System and method for cell-specific control of three-terminal cells

A system and method are described permitting a sophisticated control of a battery composed of a multiplicity of three-terminal electrochemical cells. Each cell has first and second terminals, connected with respective electrodes, one of which is a positive terminal and one of which is a negative terminal. Each cell has a third terminal connected with a grid electrode. A battery is composed of N cells. For each of the N cells, there is provided a respective capacitor switchably coupled to the second and third terminals thereof. A controller is connected through a switching matrix to the capacitors. In operation, the controller is connected sequentially to each capacitor among the multiplicity of capacitors, during which time the capacitor is momentarily uncoupled from its respective cell. When the controller is connected to one of the capacitors, it measures the voltage thereupon. The controller can then charge up or discharge the capacitor to drive it to a desired voltage level. Thereafter, the capacitor is disconnected from the controller and is coupled again to its respective cell.

When large amounts of energy are to be stored to and retrieved from a battery composed of three-terminal electrochemical cells, many competing goals need to be satisfied, including but not limited to the following. It is desired to provide a long service life for the battery. It is desired to maximize the energy density of the storage. It is desired to minimize the risk of failure of any particular cell in the battery. It is not easy to satisfy all of these goals simultaneously, and it is not easy to satisfy all of these goals at an acceptable cost. As will be described below, the invention relates generally to control of a battery composed of a multiplicity of three-terminal electrochemical cells, and can also relate to a single three-terminal electrochemical cell.

BACKGROUND

The classical voltaic cell stores electrical energy between two electrodes, and the energy is stored by electrochemical means. Such a cell is sometimes called a “Faradaic” cell to distinguish it from a classical capacitor which stores electrical energy in an electrostatic fashion. The classical cell is a two-terminal device. In most industrial and automotive applications, a multiplicity of voltaic cells are placed in series, defining a battery. In some applications, the cells are placed in series-parallel combinations, defining a battery. The battery is charged and discharged hundreds or thousands of times during its service life. Each cell has two electrodes, typically composed of non-identical metals or other conductive materials, with an electrolyte disposed between the electrodes.

From time to time it has been proposed to place a third electrode in a voltaic cell. It has been proposed that the third electrode might be used for sensing voltage, so as to arrive at an estimate of some parameter of interest in the cell. See for example P. F. Grieger et al., “Sealed battery with charge-control electrode”, U.S. Pat. No. 3,424,617 A, Jan. 28, 1969, and H. Reber, “Hermetically sealed storage battery including an auxiliary electrode”, U.S. Pat. No. 3,462,303 A, Aug. 19, 1969.

SUMMARY OF THE INVENTION

A system and method are described permitting a sophisticated control of a battery composed of a multiplicity of three-terminal electrochemical cells. Each cell has first and second terminals, connected with respective electrodes, one of which is a positive terminal and one of which is a negative terminal. Each cell has a third terminal connected with a grid electrode. A battery is composed of N cells. For each of the N cells, there is provided a respective capacitor switchably coupled to the second and third terminals thereof. A controller is connected through a switching matrix to the capacitors. In operation, the controller is connected sequentially to each capacitor among the multiplicity of capacitors, during which time the capacitor is momentarily uncoupled from its respective cell. When the controller is connected to one of the capacitors, it measures the voltage thereupon. The controller can then charge up or discharge the capacitor to drive it to a desired voltage level. Thereafter, the capacitor is disconnected from the controller and is coupled again to its respective cell.

Where possible, like reference numerals have been used in the various figures to denote like elements.

DETAILED DESCRIPTION

Turning first toFIG. 4, there is shown a three-terminal electrochemical cell. Ignoring for the moment the electrode133and the terminal134, we see a conventional electrochemical cell with positive terminal131and negative terminal132. An electrolyte, omitted for clarity inFIG. 4is disposed between the positive and negative terminals. A third terminal134sets this cell apart from the majority of electrochemical cells which have two terminals. The third terminal134connects with a grid133. The grid133lies within the electrolyte and may be compared in some ways with a grid of a triode vacuum tube, and may be compared in some ways with the gate of a field-effect transistor (“FET”). As mentioned above, it has been reported that some investigators have inserted such a grid into an electrochemical cell so as to facilitate the monitoring of electrical potential (voltage) at some point between the positive and negative terminals. In the system according to the invention, it is contemplated that the third terminal134and grid133are used not only for sensing purposes but also for control purposes, with some electrical potential applied to the grid so as to influence the function of the cell, for example during charging time or during discharging time.

The specific use to which the grid and third electrode will be put in a particular cell are a function of the particular chemistry selected for the cell (for example the composition of the positive and negative electrodes and the selection of electrolyte) and the physical structure (for example electrode surface structure). In one type of cell, the grid voltage might be employed to control (and perhaps to slow down) a charging current to permit an electrode surface to more readily absorb ions in a controlled way. This might be done to attempt to maximize the service life of the cell, or might be done to attempt to maximize the energy storage capacity of the cell.

In general, such an electrochemical cell is not employed in isolation but forms part of a battery (a number of cells in series). In such a battery the grids might be employed to maximize the performance of battery (for example, attempting to maximize service life or energy storage capacity), in which case the grids of the various cells might be driven in more or less the same way. But in such a battery, another possibility is that the grids might be employed as well as part of a cell balancing system. If a particular cell were seen to be out of balance with its neighbors, the grid of that cell might be driven in a particular way, driven rather differently than the grid drive for the neighboring cells.

It will be appreciated that the cell balancing approach described here has an advantage over a traditional cell balancing approach in which resistors are used to draw down particular cells so as to bring about balancing. The resistor approach wastes energy due to heat dissipation in the resistors.

It will be appreciated that in many implementations, the grid133is insulated chemically and electrically from the electrolyte of the cell and from the positive and negative electrodes of the cell. If it is insulated, then very little current would ever flow into or out of the cell via terminal134. The current flow would be the modest amount of current flow needed to bring about some desired potential at the grid.

Another possibility is that the grid133is selected to be some material that is nonreactive in the relevant context. Thus for some cell chemistries, the grid might be made of platinum and might not need to be insulated because platinum is nonreactive in the relevant context.

It is also possible the grid133might be neither insulated from the electrolyte nor nonreactive relative to the electrolyte.

We turn now toFIG. 1.FIG. 1shows a battery101composed of individual cells such as cell104ofFIG. 4. The battery101is connected via lines127,128with circuitry105which in a general way might include a load (such as motors in an electric car) and might include a charging system for recharging the battery101.

We now turn toFIG. 3.FIG. 3shows a typical circuitry105as inFIG. 1. Circuitry105includes a load130(for example a motor in an electric car) and a charging system129for recharging the battery.

Returning toFIG. 1, we see that each cell104,103,102in the battery101is in series with the other cells of the battery. Each cell has a “third electrode” as discussed in connection withFIG. 4. Each cell has a respective module204,203,202connected with the grid and another terminal thereof. The modules204,203,202are collectively termed a battery control apparatus207. Each of the modules such as module204is controlled so as to apply some desired electrical potential (voltage) to the grid and other terminal of the respective cell such as cell104.

Each module204,203,202may be an electronic driver, which produces a voltage according to a digital word stored in it, and which is converted to analog form through a D/A converter. As will be discussed below, other approaches may also be employed for the modules204,203,202.

Turning momentarily toFIG. 6, there is shown a controller209which controls the modules204,203,202. The controller209may receive information about current flowing into the load130or information about charging current sourced from the charging system129. The controller209controls the various modules204,203,202so as to bring about desired potentials at the respective grids of the respective cells. The controller209may also collect information from the various modules204,203,202so as to ascertain the actual potential at the respective grids. In this way the controller209may gain information for example about the state of charge of each cell or information about the chemical condition of each cell.

As was mentioned above, each module204,203,202may be an electronic driver. Turning now toFIG. 5, what is shown is a detail for a second approach for the modules ofFIG. 1. Each module204,203,202has a respective capacitor112,113,114connected between the grid terminal of its respective cell and between one of the other cell terminals. InFIG. 5it is portrayed that the capacitor is connected with the grid and with the negative terminal of the cell. It will be appreciated that the capacitor could, without departing from the invention, likewise instead be connected with the grid and with the positive terminal of the cell.

The connection of a capacitor (for example112) to its respective cell (for example104) is by means of switches (for example106,107). The switches might be traditional electromechanical switches (relays). But more likely the switches will be solid-state switches. Such switches are seen at106,107,108,109,110and111.

Also provided are switches (for example115,116,117,118,119and120) that selectively couple one or another of the capacitors112,113,114with a driver121with leads122,123(shown inFIG. 2).

During the time that a particular capacitor (for example114) is disconnected from its grid, it may be desired to maintain a particular potential at that grid. If so, then as shown inFIG. 5, optionally some additional capacitors314,313,312may be provided. As for capacitor114, when it is disconnected from its grid, capacitor314may be switched into place to maintain a particular potential at that grid. Just as each capacitor114has four switches106,107,115,116around it, so that the capacitor114may be selectively connected with the grid or with the driver121, so likewise each capacitor314has four switches around it, accomplishing similar selective connection with the grid or with the driver121. The four switches around capacitor314are unlabeled for clarity but their function is apparent to the alert reader from the present discussion.

It will be appreciated that in the arrangement ofFIG. 5, the number of switches may be around four times the number of cells if no additional capacitors314,313,312are provided. If the additional capacitors314,313,312are provided, then the number of switches may be around eight times the number of cells. But returning toFIG. 1, more generally the individual voltage control on the grids of the cells might use different circuitry than what is shown inFIG. 5. So returning briefly toFIG. 1, the number of switches might be some number other than four times the number of cells. Or the modules204,203,202might use completely different ways to isolate the grids from each other.

More will be said about the driver121in connection withFIG. 2. The driver121may have a microcontroller126with a control line135to the controller209. The controller209may be provided information about the system, such as the temperature of the battery101, the measured discharge current when the battery101powers the load130, or the measured charging current when the charging system129tries to charge up the battery101. The measurements of current are carried out by means of a current measurement process, the details of which are omitted for clarity inFIG. 1. The measurement of battery temperature is likewise carried out by a temperature sensor at the battery, omitted for clarity inFIG. 1.

Also visible inFIG. 2is a voltage driver124which is intended to be able to force the potential in one of the capacitors112,113,114to some desired potential. Also visible inFIG. 2is a sensing amplifier125. This amplifier125provides information of interest to the microcontroller126, namely the sensed potential on one or another of the capacitors112,113,114.

An exemplary voltage driver124may be relatively low impedance when it is trying to drive a capacitor to some desired potential and may be relatively high impedance if it is not trying to drive the capacitor at all. An exemplary amplifier125will have a very high input impedance at lines122,123so that it does not disturb the conditions being measured such as the potential on the capacitor such as112,113,114.

Switches316,318may be employed to permit the voltage driver124to be selectively connected or not connected with the lines122,123. Switches317,319may be employed to permit the amplifier125to be selectively connected or not connected with the lines122,123.

It will be appreciated that what is shown inFIG. 5is a switching matrix with four times as many switches as there are cells to control. The switching matrix is controlled with control lines controlled by controller209. The control lines are omitted for charity inFIGS. 1 and 2.

A typical sequence of steps in connection withFIG. 5is as follows. First, as a general rule, most of the time each capacitor112,113,114is connected with its respective cell104,103,102. As mentioned above, each grid133may be insulated, in which case the charge on the capacitor may stay substantially unchanged for some period of time. Or as mentioned above, the grid133may be a material that is electrically conductive but is not chemically reactive in its context (perhaps platinum), and again this may lead to the charge on the capacitor staying substantially unchanged for some period of time.

With the passage of time, the charge on a capacitor might change, for example due to a substantial running-down of a cell (due to discharge into a load) or due to a substantial charging-up of a cell.

A next step is that cell-side switches (for example106,107) are opened so that a selected capacitor (for example112) is no longer connected with its respective cell (for example104). After the cell-side switches are opened, then controller-side switches (for example115,116) are closed. This connects the selected capacitor with the driver121.

It is desirable that this switching be break-before-make. This protects the driver121from having to deal with the (sometimes very high) voltages present at some points in the battery101. In an electric car, for example, the battery voltage might be 400 volts or 600 volts. Preserving a break-before-make regime in the switching matrix saves having to “float” the driver121and saves having to rate its connections and insulation at such high voltages.

Once the selected capacitor is connected with the driver121, typically the first step will be to measure the voltage on the capacitor by means of sensing amplifier125, at a time when the voltage driver124has a floating (high impedance) output. If desired, the driver121may “touch up” the voltage on the capacitor by means of the voltage driver124, perhaps drawing down the voltage on the capacitor or charging up the capacitor. It will be noted that the sensing amplifier125permits the microcontroller126to know what has been accomplished by the voltage driver124. For example the voltage driver124may be “turned on” in a direction that tends to charge or discharge the capacitor, and sensing amplifier125may be employed to monitor the voltage on the capacitor, so that the microcontroller126can “turn off” the voltage driver124when the desired voltage has been reached.

The controller-side switches would then be opened, and after this, the cell-side switches would be closed, reconnecting the capacitor with its respective cell.

Depending upon the detailed goals of the battery control apparatus, it may be intended that the voltage at a particular grid remain constant or nearly constant even during the time that an associated switchable capacitor is disconnected from the grid. There are several approaches that might be followed to make this possible.

One possibility as mentioned above is to double the number of switchable capacitors so that each cell has two respective switchable capacitors, and at any given instant one capacitor or the other is being employed to force the grid to a desired voltage.

Regardless of the details of the battery control apparatus design, the process discussed in connection withFIG. 5is then repeated for the other capacitors, until all of the capacitors have been checked to see what voltage they contain and each capacitor driven up or down in voltage as desired.

The process is then repeated for all of the capacitors from time to time, so as to learn what voltages are present at the various capacitors and so as to drive the capacitors to desired voltages.

FIG. 7shows a detail of a third approach for the modules ofFIG. 1. Each module204,203,202has a respective capacitor112,113,114connected between the grid terminal of its respective cell and between one of the other cell terminals. InFIG. 7it is portrayed that the capacitor is connected with the grid and with the negative terminal of the cell. It will be appreciated that the capacitor could, without departing from the invention, likewise instead be connected with the grid and with the positive terminal of the cell.

The connection of a capacitor (for example112) to its respective cell (for example104) is by means of switches (for example106,107). The switches might be traditional electromechanical switches (relays). But more likely the switches will be solid-state switches. Such switches are seen at106,107,108,109,110and111.

Also provided are switches (for example115,116,117,118,119and120) that selectively couple one or another of the capacitors112,113,114with a driver121with leads122,123(shown inFIG. 2).

During the time that a particular capacitor (for example114) is disconnected from its grid, it may be desired to maintain a particular potential at that grid. If so, then as shown inFIG. 7, an additional capacitors315may be provided. As for capacitor114, when it is disconnected from its grid, capacitor315may be switched into place to maintain a particular potential at that grid. Just as each capacitor114has four switches106,107,115,116around it, so that the capacitor114may be selectively connected with the grid or with the driver121, so likewise capacitor315has many switches around it, accomplishing similar selective connection with any of the grids of the cells or with the driver121. The switches around capacitor315are unlabeled for clarity but their function is apparent to the alert reader from the present discussion.

It will be appreciated that in the arrangement ofFIG. 7, the number of switches may be around six times the number of cells.

The switching matrix ofFIG. 7is controlled with control lines controlled by controller209. The control lines are omitted for charity inFIG. 7.

A typical sequence of steps in connection withFIG. 7is as follows. First, as a general rule, most of the time each capacitor112,113,114is connected with its respective cell104,103,102. As mentioned above, each grid133may be insulated, in which case the charge on the capacitor may stay substantially unchanged for some period of time. Or as mentioned above, the grid133may be a material that is electrically conductive but is not chemically reactive in its context (perhaps platinum), and again this may lead to the charge on the capacitor staying substantially unchanged for some period of time.

With the passage of time, the charge on a capacitor might change, for example due to a substantial running-down of a cell (due to discharge into a load) or due to a substantial charging-up of a cell.

A next step is that cell-side switches (for example106,107) are opened so that a selected capacitor (for example112) is no longer connected with its respective cell (for example104). After the cell-side switches are opened, then controller-side switches (for example115,116) are closed. This connects the selected capacitor with the driver121.

The controller-side switches would then be opened, and after this, the cell-side switches would be closed, reconnecting the capacitor with its respective cell.

As shown inFIG. 7, it may be intended that the voltage at a particular grid remain constant or nearly constant even during the time that an associated switchable capacitor is disconnected from the grid.

The capacitor315is switched into place at a particular grid during the time that its respective capacitor is uncoupled from the grid.

The process discussed in connection withFIG. 7is then repeated for the other grids, until all of the capacitor that correspond with a grid have been checked to see what voltage they contain and each capacitor driven up or down in voltage as desired.

The process is then repeated for all of the capacitors from time to time, so as to learn what voltages are present at the various capacitors and so as to drive the capacitors to desired voltages.

In this way, the system may permit cell balancing. If it is learned that some cell is unbalanced relative to its neighbors, the selective tweaking of charge on the capacitor connected with that cell may tend to bring the cell back into balance.

The sensing by means of the grid electrodes may permit the driver121and controller209to arrive at a prediction of imminent failure of a cell, thus permitting the battery to be taken out of service chronologically prior to such failure.

The system may, in this way, permit more careful control of boundary conditions such as electrode management at extreme times (electrode nearly saturated with ions during charge, or electrode nearly empty of ions during discharge), thus permitting optimization of battery capacity or battery life.

The system and method are shown in a system in which each capacitor is connected with the grid electrode and with the negative electrode of the respective cell. But another approach is that each capacitor is connected with the grid electrode and with the positive electrode of the respective cell. One approach or the other might be optimal depending upon cell chemistry or physical configuration of the cells.

The alert reader will appreciate that other obvious changes and improvements can be made to the system and method without departing therefrom. Any such changes and improvements are intended to be encompassed by the claims which follow.