Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells – With discharge of cells or batteries
Utility Patent
1999-12-14
2001-01-02
Wong, Peter S. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
With discharge of cells or batteries
C320S116000
Utility Patent
active
06169385
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 fear 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. 3
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 V1 to V4 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. 3
, 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.
If 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
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.
In a first aspect of the present invention, a capacity leveling circuit for a battery group configured by connecting plural cells in series, a forced discharging circuit having a current-limiting element is disposed for each of the cells, and the forced discharging circuits are selectively activated, thereby leveling capacities of the cells.
According to the this configuration, for a cell in which the capacity tends to be lowered, the time period when the corresponding forced discharging circuit is activated is set to be zero or shortened, and, for a cell in which the remaining capacity is large, the time period is set to be lengthened, whereby the average currents are uniformalized. As a result, the capacities of the cells can be leveled.
In a second aspect of the present invention, in a configuration where voltage dividing circuits are connected between one of output terminals of a series connection of cells and output terminals of the cells to measure voltages of the cells, respectively, a forced discharging circuit having a current-limiting element is disposed for each of the cells, and the forced discharging circuits are selectively activated, thereby leveling capacities of the cells. Even under circumstances where the capacities tend to be varied by discharging currents flowing through the voltage dividing circuits for measuring the voltages of the cells, the capacities of the cells can be leveled. Namely, in the same manner as the invention of the first aspect, for a cell in which the capacity tends to be lowered, the time period when the corresponding forced discharging circuit is activated is set to be zero or shortened, and, for a cell in which the remaining capacity is large, the time period is set to be lengthened, whereby the average currents are uniformalized.
In a third aspect of the present invention, in the invention of the first and second aspects, each of the forced discharging circuits is configured by connecting a switch element, a reverse blocking Zener diode, and a discharging resistor in series, and a Zener voltage of the Zener diode is set to be substantially equal to a cut-off voltage of discharge of the cell.
According to this configuration, even when a switch element is short-circuited as a result of a failure of the switch element itself or a control circuit, the voltage of the corresponding cell is lowered to the cut-off voltage of discharge, and the Zener diode is then turned off, whereby the cell is prevented from being overcharged.
In a fourth aspect of the present invention, in a battery group configured by connecting plural cells in series, voltage dividing circuits being connected between one of output terminals of the series connection of the cells and output terminals of the cells to measure a voltage of the cell, respectively, a switch element is disposed in each of the voltage dividing circuits, and on/off states of the switch elements are selectively controlled, thereby leveling capacities of the cells.
According to this configuration, even when special forced discharging circuits are not disposed, forced discharging for leveling the capacities can be performed by using the voltage dividing circuits.
REFERENCES:
patent: 5818201 (1998-10-01), Stockstad et al.
patent: 5914606 (1999-01-01), Becker-Irvin
patent: 5982050 (1999-11-01), Matsui
Konishi Daisuke
Mizuta Yoshihiko
Nakamura Hideji
Japan Storage Battery Co., Ltd.
Luk Lawrence
Sughrue Mion Zinn Macpeak & Seas, PLLC
Wong Peter S.
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