of the correcting discharging circuit, the voltage dividing circuit and the correcting discharging circuit being connected to the one output terminal of a same cell.
According to this configuration, the resistance ratio of a voltage dividing circuit and a correcting discharging circuit which correspond to each other is made equal to the cell number ratio of the circuits, and hence a current flowing through a cell(s) which are connected in series to the voltage dividing circuit is equal in level to that flowing through the a cell(s) which are connected in series to the correcting discharging circuit. As a result, the amounts of discharges of the cells can be equalized with each other simply by setting the turn-on period of the voltage dividing circuit to coincide with that of the correcting discharging circuit.
<Fifth aspect of the Invention>
According to a fifth aspect of the invention, in the capacity leveling circuit for a battery group according to one of the second to fourth aspects of the invention, turn-on periods of the photo FETs
1
of all the voltage dividing circuits coincide with one another.
According to this configuration, the on/off control of the FETs
1
can be easily performed, and the correcting discha
Konishi Daisuke
Mizuta Yoshihiko
Japan Storage Battry Co., Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Tso Edward H.
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