Battery comprising a plurality of elecrochemical cells and, for each cell, a device for controlling the voltage across the terminals of said cell

A battery is provided. The battery includes a plurality of electrochemical cells connected in series with each other and adapted for each generating an electric current from an oxidation-reduction reaction between an oxidizing fluid and a reducing fluid. The battery also includes, for each electrochemical cell, a control device for controlling the voltage across the terminals of the cell. The battery also includes a voltage regulator device electrically connected to the cell so that the control device measures the voltage across the terminals of the cell, increased by an offset voltage across the terminals of the regulator device.

The present invention relates to a battery, of the type comprising a plurality of electrochemical cells, connected in series with each other and adapted for each generating an electric current from an oxidation-reduction reaction between an oxidizing fluid and a reducing fluid and for each electrochemical cell, to a device for controlling the voltage across the terminals of said cell.

BACKGROUND

Electrochemical cells are known, allowing electricity to be produced by an oxidation-reduction reaction between an oxidizing fluid and a reducing fluid. Notably, fuel cells are known allowing electricity to be produced by an oxidation-reduction reaction between a fuel, comprising hydrogen, and an oxidizer, comprising oxygen. The fuel is injected into an anode compartment and the oxidizer is injected into a cathode compartment, an electrolyte layer ensuring the seal between both of these compartments, allowing ion exchanges. Because of these ion exchanges, hydrogen contained in the fuel may react with the oxygen contained in the oxidizer giving water, by generating electrons at the anode. The result of this, during the operation of the fuel cell, is the establishment of a potential difference between both sides of the electrolyte, this potential difference may be utilized for generating an electric current.

However, the potential differences which are established within a cell of a fuel cell remain low, of the order of 0.6 to 1.0V. Also, in order to obtain a utilizable output voltage, the cells are most often stacked and electrically connected in series with each other, within what is currently called a fuel cell.

However, inside such a stack, it is important to control the proper operation of each cell independently of the operation of the other cells. Such an individual control of the cells actually gives the possibility of detecting a possible defect as early as possible, and of easily identifying the faulty cell in order to replace it.

EP 1 323 204 thus proposes a device for individually controlling the voltage of the cells of a fuel cell. This control device for each cell of the fuel cell comprises a resistor and an optocoupler connected in series to the terminals of the cell. An output signal of each optocoupler is adapted so as to establish a positive or negative voltage depending on whether the voltage on the input terminals of the optocoupler is greater or less than its threshold voltage, and to be transmitted to an interpretation unit which sends a dysfunction signal when one of the output signals has a negative voltage.

This control device has the advantage of being economical. However, it was observed that it signaled dysfunctions in an untimely manner, even when all the cells of the fuel cell were operating properly.

SUMMARY OF THE INVENTION

An object of the invention is to propose an inexpensive and reliable solution for controlling the voltage across the terminals of each electrochemical cell of a battery.

For this purpose, a battery is provided a voltage regulator device, electrically connected to said cell so that the control device measures the voltage across the terminals of the cell increased by an offset voltage across the terminals of the regulator device.

In preferred embodiments of the invention, the battery further has one or more of the following features, taken individually or according to all the technically possible combination(s):the offset voltage is set;the regulator device is a passive device, such as a diode;the regulator device is electrically connected through a first terminal, to a terminal of a consecutive cell of the battery, and to the cell whose voltage is measured, and, through a second terminal, to the other terminal of said consecutive cell;the first terminal of the regulator device is in a direct electric connection with the terminal common to the cell whose voltage is measured and to the consecutive cell;a resistor is electrically inserted between the second terminal of the regulator device and the terminal of the consecutive cell to which is connected said second terminal;the control device comprises an input connected to a terminal of the cell, the voltage of which is measured, on the one hand and to the second terminal of the regulator device on the other hand;the control device is an active optical element;the control device comprises an output for constructing an image of the measured voltage, the outputs of at least two of the control devices being a electrically connected in series with each other;the control device comprises an output for constructing an image of the measured voltage, the outputs of at least two of the control devices being electrically connected in parallel with each other;it is a fuel cell.

DETAILED DESCRIPTION

In the following, reference will be made to a fuel cell according to the invention, it being understood that the invention may also be applied to electric batteries.

A cell15of said fuel cell is illustrated inFIG. 1. It comprises a membrane-electrode assembly16inserted between an anode plate18and a cathode plate22.

The membrane-electrode assembly16comprises an ion exchange membrane26sandwiched between an anode28aand a cathode28b.

The membrane26electrically insulates the anode28afrom the cathode28b.

The membrane26is adapted for only letting through charged ions, preferably cations. The membrane26is generally a proton exchange membrane, adapted for only letting through protons. The membrane26is typically in a polymeric material.

The anode28aand the cathode28beach comprise a catalyst, typically platinum or a platinum alloy, for facilitating the reaction.

The anode plate18delimits an anode conduit20four circulating a reducing gas along the anode28aand in contact with the latter. To do this, the plate18is provided with at least one channel made in the face of the plate turned towards the membrane-electrode assembly16and closed by said membrane-electrode assembly16. The anode plate18is formed with an electrically conducting material, typically graphite. The reducing gas used is a gas comprising dihydrogen, such as for example pure dihydrogen.

The cathode plate22delimits a cathode conduit24for circulating an oxidizing gas along the cathode28band in contact with the latter. To do this, the plate22is provided with at least one channel made in the face of the plates turned towards the membrane-electrode assembly16and closed by said membrane-electrode assembly16. The cathode plate22is formed with an electrically conducting material, typically graphite. The oxidizing gas used is a gas comprising dioxygen, such as for example pure dioxygen, air, or a restored mixture of dioxygen and of a neutral gas, such as nitrogen or carbon dioxide.

The membrane26separates the oxidizing and reducing gases. It is positioned between the anode plate18and the cathode plate22of the cell15and electrically insulates the latter from each other.

The anode28ais in electric contact with the anode plate18. The cathode28bis in electric contact with the cathode plate22. During operation of the fuel cell, an oxidation of the reducing gas occurs at the anode28a, inducing the generation of electrons and protons. The electrons are then in transit via the anode plate18to the cathode28bof the cell15, or to the cathode of another cell, in order to participate in reduction of the oxidizing gas.

The cell15thus comprises two electric terminals: a negative electric terminal is formed by the anode plate18, and a positive electric terminal is formed by the cathode plate20.

The cell15is stacked with other similar cells, the anode plate18of each cell being in contact with the cathode plate22of the neighboring cell. The anode and cathode plates18,22thus ensure the transfer of the electrons from the reducing gas circulating in a cell towards the oxidizing gas circulating in another cell. The anode18and cathode22plates of two neighboring cells of the stack are preferably made simultaneously in the same material and form together a bipolar plate.

With reference toFIGS. 2 and 3, the fuel cell comprises for each cell15A, a device30four controlling the voltage at the terminals of said cell15A. It further comprises a voltage regulator device32, laid out so that the control device30measures the voltage VAat the terminals of the cell15A, increased by a fixed offset voltage ΔV.

The voltage VAis typically comprised between 0.5 and 1.0V, during normal operation of the cell15A. The offset voltage ΔV is the voltage across the terminals of the regulator device32. Preferably, the offset voltage ΔV is substantially equal to 0.3V.

The control device30is formed by an active optical device, typically an optocoupler. It comprises an input34, electrically connected to the cell15A, for measuring its voltage VA, and an output36, for constructing an image of the measured voltage VA. The input34and the output36are electrically insulated.

The entry34comprises a light-emitting diode38, adapted for emitting photons when the voltage on its terminals is greater than a threshold voltage Vmin. During normal operation of the cell15A, the threshold voltage Vminis less than the sum of the voltages VAand ΔV, so that the light-emitting diode38emits photons. The threshold voltage Vminis typically comprised between 0.8 and 1.2V.

The output36comprises a phototransistor40adapted so as to be in a closed configuration, i.e. electrically connecting its collector41A and its emitter41B to each other, when its base41C receives photons, and to be in an open configuration, i.e. electrically insulating its collector41A and its emitter41B from each other, when its base41C does not receive any photons.

The phototransistor40is optically coupled with the light-emitting diode38. In other words, the light-emitting diode38and the phototransistor40are laid down so that the photons emitted by the diode38reach the base41C of the phototransistor40. The passing of the phototransistor40from its open configuration to its closed configuration is thus controlled by the state of the light-emitting diode38.

The regulator device32is a passive electric device. In particular, the regulator device32is a diode, typically a Schottky diode, advantageous because inexpensive and giving a fixed and easily controllable offset voltage. The diode is oriented in order to let through currents from high potential areas to low potential areas. Thus, when operating normally, the voltage across the terminals of the regulator device32, which is also the offset voltage ΔV, is equal to the threshold voltage of the diode.

The regulator device32comprises a first terminal42, electrically connected to a terminal44of a cell15B of the stack consecutive to the cell15A. In other words, the first terminal42is connected to the bipolar plate forming the separation between the cells15A and15B. The first terminal42is in the director electric connection with the terminal44, and to the cells15A,15B, i.e. there is no electric compound interposed between the terminals42and44.

The regulator device32also comprises a second terminal46, electrically connected to the other terminal48of the consecutive cell15B. In other words, the second terminal46is electrically connected to the plate18,22of the cell15B opposite to the face of the cell15B in contact with the cell15A. A resistor50is inserted between the second terminal46and the terminal48in order to limit the intensity of the current flowing through the regulator device32.

The input34of the control device30is connected to the second terminal46of the regulator device32on the one hand and to the terminal52of the cell15A other than the common terminal44.

In the example illustrated inFIG. 2, the common terminal44is the terminal of the cell15A having the highest potential. The consecutive cell15B is then an upper cell of the stack. The regulator device32is oriented so that its terminal42is its lowest potential terminal. Thus the voltage across the terminals of the input34is equal to the sum of the voltages VAand ΔV.

In the example illustrated inFIG. 3, the common terminal44is the terminal of the cell15A having the lowest potential. The consecutive cell15B is then a lower cell of the stack. The regulator device32is oriented so that its terminal42is its highest potential terminal. Thus, the voltage across the terminals of the input34is equal to the sum of the voltages VAand ΔV.

As the control device30measures the voltage VAof the cell15A increased by the offset voltage ΔV, the voltage VAmay vary over a wider range before the voltage across the terminals of the input34of the control device30passes under the threshold voltage Vmin. Untimely detections of dysfunction of the cell15A are thereby avoided.

With reference toFIGS. 4 and 5, the fuel cell comprises four cells15C,15D,15E,15F connected in series with each other. It will be noted that this number of cells is only given as an example and that, in alternatives of the invention, the fuel cell comprises another number of cells, this number depending on the alternatives being greater or less than four.

The voltage VC, VD, VE, VFof each cell15C,15D,15E,15F is measured by a control device,30C,30D,30E,30F respectively. In the same way as for the cell15A illustrated inFIGS. 2 and 3, a regulator device,32C,32D,32E,32F respectively, is provided for each cell15C,15D,15E,15F, so that the associated control device30C,30D,30E,30F measures the voltage VC, VD, VE, VFincreased by an offset voltage ΔV.

For each of the cells15D,15E,15F, the cell consecutive to the terminals of which is connected the regulator device32D,32E,32F, is formed by the upper consecutive cell,15C,15D,15E respectively. For the cell15C, the cell consecutive to the terminals of which is connected the regulator device32C is formed of by the lower consecutive cell15D.

In the alternative shown inFIG. 4, the outputs36of the control devices30C,30D,30E,30F are connected in series with each other, between a positive potential V+line60(typically 5 volts) and an output line62. A resistor64is electrically interposed between the mine62and a reference potential V0line66.

Thus, as long as all the phototransistors40of the control devices30C,30D,30E,30F are closed, the potential of the output line62is equal to the positive potential V+. If on the other hand the potential of the output line62passes to the reference potential V0, it is the sign that one of the phototransistors40has opened, and therefore that one of the cells15C,15D,15E,15F is subject to dysfunction.

In the alternative shown inFIG. 5, the outputs36of the control devices30C,30D,30E,30F are connected in parallel with each other, between a positive potential V+line70(typically 5 volts) and a reference potential V0line72. In particular, each collector41A is electrically connected to the line70and each emitter41B is electrically connected to the line72. Further, an output line74C,74D,74E,74F extends each emitter41B. Finally, a resistor76is inserted between each emitter41B and the line72.

Preferably, a multiplexer80is provided for combining the signals of the different output lines74C,74D,74E,74F and for transmitting them via a single line82.

Thus, as long as the phototransistor40of the control device30C,30D,30E,30F associated with an output line74C,74D,74E,74F, respectively is closed, the potential of said output line74C,74D,74E,74F is equal to the positive potential V+. If on the other hand the potential of the output line74C,74D,74E,74F passes to the reference potential V0, this is the sign that that the associated phototransistor40has opened, and therefore that the associated cell15C,15D,15E,15F respectively, is subject to dysfunction.

This embodiment allows more accurate control of the fuel cell by providing more detailed information than in the embodiment ofFIG. 4, for a hardly higher cost.

In a third alternative of the invention (not shown), the outputs36of a first pair of control devices30C,30D, are connected in series with each other, and the outputs36of a second pair of control devices30E,30F are connected in series with each other, between a line with a positive potential V+and an output line. The output lines are connected in parallel with each other to a multiplexer, provided for combining the signals of the different output lines.

Thus, as long as the phototransistors40of the control devices30C,30D,30E,30F associated with an output line are closed, the potential of said output line is equal to the positive potential. If on the other hand the potential of the output line passes to the reference potential V0, it is the sign that one of the associated phototransistors40has opened, and therefore that one of the associated cells15C,15D,15E,15F is subject to dysfunction.

It is therefore possible to identify dysfunctions at pairs of cells, without however being able to identify a dysfunction of each cell independently of the others, like in the embodiment ofFIG. 5. This third alternative is advantageous in the case when the stack comprises a large number of cells.

It will be noted that, in the example of the third alternative given above, the control devices30are grouped pairwise, but the control devices30may be grouped batchwise each comprising more than two control devices30, the number of control devices30within a same batch may vary from one batch to the other.

By means of the invention, it is therefore possible to control that the voltage across the terminals of each cell of the stack remains in the tolerated value range, reliably and at a lower cost.