Method for detecting a loosening of an electrical connection screwed into an electric battery

Abstract

The invention relates to a monitoring method implemented in a system comprising a unit formed as an assembly of a plurality of N electrical energy storage modules (M(k)), each module comprising a positive terminal (BP) and a negative terminal (BN), the system comprising sensors for determining a temperature denoted TPos(k) at each of the positive terminals, and a temperature denoted TNeg(k) at each of the negative terminals, the method comprising a step of acquiring current temperature values TPos(k) and TNeg(k) for each module M(k) of rank k, with an alert being generated in the case where one of the following conditions [C1] or [C2] becomes true; [C1]: (TPos(k) – TNeg(k)) > SL1 and (TNeg(k+1) - TNeg(k)) > SL2, where SL1 and SL2 are temperature thresholds, and [C2]: (TNeg(k) – TNPos(k)) > SL1 and (TPos(k-1) - TPos(k)) > SL2.

Description

DESCRIPTION TITLE OF THE INVENTION: METHOD FOR DETECTING LOOSENING OF A SCREWED ELECTRICAL CONNECTION IN AN ELECTRIC BATTERY

[0001] The present invention claims priority from French application No. 2302678 filed on 03 / 22 / 2023, the content of which (text, drawings and claims) is incorporated herein by reference.

[0002] The invention relates to a method and a system for monitoring a screwed electrical connection and detecting a loosening thereof in a battery pack, i.e. an assembly formed as a series assembly of a plurality of electrical energy storage modules.

[0003] A battery pack consists of a series of modules connected to each other by a system of connecting elements called bus bars (or bus bars or 'busbars') in the trade, generally screwed to each electrical terminal of the modules.

[0004] Furthermore, throughout the text of this document, the term "electrical energy storage module", otherwise known as "battery module", refers to an electrochemical cell or several electrochemical cells connected together. Each electrochemical cell can store electrical energy and can generate current by chemical reaction. The electrochemical cell can be, for example, of the lithium-ion (or Li-ion) type, of the Ni-Mh, or Ni-Cd type, without excluding lead.

[0005] Screwing contributes to the mechanical maintenance of the modules, but also contributes to good electrical contact between the terminal and the bus bar system, the screwing ensuring the lowest possible electrical contact resistance.

[0006] In practice, this screw connection can sometimes loosen, for example due to vibrations transmitted to the battery. This increases the electrical contact resistance between the terminal and the bus bar system. This is very harmful because, with the high current flowing through this contact resistance, a temperature rise can occur, creating an unwanted hot spot, which can locally cause material damage, even leading to the start of combustion.

[0007] It is therefore desirable to be able to detect a loosening of a screw connection as considered here. Detection should preferably be early and reliable. False detections or no detection at all should also be avoided.

[0008] It is known from document FR3115607 to measure potential differences between several parts of the modules constituting the battery pack, in order to deduce a loosening. However, measurements of potential differences are subject to electrical interference and ground deviations under high current which disrupt the measurements.

[0009] The inventors sought to develop another solution to detect loosening with good reliability.

[0010] To do this, the invention thus relates, in its broadest sense, to a method implemented in a system comprising an assembly formed as an assembly of a plurality of N modules (M(k)) for storing electrical energy, each module comprising a positive terminal and a negative terminal, the modules being connected to each other in series by screw connections, the system comprising sensors for determining a temperature denoted TPos(k) prevailing at the location of each of the positive terminals of the modules (M(k)), and a temperature denoted TNeg(k) prevailing at the location of each of the negative terminals of the modules (M(k)), the method being characterized in that it comprises a step of recurrent acquisition of temperature values ​​TPos(k) and TNeg(k) for each module M(k) of rank k, and in that it generates an alert in the case where one of the following conditions [C1] or [C2] becomes true: [C1]: (TPos(k) - TNeg(k)) > SL1 and (TNeg(k+1 ) - TNeg(k)) > SL2, for k from 1 to N-1 , SL1 being a first predefined temperature threshold, SL2 being a second predefined temperature threshold, [C2]: (TNeg(k) - TNPos(k)) > SL1 and (TPos(k-1 ) - TPos(k)) > SL2, for k from 2 to N, said alert being representative of a loosening of a screwed connection.

[0011] Thanks to these provisions, by focusing on temperature differences between neighboring terminals, we are free from general temperature changes. In other words, we work in local differential mode and the absolute temperature values ​​have no influence on the conditions that trigger the alert.

[0012] According to the first condition C1, we are interested in a temperature difference concerning a negative terminal.

[0013] According to the second condition C2, we are interested in a temperature difference concerning a positive terminal.

[0014] Triggering an alert following the occurrence of a true result on one of the conditions C1 or C2 makes it possible to unambiguously identify the suspect connection terminal.

[0015] More precisely, if condition C1 becomes true, for a given index k then it is the negative terminal of the module of index k which is suspected of a loosening of its screw connection. Conversely, in a similar manner, if condition C2 becomes true, for a given index k then it is the positive terminal of the module of index k which is suspected of a loosening of its screw connection.

[0016] This information is stored for future maintenance.

[0017] We note that in each condition C1 or C2, the first difference term measures the temperature difference between the 2 terminals of the same module while the second difference term between two neighboring terminals belonging to two neighboring modules connected by an electrical connection element which participates in two screwed electrical connections.

[0018] In nominal operation, neighboring terminals have very similar temperatures, regardless of the outside temperature conditions and changes in outside temperatures over time.

[0019] If a connecting element contributing to two screw connections loosens, then the two terminals in thermal contact with this connecting element see their temperature increase relative to the temperature of the other terminals of the battery pack in question.

[0020] Both screw connections of a connecting element are hotter than the neighboring ones and the loose one is hotter than the other belonging to the same connecting element.

[0021] It is noted that said 'assembly comprising the plurality of N modules' is called in the trade 'battery pack'. It is noted that several battery packs can be connected in parallel to form a complete battery.

[0022] If an alert is generated for a vehicle, it is communicated to the driver or maintenance manager of the vehicle in which the battery pack is located. This is an incentive to repair or to go to the garage for checking and possibly tightening the screw connection. It should be noted that the index of the module on which the fault occurs, as well as the positive or negative terminal, can be memorized and returned to the maintenance technician who will work on the vehicle, which facilitates checking and, if necessary, repair.

[0023] Advantageously, the method may further comprise a step of disconnecting the assembly following the generation of an alert. It is thus possible to isolate the assembly (the battery pack) in question and allow the other battery packs to operate normally. As in practice, there are several battery packs in parallel, the supply of energy can still continue.

[0024] Advantageously, according to one option, the method may further comprise a step of inhibiting the assembly following the generation of an alert. The battery pack is set aside until a repair is validated, and it is avoided to request it again before verification or repair.

[0025] According to one option, it can be provided that the first temperature threshold SL1 has a value of 4°C. This threshold is small enough to cause early detection. And at the same time the inventors noticed that it was large enough to be indicative of a degradation of a screw connection on one of the terminals of a module. A difference in temperatures between the two ends of an electrical connection element is indicative of an imbalance in tightening / screwing between the two ends and consequently a degraded tightening / screwing can be detected thanks to the threshold SL1.

[0026] According to one option, it can be provided that the second temperature threshold SL2 has a value of 2 °C. Under this condition also, the detection is early, and the threshold SL2 is sufficiently large to be indicative of a deterioration of a screw connection on one of the screw terminals. A difference in temperatures between the two ends of an electrical connection element is indicative of an imbalance in tightening / screwing between the two ends and consequently a degraded tightening / screwing can be detected thanks to the threshold SL2.

[0027] The invention also relates to an assembly forming part of a battery of an electric or hybrid motor vehicle, the assembly being formed as a series-mode assembly of a plurality of N modules (M(k)) for storing electrical energy, each module comprising a positive terminal and a negative terminal, the modules being connected to each other in series by screw connections, the assembly comprising sensors for determining a temperature denoted TPos(k) prevailing at the location of each of the positive terminals of the modules and a temperature denoted TNeg(k) prevailing at the location of each of the negative terminals of the modules, the assembly further comprising a computer configured to implement the monitoring method as described above, in order to detect a loosening of a screw connection.

[0028] According to one option, the assembly comprises a first end module forming a first end of the assembly, a second end module forming a second end of the assembly and intermediate modules, the positive terminal of each intermediate module M(k) being electrically connected to the negative terminal of one of its two neighbors by a connecting element fixed by screwing, the negative terminal of each intermediate module M(k) being electrically connected to the positive terminal of the other of its two neighbors by another connecting element fixed by screwing, said connecting elements each being formed as a perforated metal strip.

[0029] The assembly of the modules thus forms a chain, the modules being the links connected to each other by the electrical connection elements. The connection element, formed as a perforated metal strip, has a very low electrical resistance of the order of a few micro-ohms, given its cross-section and its very good conductive material. The connection element can also be described as a flat bar, hence the name bus bar.

[0030] According to one option, the sensors intended to determine a temperature comprise, for each positive connection terminal of the modules and each negative connection terminal of the modules, a thermistor or a thermocouple in intimate thermal coupling with said connection terminal and connected to the computer.

[0031] The thermistor or thermocouple can be attached directly to the terminal. Alternatively, it can be attached to the electrical connection element in the immediate vicinity of the terminal.

[0032] According to one option, the assembly further comprises a power switch arranged in series on a conductive element for connecting the assembly to a network.

[0033] This makes it possible to isolate the faulty battery pack concerned and allow the other battery packs to operate normally.

[0034] The invention also relates to an electric or hybrid motor vehicle, comprising at least one assembly forming part of a battery, the assembly being as described above.

[0035] The invention will be further detailed by the description of non-limiting embodiments, and on the basis of the appended figures illustrating variants of the invention, in which: - [Fig.1] schematically and partially illustrates, in elevation, battery modules connected together and forming part of an assembly called a 'battery pack' in which the present invention is implemented; - [Fig.2] schematically illustrates in top view a bolted connecting element connecting two neighboring modules; - [Fig.3] schematically illustrates a battery pack system with modules and a computer configured to implement the method. - [Fig.4] illustrates in perspective an electrical connection element; - [Fig.5] schematically illustrates a temperature profile along a module assembly in the case of a negative terminal with a loosening;

[0036] In the various figures, the same references designate identical or similar elements. For reasons of clarity of the presentation, certain elements are not necessarily represented to scale.

[0037] With reference to figure 3, a battery pack 1 is shown which comprises a series of modules connected to each other by a system of electrical connection elements 2.

[0038] Here a module is an electrical energy storage module, otherwise called a battery module. Each module includes one or several electrochemical cell(s). The modules are identical, they are generally identified M(k), k being a rank or naming index. With a number N of modules, k varies from 1 to N.

[0039] In the illustrated example, N=8, the modules M1, M2, M3, M4, M5, M6, M7, M8 are arranged next to each other, in a line. There is no limitation on the number of modules N, the present invention works even with a very large number N of modules.

[0040] Referring to Figures 1 and 3, each module M(k) comprises a positive terminal BP and a negative terminal BN.

[0041] The N modules are electrically connected in series configuration, the positive terminal BP of each intermediate module M(k) is electrically connected to the negative terminal BN of one of its neighbors (here the one on the right M(k+1 )) by a connecting element 2, the negative terminal BN of each intermediate module M(k) is electrically connected to the positive terminal BP of the other of its neighbors (here the one on the left M(k-1 )) by another connecting element 2.

[0042] For example, the negative terminal BN of module M2 is connected to the positive terminal BP of module M1 and the positive terminal BP of module M2 is connected to the negative terminal BN of module M3. The same logic applies mutatis mutandis to the other modules.

[0043] The assembly includes an intermediary electrically connected on either side to its two immediate neighbors.

[0044] Each connecting element 2 is in the form of a flat bar, also a metal strip, for example obtained by press cutting. With reference to Figure 4, the thickness E2 of such a metal strip is generally uniform, between 1 mm and 4 mm. For example, the width W is between 10 mm and 20 mm. For example, the length L is between 50 mm and 100 mm. The material constituting each connecting element 2 is metallic, for example an aluminum or copper alloy, or a steel alloy.

[0045] Each connecting element 2 comprises a first end 21 with a first hole 23, and a second end 22 with a second hole 24.

[0046] In other words, each connecting element 2 is a perforated metal strip. It is fixed by screwing to the terminals of the modules.

[0047] More precisely, the first hole 23 receives the rod 4 of a terminal of a module and the second hole 24 receives the rod 4 of another terminal of a module.

[0048] The inner diameter D of each hole can be between 6 mm and 16 mm. Preferably, an inner diameter between 8 mm and 12 mm can be chosen.

[0049] A nut 3 is screwed onto each terminal rod. The rod 4 may include an external thread forming a counterpart to the internal thread of the nut. Alternatively, a conical sleeve may be provided, with an external thread on the sleeve and radially internal teeth for hooking onto the rod 4.

[0050] A washer 41, for example a Grower type washer, may be provided between the nut 3 and the connecting element 2.

[0051] Each connecting element 2 bears on a shoulder 42 of the terminal to which it is screwed.

[0052] The electrical connection elements could, however, have a different shape while fulfilling the same function.

[0053] In addition to the example presented here, we note more generally that any other screwing method may be suitable.

[0054] Each connecting element 2 participates in two screw connections, namely a first screw connection at the location of the negative terminal and a second screw connection at the location of the positive terminal. The screw connection itself comprises the rod 4 of the terminal, the end zone of the connecting element 2 with the corresponding hole and the nut 3. The nut 3 tightens the end zone of the connecting element on the terminal.

[0055] Sensors are provided to measure temperatures.

[0056] Generally, the assembly comprises sensors 11, 12 for determining a current temperature denoted TPos(k) prevailing at the location of each of the positive terminals of the modules and a current temperature denoted TNeg(k) prevailing at the location of each of the negative terminals of the modules.

[0057] More specifically, a capture element 11 is provided on each positive connection terminal BP of each module of each module, and a capture element 12 on each negative connection terminal BN of each module of each module.

[0058] As for the sensing element, one can choose in particular a thermocouple, a variable resistor with a positive coefficient (PTC) or a variable resistor with a negative coefficient (NTC).

[0059] Each thermistor is connected to a computer 5, by a pair of conductors 28. Alternatively, a single wire may be sufficient to connect a thermistor to the computer if the other pole of the thermistor can be connected locally to the vehicle ground.

[0060] The calculator 5 can be called BMS ('Battery Management System').

[0061] The calculator 5 carries out the recurrent acquisition of the temperature values ​​TPos(k) and TNeg(k).

[0062] Calculator 5 performs the following four differences in a recurring manner: (TPos(k) - TNeg(k)), for k from 1 to N (TNeg(k+1) - TNeg(k)), for k from 1 to N-1 (TNeg(k) - TNPos(k)) for k from 1 to N (TPos(k-1 ) - TPos(k)) for k from 2 to N.

[0063] Calculator 5 compares these four differences to configurable thresholds SL1 and SL2, and generates an alert in the event that one of the following conditions [C1] or [C2] becomes true.

[0064] [C1]: (TPos(k) - TNeg(k)) > SL1 and (TNeg(k+1) - TNeg(k)) > SL2,

[0065] [C2]: (TNeg(k) - TNPos(k)) > SL1 and (TPos(k-1) - TPos(k)) > SL2

[0066] If the first condition C1 becomes true, the logic implemented in the calculator deduces that the negative terminal BN of the module of index k, namely M(k), may be the site of a loosening of the screw connection.

[0067] If the first condition C2 becomes true, the logic implemented in the calculator deduces that the negative terminal BN of the module of index k, namely M(k), may be the site of a loosening of the screw connection.

[0068] Note that it is statistically very unlikely that two screw connections will loosen at the same time.

[0069] The short length and good thermal conductivity of the connecting elements are used to assume that the screw connection that is not loosened also experiences an increase in temperature.

[0070] Turning to Figure 5, we see that the negative terminal BN of module k, i.e. TNeg(k) exceeds TPos(k) by a difference greater than the first threshold SL1, so the first half of condition C2 is true. In addition, we see that TPos(k-1) exceeds TPos(k) by a difference greater than the first threshold SL2. Condition C2 is therefore true for index k. On the other hand, condition C2 remains false for index k-1 and for index k+1. It is deduced that terminal TNeg(k) is suspected of loosening.

[0071] On the side of condition C1, it is not verified, and this is the case for all the Mk modules illustrated.

[0072] We also note that a slow evolution of temperatures along the series of modules as illustrated on the right part of figure 5, does not lead to the verification of condition C1 or C2, thus avoiding an untimely detection. At each calculation, the reference taken is local, we use a differential calculation which allows to accept a certain spatial gradient, which spatial gradient can be caused by the organs surrounding the battery pack.

[0073] According to an advantageous option, the assembly further comprises a power switch 82 arranged in series on the negative collector bus bar 80 for connection to an external network.

[0074] Optionally, it is provided that the method includes a step of disconnecting the battery pack presenting a suspected fault.

[0075] Optionally, it is further provided that the battery pack showing a suspected fault is inhibited for the following operating cycles until the repair is validated or the proper tightening is verified.

[0076] According to an advantageous option, the assembly further includes a fuse 83, in particular to protect the battery pack from a charging short circuit at the output.

[0077] It should be noted that the method promoted here can be used for a battery pack in a stationary configuration, for example a buffer battery coupled to an intermittent renewable energy capture system (solar or wind).

Claims

CLAIMS 1. Monitoring method implemented in a system comprising an assembly formed as an assembly of a plurality of N electrical energy storage modules (M(k)), each module comprising a positive terminal (BP) and a negative terminal (BN), the modules being connected to each other in series by screw connections, the system comprising sensors for determining a temperature denoted TPos(k) prevailing at the location of each of the positive terminals of the modules (M(k)), and a temperature denoted TNeg(k) prevailing at the location of each of the negative terminals of the modules (M(k)), the method being characterized in that it comprises a step of recurrent acquisition of temperature values TPos(k) and TNeg(k) for each module M(k) of rank k, and in that it generates an alert in the case where one of the following conditions [C1] or [C2] becomes true: [C1]: (TPos(k) - TNeg(k)) > SL1 and (TNeg(k+1 ) - TNeg(k)) > SL2, for k from 1 to N-1 , SL1 being a first predefined temperature threshold, SL2 being a second predefined temperature threshold [C2]: (TNeg(k) - TNPos(k)) > SL1 and (TPos(k-1 ) - TPos(k)) > SL2, for k from 2 to N, said alert being representative of a loosening of a screwed connection.

2. Method according to claim 1, characterized in that it further comprises a step of disconnecting the assembly following the generation of an alert.

3. Method according to claim 2, characterized in that it further comprises a step of inhibiting the assembly following the generation of an alert, the assembly being set aside until validation of a repair.

4. Method according to any one of claims 1 to 3, characterized in that the first temperature threshold SL1 has a value of 4°C.

5. Method according to any one of claims 1 to 4, characterized in that the second temperature threshold SL2 has a value of 2°C.

6. Assembly forming part of a battery of an electric or hybrid motor vehicle, the assembly being formed as a series mode assembly of a plurality of N electrical energy storage modules (M(k)), each module comprising a positive terminal (BP) and a negative terminal, the modules being connected to each other in series by screwed connections, the assembly comprising sensors for determining a temperature noted TPos(k) prevailing at the location of each of the positive terminals of the modules and a temperature noted TNeg(k) prevailing at the location of each of the negative terminals of the modules, the assembly further comprising a computer (5) configured to implement the monitoring method according to one of claims 1 to 5, in order to detect a loosening of a screwed connection.

7. Assembly according to claim 6, the assembly comprising a first end module (M(1)) forming a first end of the assembly, a second end module (M(N)) forming a second end of the assembly and intermediate modules, the positive terminal (BP) of each intermediate module M(k) being electrically connected to the negative terminal of one of its two neighbors by a connecting element (2) fixed by screwing, the negative terminal (BN) of each intermediate module M(k) being electrically connected to the positive terminal of the other of its two neighbors by another connecting element (2) fixed by screwing, said connecting elements each being formed as a perforated metal strip.

8. Assembly according to one of claims 6 to 7, in which the sensors intended to determine a temperature comprise, for each positive connection terminal (BP) of the modules and each negative connection terminal (BN) of the modules, a thermistor or a thermocouple in thermal coupling with said connection terminal and connected to the computer.

9. Assembly according to one of claims 6 to 8, characterized in that the assembly further comprises a power switch arranged in series on a conductive element for connecting the assembly to a network.

10. Electric or hybrid motor vehicle, comprising at least one assembly forming part of a battery, the assembly being in accordance with one of claims 6 to 9.

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