1. Field of the Invention
The present invention relates generally to an electricity accumulator formed as a storage battery comprising a large number of battery cells connected in series, and more particularly to an electricity accumulator suitable for use in electric automobiles.
2. Description of Background Art
Techniques have recently been developed for an enhancement in the practical use of electric automobiles, and as the present power supply for electric automobiles, a storage battery comprising a large number of battery cells connected in series has been used.
In the case of such a storage battery comprising a large number of battery cells connected in series, the output of the storage battery depends upon the battery cell whose voltage is lowest, so the storage battery cannot use all the battery cells equally and exhibit the capability of each battery cell maximally.
Incidentally, as in a lithium-ion battery cell, in a battery cell in which its output voltage is determined in dependence on its discharge amount (see FIG. 4), the discharge amounts of all battery cells (conversely speaking, charge amounts or residual capacities) can be equalized with one another by equalizing the voltages of all battery cells. Therefore, if charging or discharging is performed so that the voltages of all battery cells become equal to one another, the storage battery can exhibit the capability of each battery cell maximally.
Hence, a voltage equalizing circuit for a storage battery has been provided, and it is constructed as shown in FIG. 5.
The circuit shown in FIG. 5 is a voltage equalizing circuit corresponding to one battery cell (or one module) of a storage battery, and therefore, the same circuit is provided in each battery cell.
In such a circuit, a charging operation is first performed and then a discharging operation is performed at the end of the charging operation.
That is, the terminal voltage of a battery cell 101 is raised by the charging operation. This raised state is monitored by a voltage monitoring circuit (voltage detection circuit) 104, and if the voltage VB across both terminals of the battery cell 101 exceeds a set voltage, a discharging switch 102 will be caused to be in an ON state (closed state).
With the above operation, current conduction to a discharging resistor 103 is performed, whereby electric energy is converted to heat and dissipated. With this dissipation, if the cell voltage VB becomes equal to or less than the set voltage, the discharging switch 102 will be caused to be in an OFF state (open state).
By reiterating such on-off control of the discharging switch 102, the voltage VB of the battery cell is adjusted to the set voltage.
Note that in the actual circuit of the voltage equalizing circuit, instead of the discharging switch 102 a power device such as a power transistor is generally used to adjust voltage by linear control rather than by on-off control.
The aforementioned conventional electricity accumulator, however, have the following disadvantages.
That is, in the above-mentioned circuit, the energy exceeding a set voltage is dissipated in the form of heat by the discharging resistor 103. For this reason, there are significant problems that power loss becomes great and that a countermeasure to radiate heat has to be considered.
In addition, the voltage equalization is possible only when the cell voltage VB at the end of the charging operation rises, and therefore, there is a problem that the voltage equalization can not be performed during discharge or for a vacant time during which a vehicle is not in use.
The aforementioned conventional electricity accumulator, therefore, cannot be utilized in electric automobiles that are not charged to its full charge during travel while generating electrical energy, as in hybrid electric automobiles.
Furthermore, since large-capacity components, such as a discharge resistor, a heat radiating plate, and a switching element, must be used, the overall device size is increased and a cooling unit for heat radiation is required, so that there is also a problem that the entire device becomes structurally complicated.
Hence, an equalizing circuit which is not required of a discharging method is necessary, and as the example, a technique disclosed in Japanese Laid-Open Patent Application No. HEI 6-319287 has been proposed.
In the aforementioned technique, both ends of a storage battery comprising a large number of battery cells connected in series are connected to a capacitor so that all battery cells are uniformly charged. However, a capacitor with a large capacitance is required and the control of detecting the terminal voltage of each battery cell and selecting a battery cell to be charged is logically complicated.
Hence, it is conceivable that a plurality of capacitors corresponding in number to battery cells connected in series are provided. And a first connection mode in which all capacitors are respectively connected in parallel with the corresponding battery cells and a second connection mode in which the capacitors are respectively connected in parallel with the battery cells adjacent to the corresponding battery cells are alternately switched, whereby the voltages of all battery cells are equalized with one another.
In the above case, the voltage equalization between battery cells is performed by moving electric charge between battery cells through capacitors.
However, in the aforementioned constitution, since electric charge is moved only between adjacent battery cells, there is a problem that the time required for voltage equalization is increased as the number of battery cells is increased. Also, as each battery cell requires a capacitor, the voltage equalizing circuit is increased in size, so that there is also a problem that the production cost is increased.
For the storage battery constituted by a plurality of battery cells connected in series, the aforementioned Japanese Laid-Open Patent Publication No. HEI 6-319287 discloses a technique in which a battery cell to be charged is selected, while the terminal voltage of each battery cell and the voltage of a capacitor are being monitored in performing a charging operation by the capacitor charged by regenerative current.
In the above technique, some of a plurality of battery cells are selected and charged so that the sum total of the terminal voltages of the selected battery cells becomes lower than the capacitor voltage. With this, even if the capacitor voltage were reduced, charging can be performed, and by charging a battery cell whose terminal voltage is low, a plurality of battery cells are uniformly charged.
Although the aforementioned technique can equalize the voltages of all battery cells in charging the storage battery, it cannot perform the voltage equalization except during charge.
The present invention has been made in view of the above-mentioned problems. Accordingly, it is an object of the invention to provide an electricity accumulator which is capable of equalizing the voltages of a plurality of electricity accumulating means with inexpensive and simple constitution and quickly even in a state which is not a fully charged state, while preventing dissipation of electrical energy.
To achieve this end and in accordance with one aspect of the present invention, there is provided an electricity accumulator comprising: a plurality of electricity accumulating means connected in series; a capacitor connectable in parallel with at least two of the plurality of electricity accumulating means; and connection switching means to switch selectively a first connection mode in which the capacitor is connected to first electricity accumulating means of the plurality of electricity accumulating means so that the capacitor is charged or discharged through the first electricity accumulating means and a second connection mode in which the capacitor is connected to second electricity accumulating means differing from the first electricity accumulating means so that the capacitor is charged or discharged through the second electricity accumulating means; wherein the switch of the first and second connection modes by the connection switching means is reiteratedly performed.
With such constitution, if the switch of the first and second connection modes is reiteratedly performed by the connection switching means, voltage equalization will be performed between the first and second electricity accumulating means through the capacitor and therefore the differential voltage between the first and second electricity accumulating means will not be dissipated as power consumption through radiation of heat, so that there will be an advantage that voltage equalization can be performed, while power loss is being suppressed. For this reason, the storage battery can exhibit its capability efficiently. Also, the additional advantage that a measure to counter radiation of heat can be alleviated by a reduction in the loss of heat radiation is obtainable. Of course there is an advantage that voltage equalization can be performed regardless of operating states such as traveling, charging, discharging, etc.
In accordance with another aspect of the present invention, there is provided an electricity accumulator comprising: a plurality of electricity accumulating means connected in series; a capacitor connectable in parallel with each of the plurality of electricity accumulating means; voltage monitoring means to monitor respective voltages of the plurality of electricity accumulating means; and connection switching means to select two electricity accumulating means from among the plurality of electricity accumulating means in accordance with the voltages detected by the voltage monitoring means and then to switch selectively a first connection mode in which the capacitor is connected to first electricity accumulating means of the two electricity accumulating means so that the capacitor is charged or discharged through the first electricity accumulating means and a second connection mode in which the capacitor is connected to second electricity accumulating means differing from the first electricity accumulating means so that the capacitor is charged or discharged through the second electricity accumulating means; wherein the switch of the first and second connection modes by the connection switching means is reiteratedly performed.
With such constitution, if the switch of the first and second connection modes is reiteratedly performed by the connection switching means, voltage equalization will be performed between the first and second electricity accumulating means selected according to the voltages detected by the voltage monitoring means through the capacitor. Therefore, according to a voltage state, electricity accumulating means in which a request of voltage equalization is high is selected, whereby voltage equalization can be quickly performed. For this reason, the storage battery can exhibit its capability efficiently. Of course, the differential voltage between the first and second electricity accumulating means will not be dissipated as power consumption through radiation of heat, and there is an advantage that voltage equalization can be performed, while power loss is being suppressed. Also, the additional advantage that a measure to counter radiation of heat can be alleviated by a reduction in the loss of heat radiation is obtainable. Furthermore, there is an advantage that voltage equalization can be performed regardless of operating states such as traveling, charging, discharging, etc.
In a preferred form of the present invention, two electricity accumulating means in which a voltage difference therebetween is large are selected as the first and second electricity accumulating means by the connection switching means in accordance with the voltages detected by the voltage monitoring means.
In accordance with still another aspect of the present invention, there is provided an electricity accumulator comprising: a plurality of electricity accumulating means connected in series; selection means to partition the plurality of electricity accumulating means to into a plurality of electricity accumulating blocks and select two electricity accumulating blocks from among the plurality of electricity accumulating blocks, each electricity accumulating block being constituted by one electricity accumulating means or a plurality of electricity accumulating means; a capacitor provided so that it is connectable in parallel with an arbitrary block of the plurality of electricity accumulating blocks; and connection switching means to switch selectively a first connection mode in which the capacitor is connected to a first electricity accumulating block of the two electricity accumulating blocks selected by the selection means so that part of the electric power of the first electricity accumulating block is transferred to the capacitor and a second connection mode in which the capacitor is connected to a second electricity accumulating block differing from the first electricity accumulating block so that the second electricity accumulating block is charged; wherein the selection of the two electricity accumulating blocks by the selection means and the switch of the first and second connection modes by the connection switching means are reiteratedly performed.
With such constitution, if the switch of the first and second connection modes is reiteratedly performed by the connection switching means, voltage equalization will be performed between the first and second electricity accumulating blocks selected according to the voltages by the voltage monitoring means through the capacitor. Therefore, according to a voltage state, electricity accumulating blocks in which a request of voltage equalization is high is selected, whereby voltage equalization can be quickly performed. For this reason, the storage battery can exhibit its capability efficiently. The differential voltage between the first and second electricity accumulating block will not be dissipated as power consumption through radiation of heat, so that there is an advantage that voltage equalization can be performed, while power loss is being suppressed. Also, the additional advantage that a measure to counter radiation of heat can be alleviated by a reduction in the loss of heat radiation is obtainable. Furthermore, there is an advantage that voltage equalization can be performed regardless of operating states such as traveling, charging, discharging, etc.
In a preferred form of the present invention, the capacitor is connectable in parallel with all electricity accumulating means of the electricity accumulating block which becomes an object of selection, selected by the selection means.
In another preferred form of the present invention, the selection means has voltage monitoring means to monitor respective voltages of the plurality of electricity accumulating means and, based on the voltages detected by the voltage monitoring means, the selection means sets the plurality of electricity accumulating blocks and also selects two electricity accumulating blocks from among the set plurality of electricity accumulating blocks.
In still another preferred form of the present invention, the selection means has voltage monitoring means to monitor respective voltages of the plurality of electricity accumulating means and sets, as a single electricity accumulating block, single electricity accumulating means in which the voltage detected by the voltage monitoring means is in a predetermined state.
In a further preferred form of the present invention, electricity accumulating means in which the voltage detected by the voltage monitoring means is in a highest voltage state is set as the first electricity accumulating block from among the plurality of electricity accumulating means by the selection means.
In a further preferred form of the present invention, electricity accumulating means in which the voltage detected by the voltage monitoring means is in a lowest voltage state is set as the first electricity accumulating block from among the plurality of electricity accumulating means by the selection means.
The selection means may set as a single electricity accumulating block a plurality of electricity accumulating means in which the voltages detected by the voltage monitoring means are within a predetermined range.
In this case, the selection means may compute a mean voltage value of the voltages of the plurality of electricity accumulating means detected by the voltage monitoring means, and may set, as the first electricity accumulating block, electricity accumulating means in which a voltage difference with the mean voltage is equal to or greater than a first predetermined voltage.
Furthermore, the selection means may set, as the second electricity accumulating block, electricity accumulating means in which a voltage difference with the mean voltage is equal to or less than a second predetermined voltage.
Moreover, the connection switching means may perform the switch of the first and second connection modes reiteratedly once or a predetermined number of times.
It is preferable that the selection means select a subsequent electricity accumulating block when the switch of the first and second connection modes by the connection switching means has been performed the predetermined number of times, and it is also preferable that the selection of the subsequent electricity accumulating block by the selection means and the switch of the first and second connection modes by the connection switching means be reiteratedly executed.
Furthermore, the selection means may have voltage monitoring means to monitor respective voltages of the plurality of electricity accumulating means, and based on the voltages detected by the voltage monitoring means, the selection means may set the plurality of electricity accumulating blocks and also selects two electricity accumulating blocks from among the set plurality of electricity accumulating blocks.
The selection means may compute a mean voltage value of the voltages of the plurality of electricity accumulating means detected by the voltage monitoring means. When the voltages of all electricity accumulating means within the first and second electricity accumulating blocks with respect to the mean voltage are caused to be within a third predetermined voltage difference by the switch of the first and second connection modes performed by the connection switching means, the selection means may end the switch of the first and second connection modes performed by the connection switching means and selects a subsequent electricity accumulating block, and the selection of the subsequent electricity accumulating block by the selection means and the switch of the first and second connection modes by the connection switching means may be reiteratedly executed.