Circuit for testing a disconnecting system for a battery, and associated devices and method

Abstract

The present invention relates to a test circuit (30) for a system for disconnecting a high-power electrical source (12), the disconnecting system comprising a plurality of arms arranged in parallel, wherein each arm comprises contactors placed head-to-tail and separated by an intermediate point (P1, PI, PN), and wherein each contactor comprises a transistor, the test circuit (30) comprising: - for each intermediate point (P1, PI, PN), a respective current source (32_1, 32_l, 32_N) connected to the intermediate point (P1, PI, PN), wherein each current source (32_1, 32_l, 32_N) is able to supply a current; and - a reading unit (34) able to deliver a voltage that is representative of the sum of the currents supplied by each current source (32_1, 32_l, 32_N).

Description

[0001] Test circuit for a battery cut-off system, associated devices and method

[0002] The present invention relates to a test circuit for a cut-off system of a high-power electrical source, in particular a battery. The present invention also relates to a battery cut-off system and a battery comprising such a test circuit. The present invention also relates to a vehicle comprising such a cut-off system. The present invention also relates to a corresponding test method.

[0003] Typically, a battery comprises one or more current accumulators, also called electrochemical generators, cells, or elements. An accumulator is an electricity-generating device in which chemical energy is converted into electrical energy. The chemical energy comes from electrochemically active compounds deposited on at least one face of electrodes arranged in the accumulator. The electrical energy is produced by electrochemical reactions during a discharge of the accumulator. The electrodes, arranged in a container, are electrically connected to current output terminals that ensure electrical continuity between the electrodes and an electrical consumer with which the accumulator is associated.

[0004] In order to increase the electrical power delivered, several sealed accumulators can be combined together to form a battery. Thus, a battery can be divided into modules, each module being composed of one or more accumulators connected together in series and / or in parallel. Thus, a battery can, for example, comprise one or more parallel branches of accumulators connected in series and / or one or more parallel branches of modules connected in series.

[0005] A charging circuit is usually provided to which the battery can be connected to recharge the accumulators.

[0006] Furthermore, an electronic management system comprising measurement sensors and an electronic control circuit, more or less advanced depending on the applications, can be associated with the battery. Such a system makes it possible in particular to organize and control the charging and discharging of the battery, to balance the charging and discharging of the different accumulators in the battery in relation to each other.

[0007] For high power applications, typically batteries with a capacity greater than 25-30 Ah and / or operating at currents greater than 100 A, a cut-off system is also used comprising a plurality of parallel branches, each branch comprising at least one silicon-type contactor. Because the cut-off system provides insulation between the battery and its load, the correct operation of each contactor must be checked.

[0008] To achieve this, it is known to perform such detection using specific tests. A parallel circuit is used to route the voltage upstream or downstream of the contactors depending on the group tested. A chaser then allows the opening or closing of each arm of each group to be controlled one after the other to verify their proper functioning. Such an assembly is complex to control and involves a large number of additional components, therefore compromising the reliability of the complete system.

[0009] There is a need for a disconnection system test circuit that can more easily check the proper operation of contactors.

[0010] For this purpose, the description describes a test circuit for a system for cutting off a high-power electrical source, the cutting off system comprising a plurality of arms arranged in parallel, each arm comprising contactors placed head to tail and separated by an intermediate point, each contactor comprising a transistor, the test circuit comprising:

[0011] - for each intermediate point, a respective current source connected to the intermediate point, each current source being capable of supplying a current, and

[0012] - a reading unit capable of delivering a voltage representing the sum of the currents supplied by each current source.

[0013] According to particular embodiments, the test circuit has one or more of the following characteristics, taken in isolation or in all technically possible combinations:

[0014] - each transistor is an insulated gate field effect transistor.

[0015] - the reading unit has a resistor.

[0016] - the reading unit includes an amplifier.

[0017] - the current source comprises a field effect transistor with junction.

[0018] - the contactors connected to an output of the high-power electrical source form a first set, the first set being controlled by a first control unit.

[0019] - the contactors connected to an input of the system to be supplied forming a second set, the second set being controlled by a second control unit, the second control unit being distinct from the first control unit.

[0020] The description also describes a system for cutting off a high-power electrical source comprising a plurality of arms arranged in parallel, each arm comprising contactors placed head to tail and separated by an intermediate point, each contactor comprising a transistor, the cutting system comprising a test circuit.

[0021] The description also provides a high power electrical source provided with a cut-off system, the high power electrical source preferably being a battery.

[0022] The description also relates to a vehicle comprising a high-power electrical source, the vehicle being chosen from the list consisting of an aircraft, a means of rail transport, a means of road transport, a means of maritime or river transport.

[0023] The description also describes a method for testing a cut-off system for a high-power electrical source, the cut-off system comprising a plurality of arms arranged in parallel, each arm comprising contactors placed head to tail and separated by an intermediate point, each contactor comprising a transistor, the test method being implemented by a test circuit comprising:

[0024] - for each intermediate point, a respective current source connected to the intermediate point, and

[0025] - a reading unit, the method comprising a step of:

[0026] - supply of a current by each current source, and

[0027] - delivery by the reading unit of a voltage image of the sum of the currents supplied by each current source.

[0028] In this description, the expression "suitable for" means indifferently "adapted for", "adapted to" or "configured for".

[0029] Characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of non-limiting example, and made with reference to the appended drawings, in which:

[0030] - Figure 1 is a schematic representation of an installation comprising a battery, a cut-off system and a system to be supplied, and

[0031] - Figure 2 is an electrical diagram of a test circuit of the cut-off system of Figure 1.

[0032] An installation 10 is shown schematically in Figure 1.

[0033] The installation 10 comprises a battery 12 equipped with a cut-off system 14 and a system to be supplied 16.

[0034] As is known per se, a battery is generally an arrangement of a plurality of electrochemical elements, but for the sake of simplification, a case with a single electrochemical element is described below, knowing that the transposition to other arrangements is immediate.

[0035] The battery 12 comprises at least one electrochemical element and a system for managing the electrochemical element.

[0036] As explained earlier, an electrochemical cell is an electricity-generating device in which chemical energy is converted into electrical energy.

[0037] The electrochemical element therefore delivers a current and a voltage between two terminals.

[0038] The management system is a system specific to managing the electrochemical element.

[0039] In this case, battery 12 is a high-power battery, typically capable of supplying around ten kW

[0040] The system to be powered 16 here designates any load requiring such power.

[0041] Depending on the case, the system to be supplied 16 may include one or more pieces of equipment.

[0042] According to a particular example, the system to be powered 16 is a vehicle, so that the battery 12 is an on-board power supply.

[0043] According to one embodiment, the vehicle is chosen from the list consisting of an aircraft, a means of rail transport, a means of road transport, a means of maritime or river transport.

[0044] For the rest, an active sign convention is chosen for battery 12.

[0045] Thus, the discharge current, that is to say a current flowing from the battery 12 to the system to be supplied 16, is positive while the charge current, that is to say a current flowing from the system to be supplied 16 to the battery 12, is negative.

[0046] As seen in Figure 1, the cut-off system 14 is interposed between the battery 12 and the system to be powered 16.

[0047] This means that the cut-off system 14 is connected on the one hand to an output 12S of the battery 12 and on the other hand to an input 16S of the system to be supplied 16.

[0048] The cut-off system 14 is suitable for ensuring isolation between the battery 12 and the system to be supplied 16 when it is desired that no current flows between the system to be supplied 16 and the battery 12.

[0049] The cut-off system 14 is also capable of allowing the circulation of a current between the battery 12 and the system to be supplied 16.

[0050] The cutting system 14 comprises a plurality of arms 18 arranged in parallel. In the example of FIG. 1, the cutting system 14 comprises N arms, only the first arm 18_1, the second arm 18_2, the third arm 18_3 and the N-th arm 18_N being represented.

[0051] The number N is an integer preferably greater than or equal to 10.

[0052] As regards the arrangement, each arm 18_1 to 18_N is connected on the one hand to the output 12S of the battery 12 and on the other hand to the input 16S of the system to be powered 16.

[0053] Each arm 18_1 to 18_N has the same structure, i.e. two contactors 20_1 and 20_2 in series.

[0054] For the remainder of this document, the contactors connected to the battery 12 are called first contactors 20_1 while the contactors connected to the system to be supplied 16 are called second contactors 20_2.

[0055] In the chosen convention, the first contactors 20_1 can be referred to as upper contactors as opposed to the second contactors 20_2 which can be referred to as lower contactors.

[0056] A contactor is a controlled electronic component used to open or close a connection between two elements.

[0057] According to the example described, each contactor 20_1 or 20_2 is a semiconductor contactor.

[0058] A semiconductor contactor is more often referred to by the abbreviation SSR, which refers to the corresponding English term "solid-state relay", generally translated as contactor or static relay.

[0059] Such a contactor is a circuit allowing a connection to be opened or closed between two elements without using a mechanical or electromechanical element.

[0060] In the example described, each contactor 20_1 or 20_2 comprises a transistor 22_1 or 22_2.

[0061] This does not exclude the presence of other components if necessary.

[0062] Here, each transistor 22_1 or 22_2 is an insulated gate field effect transistor.

[0063] Such a transistor is more often referred to as a MOSFET transistor.

[0064] The acronym MOSFET refers to the English term "metal-oxide-semiconductor field-effect transistor" which can be literally translated as "metal-oxide-semiconductor field-effect transistor".

[0065] In the following, each first transistor is denoted first transistor T1 and each second transistor is denoted second transistor T2. The use of MOSFET transistors in this context makes it possible to obtain a low series resistance with low losses to allow the arms 18_1 to 18_N to dissipate the high power of the battery 12.

[0066] Thus, according to the case of figure 1, each transistor T1 or T2 has a drain, a source and a gate.

[0067] The drain, source and gate of the first transistor T1 are denoted respectively T1D, T1S and T1G while the drain, source and gate of the second transistor T2 are denoted respectively T2D, T2S and T2G.

[0068] Transistor T1 or T2 is used to modulate the current flowing between the drain and the source according to the control signal applied to the gate.

[0069] Depending on the nature of the majority carriers forming the current flowing between the drain and the source, the MOSFET transistor is called a P-type MOSFET or an N-type MOSFET.

[0070] In the case of Figure 1, each transistor T1 or T2 is an N-type MOSFET.

[0071] Furthermore, by constructing a MOSFET transistor, each transistor T1 or T2 comprises an intrinsic diode 24 arranged in antiparallel (anode connected to the source T1 S or T2S of the transistor T1 or T2 and cathode connected to the drain T1 D or T2D of the transistor T1 or T2).

[0072] Alternatively, each transistor T1 or T2 is of a type other than a MOSFET.

[0073] For example, each transistor T1 or T2 is a different field effect transistor than a MOSFET.

[0074] Such a transistor is more often referred to as an FET transistor, the acronym FET referring to the corresponding English term for "field-effect transistor".

[0075] In another example, each transistor T1 or T2 is an insulated gate bipolar transistor.

[0076] Such a transistor is better known as an IGBT transistor, the abbreviation IGBT corresponding to the corresponding English name for "insulated-gate bipolar transistor". In the example described, the contactors 20_1 and 20_2 are mounted head to tail.

[0077] In the mentioned case, the head-to-tail arrangement implies that, for each arm 18, the drain T1 D of the first transistor T1 is connected to the drain T2D of the second transistor T2.

[0078] As discussed more specifically with reference to the operation of the cut-off system 14, the head-to-tail arrangement ensures a good cut-off between the battery 12 and the system to be powered 16.

[0079] More specifically, this allows a bidirectional cutoff (allowing opening and closing upstream and downstream). An intermediate point P1, P2, P3, ... PN is also defined for each arm 18_1, 18_2, 18_3, ... 18_N, the intermediate point P1, P2, P3 or PN being a point located between the drain T1 D of the first transistor T1 and the drain T2D of the second transistor T2.

[0080] The intermediate point P1, P2, P3 or PN constitutes a floating node here.

[0081] In the case of P-type MOSFET transistors, the sources of each transistor are connected and the intermediate point P1, P2, P3 or PN is located between the two sources.

[0082] The cut-off system 14 further comprises two separate control units 26 and 28.

[0083] The first control unit 26 is capable of controlling a first set E1 of contactors.

[0084] The first set E1 brings together the first contactors 20_1.

[0085] According to the example described, the first control unit 26 is a gate control unit.

[0086] More precisely, the first control unit 26 is capable of controlling the gate T1 G of each first contactor 20_1.

[0087] For this, the first control unit 26 is connected to the grid T1G of each first contactor 20_1.

[0088] In operation, the first assembly E1 allows charging, that is to say the supply of a current from the system to be powered 16 to the battery 12.

[0089] The second control unit 28 is capable of controlling a second set E2 of contactors.

[0090] The second set E2 brings together the second contactors 20_2.

[0091] According to the example described, the second control unit 28 is a gate control unit.

[0092] More precisely, the second control unit 28 is capable of controlling the grid T2G of each second contactor 20_2.

[0093] For this, the second control unit 28 is connected to the grid T2G of each second contactor 20_2.

[0094] In operation, the second assembly E2 allows discharging, that is to say the supply of a current from the battery 12 to the system to be powered 16.

[0095] The cut-off system 14 includes a test circuit 30.

[0096] As will be detailed later, the test circuit 30 is suitable for testing each branch 18_1 to 18_N.

[0097] The test circuit 30 is shown more specifically in Figure 2. The test circuit comprises current sources 32_1 to 32_N and a reading unit 34.

[0098] Each current source 32_1 to 32_N is connected to a respective intermediate point P1 to PN.

[0099] The test circuit 30 thus comprises as many current sources 32_1 to 32_N as the cut-off system 14 comprises arms 18_1 to 18_N to be tested.

[0100] In this case, it is assumed that all arms 18_1 to 18_N of the cut-off system are to be tested.

[0101] However, for the sake of clarity, only three current sources are shown in Figure 2, the first current source 32_1, the i-th current source 32_i and the N-th current source 32_N.

[0102] Each current source 32_1 to 32_N is capable of supplying a current.

[0103] For certain applications, in order to reduce consumption, it may be interesting for the current delivered by each current source 32_1 to 32_N to be relatively low, typically of the order of milliamperes.

[0104] As seen in Figure 2, each current source 32_1 to 32_N comprises a field effect transistor with junction 36 in series with a resistor 38.

[0105] Such a transistor 36 is more often called a JFET transistor 36, the abbreviation JFET referring to the corresponding English name of “junction field effect transistor”.

[0106] The JFET transistor 36 has a drain 36D, a source 36S and a gate 36G while the resistor 38 has a first terminal 38B1 and a second terminal 38B2.

[0107] The first terminal 38B1 of resistor 38 is connected to the drain 36D of JFET transistor 36 while the second terminal 38B2 of resistor 38 is connected to both the intermediate point P1, Pi or PN and the gate 36G of JFET transistor 36.

[0108] More generally, each current source 32_1 to 32_N is produced by any electrical circuit making it possible to carry out the current supply function, such as a resistor supplied by a voltage source, an integrated circuit or a source diode.

[0109] The reading unit 34 is capable of delivering a voltage which is an image of the sum of the currents supplied by each current source 32_1 to 32_N.

[0110] According to the example of figure 2, the reading unit 34 comprises a resistor 40 called polarization resistor 40.

[0111] The bias resistor 40 has two terminals 40B1 and 40B2.

[0112] The first terminal 40B1 is connected to a power supply 41, and more precisely to the positive pole of the power supply 41, the negative pole being connected to ground. The power supply 41 is used here to polarize the polarization resistor 40 and therefore delivers a sufficiently high voltage between its two poles to allow such an effect. A voltage typically greater than 30 V makes it possible to obtain the polarization.

[0113] On the side of the second terminal 40B2, the bias resistor 40 is connected in series with each current source 32_1 to 32_N, and more precisely to the source 36S of the corresponding JFET transistor 36.

[0114] The polarization resistor 40 is thus traversed by the sum of the currents delivered by each current source 32_1 to 32_N.

[0115] The bias resistor 40 converts the current flowing through it into a voltage.

[0116] In this sense, thus positioned, the polarization resistor 40 serves as an adder.

[0117] The reading unit 34 also includes an amplifier 42.

[0118] According to the example described, the amplifier 42 is an operational amplifier 44 according to a differential assembly.

[0119] The operational amplifier 44 has a first input 44E1, a second input 44E2 and an output 44S.

[0120] Amplifier 32 further comprises four resistors R1, R2, R3 and R4 as well as two capacitors C1 and C2.

[0121] The first resistor R1 has a first terminal R1B1 connected to the first terminal R40B1 of the bias resistor 40 and a second terminal R1B2 connected to a second terminal C1 B2 of the first capacitor C1.

[0122] The second resistor R2 has a first terminal R2B1 connected to the second input 44E2 of the operational amplifier 44 and a second terminal R2B2 connected to an output 44S of the operational amplifier 44.

[0123] The third resistor R3 has a first terminal R3B1 connected to the second terminal R40B2 of the bias resistor 40 and a second terminal R3B2 connected to the second input 44E2 of the operational amplifier 44 as well as to the first terminal R2B1 of the second resistor R2.

[0124] The fourth resistor R4 has a first terminal R4B1 connected to a first terminal C1 B1 of the first capacitor C1 and a second terminal R4B2 connected to the first input 44E1 of the operational amplifier. The second terminal R4B2 of the fourth resistor R4 is, moreover, connected to the second terminal C1 B2 of the first capacitor C1 and to the second terminal R1B2 of the first resistor R1.

[0125] The first terminal C1B1 of the first capacitor C1 is connected to ground while the second terminal C1B2 of the first capacitor C1 is connected to both the second terminal R1B2 of the first resistor R1, the second terminal R4B2 of the fourth resistor R4 and the first input 44E1 of the operational amplifier. The second capacitor C2 has a first terminal C2B1 connected to the first terminal R2B1 of the second resistor R2 and a second terminal C2B2 connected to the second terminal R2B2 of the second resistor R2.

[0126] Such an amplifier 42 makes it possible to obtain at output a voltage such that:

[0127] OR :

[0128] • V s denotes the voltage at the 44S output of the operational amplifier,

[0129] • R1, R2, R3 and R4 respectively designate the values ​​of the first resistor R1, the second resistor R2, the third resistor R3 and the fourth resistor R4,

[0130] • V + denotes the voltage on the first input 44E1 , and

[0131] • y_ denotes the voltage on the second input 44E2.

[0132] The example of reading unit 34 described previously is not limiting.

[0133] In particular, the presence of the amplifier 42 is justified here by the implementation illustrated with currents supplied by the current sources 32_1 to 32_N which are relatively weak. Its presence is optional in the presence of a current strong enough to be read.

[0134] Furthermore, the reading unit 34 can be any circuit making it possible to read in voltage form the sum of the currents of the current sources 32_1 to 32_N.

[0135] The operation of the cut-off system 14 is now described with reference to several use cases.

[0136] In operation, the control units 26 and 28 control the gates T1G and T2G of the transistors T1 or T2 to change their state from an on state to a off state or vice versa.

[0137] In the on state, transistor T1 or T2 allows current to flow between source T1S or T2S and drain T1 D or T2D whereas, in the off state, transistor T1 or T2 interrupts the flow of current from drain T1 D or T2D to source T1S or T2S but part of the current can also pass through diode 24.

[0138] With the head-to-tail arrangement, the control of the two transistors T1 or T2 of each arm 18_1 to 18_N ensures that, in the on state, the current flows in the arm 18_1 to 18_N (closed arm configuration) while, in the off state, no current can flow, the current being blocked by one of the transistors T1 or T2 depending on the direction of current flow (open arm configuration).

[0139] When one of the two transistors T1 or T2 is short-circuited or has a fault, the arm 18_1 to 18_N will therefore be in a malfunction situation corresponding to the fact that, in the closed arm configuration, no current flows in the arm 18_1 to 18_N or that in the open arm configuration, part of the current flows in the arm 18_1 to 18_N.

[0140] In the event of a malfunction of an arm 18_1 to 18_N, the test circuit 30 can detect a difference in behavior between the measured current flow and the expected current flow due to the command sent to the gates T1G or T2G of the two transistors T1 and T2 of the arm 18_1 to 18_N.

[0141] In fact, the voltage value measured at the polarization resistor 40 reflects the number of arms 18_1 to 18_N closed or open.

[0142] With a suitable command sent by the first control unit 26 and the second control unit 28, it is possible to test the transistors T1 (of the set E1) and the transistors T2 (of the set E2).

[0143] Measuring the sum of the currents makes it possible to determine the number of functional transistors T1 (of the set E1) and the number of functional transistors T2 (of the set E2).

[0144] The test circuit 30 thus makes it simpler to test a plurality of arms 18_1 to 18_N in parallel comprising contactors 20_1 and 20_2.

[0145] The number of components involved in the test function is reduced while still allowing for complete testing.

[0146] In fact, the test circuit 30 makes it possible to determine the presence of short-circuited or faulty contactors 20_1 and 20_2 by indicating the number of arms 18_1 to 18_N affected.

[0147] Other applications of the test circuit 30 may be envisaged.

[0148] The test circuit 30 can thus be used to test a cut-off system for isolating a battery energy storage system from an electrical network.

[0149] A battery energy storage system is a system generally comprising a plurality of banks, each bank containing a set of batteries in connection with a respective conversion system.

[0150] Such a storage system is more often referred to as a BESS system, referring to the corresponding English term “Battery Energy Storage System”.

[0151] The cut-off system 14 is more generally usable for isolating a high-power electrical source from any system to be supplied involving the use of high power.

[0152] Here, a high-power electrical source is understood to mean any source capable of providing power greater than 1 kW. The cut-off system 14 is nevertheless capable of ensuring its cut-off functionality for much higher powers, typically several tens of kW.

Claims

CLAIMS 1. Test circuit (30) of a cut-off system (14) of a high-power electrical source (12), the cut-off system (14) comprising a plurality of arms (18_1, 18_2, 18_3, 18_N) arranged in parallel, each arm (18_1, 18_2, 18_3, 18_N) comprising contactors (20_1, 20_2) placed head to tail and separated by an intermediate point (P1, P2, P3, PI, PN), each contactor (20_1, 20_2) comprising a transistor (22_1, 22_2), the test circuit (30) comprising: - for each intermediate point (P1, P2, P3, PI, PN), a respective current source (32_1, 32_l, 32_N) connected to the intermediate point (P1, P2, P3, PI, PN), each current source (32_1, 32_l, 32_N) being capable of supplying a current, and - a reading unit (34) capable of delivering a voltage which is an image of the sum of the currents supplied by each current source (32_1, 32_l, 32_N).

2. Test circuit according to claim 1, wherein each transistor (22_1, 22_2) is an insulated gate field effect transistor.

3. Test circuit according to claim 1 or 2, wherein the reading unit (34) comprises a resistor (40).

4. Test circuit according to any one of claims 1 to 3, in which the reading unit (34) comprises an amplifier (34).

5. Test circuit according to any one of claims 1 to 4, wherein the current source (32_1, 32_l, 32_N) comprises a junction field effect transistor (36).

6. Test circuit according to any one of claims 1 to 5, wherein the contactors (20_1) connected to an output (12S) of the high power electrical source (12) form a first set (E1), the first set (E1) being controlled by a first control unit (26).

7. Test circuit according to claim 6, in which the contactors (20_2) connected to an input (16E) of the system to be supplied (16) form a second assembly (E2), the second assembly (E2) being controlled by a second control unit (28), the second control unit (28) being distinct from the first control unit (26).

8. Cut-off system (14) of a high-power electrical source (12) comprising a plurality of arms (18_1, 18_2, 18_3, 18_N) arranged in parallel, each arm (18_1, 18_2, 18_3, 18_N) comprising contactors placed head to tail and separated by an intermediate point (P1, P2, P3, PI, PN), each contactor (20_1, 20_2) comprising a transistor (22_1, 22_2), the cut-off system (14) comprising a test circuit (30) according to any one of claims 1 to 7.

9. High power electrical source (12) provided with a cut-off system (16) according to claim 8, the high power electrical source (12) preferably being a battery (12).

10. Vehicle comprising a high-power electrical source (12) according to claim 9, the vehicle being chosen from the list consisting of an aircraft, a means of rail transport, a means of road transport, a means of maritime or river transport.

11. A method for testing a cut-off system (14) of a high-power electrical source (12), the cut-off system (14) comprising a plurality of arms (18_1, 18_2, 18_3, 18_N) arranged in parallel, each arm (18_1, 18_2, 18_3, 18_N) comprising contactors (20_1, 20_2) placed head to tail and separated by an intermediate point (P1, P2, P3, PI, PN), each contactor (20_1, 20_2) comprising a transistor (22_1, 22_2), the test method being implemented by a test circuit (30) comprising: - for each intermediate point (P1, P2, P3, PI, PN), a respective current source (32_1, 32_l, 32_N) connected to the intermediate point (P1, P2, P3, PI, PN), and - a reading unit (34), the method comprising a step of: - supply of a current by each current source (32_1, 32_l, 32_N), and - delivery by the reading unit (34) of a voltage representing the sum of the currents supplied by each current source (32_1, 32_l, 32_N).

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