Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells – With discharge of cells or batteries
Reexamination Certificate
2002-06-20
2003-12-09
Luk, Lawrence (Department: 2838)
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
With discharge of cells or batteries
C320S116000
Reexamination Certificate
active
06661198
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a circuit for adjusting the charging rate of cells in combination which have a high voltage, for example, for use as a power source for drive motors for electric motor vehicles such as hybrid cars. The term “charging rate” as used herein means the percentage to which the cells in combination are charged relative to the full capacity thereof.
BACKGROUND OF THE INVENTION
Power sources conventionally mounted in electric motor vehicles, such as hybrid cars, for drive motors comprise secondary cells connected in series for use in combination. Since combinations of such cells need to produce a high voltage usually of 200 to 300 V, for example, 60 to 80 lithium secondary cells each having an output of about 3.6 V are connected in series, or about 200 NiMH secondary cells each having an output of about 1.2 V are connected in series for use in combination.
It is desired that all the secondary cells in combination be equivalent in charged state. Suppose one secondary cell is 70% in charging rate, and another secondary cell is 50% in charging rate. In this case, the amount of electricity chargeable into these cells in combination is 30% which corresponds to the amount of charge for the cell with the charging rate of 70% when it is to be charged to the full. If the two cells are charged to an amount in excess of 30%, the secondary cell with the charging rate of 70% will be charged more than 100% to become greatly shortened in life. Consequently the combination of cells is also shortened in life. Accordingly, it is practice to monitor the voltages of the individual cells in combination using a voltage monitoring device having the construction of FIG.
4
. In the case of the illustrated device, a plurality of cells are connected in series to provide a cell module
11
, and such cell modules
11
are connected in series to provide a cell combination
1
.
Voltage detecting lines extend from the opposite terminals of the combination
1
and from the points of connection between the cell modules
11
and are connected to a voltage detecting circuit
7
. The voltages of the cell modules
11
detected by the circuit
7
are fed to an entire control circuit
8
. The temperature of the cells is detected by a temperature sensor circuit
81
, and the current flowing through the cells is detected by a current sensor circuit
82
. The results of detection are fed to the entire control circuit
8
, which calculates the amounts of electricity remaining in the cells and checks the cells for abnormalities based on the input data. The result of monitoring is sent to a control system (not shown) through a communication line. Although the voltage of nickel-hydrogen secondary cells can be monitored in groups of 5 to 10 cells, lithium secondary cells are checked for voltage individually because overcharging or overdischarging leads to a markedly shortened life.
Variations in the amount of electricity remaining in the secondary cells in combination are dependent on the efficiency (charge-discharge efficiency) of the individual cells. For example, suppose the secondary cells in combination are all 100% in charge efficiency and 99.0 to 99.5% in discharge efficiency. If the cells are charged at 10 Ah, charge of 10 Ah is stored in each cell. When the cells are subsequently discharged at 10 Ah, charge of 10.1 Ah (=10 Ah/0.990) is delivered from the cell with a discharge efficiency of 99.0%, and charge of 10.05 Ah (=10 Ah/0.995) is delivered from the cell with a discharge efficiency of 99.5%. Charge which is 0.05 Ah greater will then remain in the cell with the higher discharge efficiency of 99.5%. Accordingly, the amount of remaining electricity varies from cell to cell owing to repetition of charge and discharge. Especially in the case of lithium ion secondary cells which are exceedingly high in charge-discharge efficiency, slight variations in charge-discharge efficiency result in a pronounced tendency for the cells to vary in the amount of remaining electricity.
To overcome this problem, it has been proposed to discharge secondary cells having a greater amount of charge with use of a discharge resistor and thereby give them the same amount of remaining electricity as those of smaller amount of charge (JP-A No. 8-19188/1996, No. 10-322925/1998, etc.).
FIG. 5
shows a basic circuit for practicing this method. The illustrated circuit is connected to each cell module
11
of a cell combination
1
. When a photocoupler
54
is turned on, an on-off switch
31
comprising a MOSFET is closed, causing a current to flow from the cell module
11
to a discharge resistor
21
, whereby the charging rate of the cell module
11
can be lowered. Accordingly, with a voltage monitoring device as shown in
FIG. 4
, all the cell modules
11
can be made equivalent in charging rate within a specified range by measuring the charging rate of each cell module
11
, specifying the cell module(s)
11
of high charging rate based on the measurements, turning on the photocoupler
11
connected to each module
11
of high rate and reducing the module
11
in charging rate.
The motor vehicle control system is adapted to operate when the ignition switch is on, and only operations involving very small current consumption are allowed while the ignition switch is off to thereby minimize the power consumption of the lead battery. This is because the lead battery needs to have electric power remaining therein in an amount required for starting up the engine when the ignition switch is thereafter turned on.
Since the circuit of
FIG. 5
operates utilizing the voltage of the cell module
11
, the control signal for operating the on-off switch
31
needs to be electrically insulated, so that the circuit has the photocoupler
54
. For the circuit to effect a sustained discharge, the photocoupler
54
must be held on. Because electric motor vehicles are in a running state (with the ignition switch on) for a considerably shorter period of time than when the vehicle is in a standby state (with the ignition switch off), discharging a cell module of considerable capacity within a limited period of time requires a discharge at a great current value by giving the discharge resistor
21
as low a resistance value as possible. This entails the problem of how to dissipate the heat of the discharge resistor
21
.
FIG. 6
shows a circuit therefore proposed (see JP-A No. 10-322925/1998). This circuit has a flip-flop circuit
9
for holding an on-off switch
31
on or off. The switch is held on or off from outside using a first photocoupler
55
for an on state, and a second photocoupler
56
for an off state. Accordingly, discharging can be sustained with the voltage of the cell module
11
even when the drive current for the photocouplers
55
,
56
is absent, or a continued discharge can be effected even if the ignition switch is off. Thus, discharging can be continued for a long period of time to diminish the discharge current.
The circuit of
FIG. 6
is smaller than the circuit of
FIG. 5
in power consumption and is advantageous for providing a smaller device. With the circuit of
FIG. 6
, the control system connected to the two photocouplers
55
,
56
is periodically initiated into operation for monitoring the state of discharge. For example, in the case where the monitoring time is 10 seconds and the system is started up at an interval of 30 minutes, the power consumption of the control system is 1/180 to diminish the depletion of lead battery more effectively than is the case with the circuit of FIG.
5
.
However, the circuit of
FIG. 6
operates on condition that instructions are given to the first and second photocouplers
55
,
56
periodically, so that when the power supply to the control system is discontinued, for example, for maintenance, an input signal for cutting off the flip-flop circuit
9
is no longer available, hence the problem that the cell module
11
is discharged continuously until the capacity becomes nearly zero.
SUMMARY OF THE INVENTION
An object of the present in
Armstrong Westerman & Hattori, LLP
Luk Lawrence
Sanyo Electric Co., LTD
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