BATTERY PACK COMPRISING A PLURALITY OF ACCUMULATORS ELECTRICALLY CONNECTED TO EACH OTHER AND A DIELECTRIC FLUID CIRCULATION SYSTEM ENSURING BOTH THE COOLING OF THE ACCUMULATORS AND THEIR TIGHTENING

Abstract

Battery pack comprising a plurality of accumulators electrically connected to each other and a dielectric fluid circulation system ensuring both the cooling of the accumulators and their clamping Battery pack (P) comprising: - a sealed casing (10), delimiting a reservoir; - a plurality of electrochemical accumulators (A1, A2…A8) each comprising at least one electrochemical cell C formed of a cathode (2), an anode (3) and an electrolyte intercalated between the cathode and the anode, and a packaging (6) arranged to contain the electrochemical cell in a sealed manner;- a fluid circuit (20), configured to circulate a heat transfer dielectric fluid inside the tank, including in the spaces between the accumulators, the fluid circuit comprising at least one means for raising the pressure level (23) applied by the fluid within the tank, so as to compress each electrochemical accumulator cell by the fluid. Figure for the abstract: Fig. 7;

Description

Title of the invention: BATTERY PACK COMPRISING A PLURALITY OF ELECTRICALLY CONNECTED ACCUMULATORS TRICALLY BETWEEN THEM AND A DIELECTRIC FLUID CIRCULATION SYSTEM ENSURING BOTH THE COOLING OF THE ACCUMULATORS AND THEIR TIGHTENING Technical field

[0001] The present invention relates to the field of electrochemical accumulators, and more particularly to metal-ion accumulators.

[0002] More particularly, the invention relates to a battery pack comprising a plurality of accumulators and a dielectric fluid circulation system ensuring the cooling of the accumulators.

[0003] The invention aims firstly to improve the tightening of the accumulators within a battery pack, in order to increase its lifespan.

[0004] Although described with reference to a lithium-ion accumulator, the invention applies to any metal-ion electrochemical accumulator, that is to say also Sodium-ion, Magnesium-ion, Aluminum-ion...or more generally to any electrochemical accumulator.

[0005] A battery pack according to the invention may be on-board or stationary. For example, the fields of electric and hybrid transportation and grid-connected storage systems may be considered within the scope of the invention. Prior Art

[0006] As illustrated schematically in Figures 1 and 2, a lithium-ion battery or accumulator usually comprises at least one electrochemical cell C consisting of an electrolyte constituent 1 between a positive electrode or cathode 2 and a negative electrode or anode 3, a current collector 4 connected to the cathode 2, a current collector 5 connected to the anode 3 and finally, a packaging 6 arranged to contain the electrochemical cell with sealing while being crossed by a part of the current collectors 4, 5.

[0007] The architecture of conventional lithium-ion batteries comprises an anode, a cathode and an electrolyte. Several types of conventional architecture geometry are known:

[0008] - a cylindrical geometry as disclosed in the patent application US 2006 / 0121348,

[0009] - a prismatic geometry as disclosed in US patents 7348098, US 7338733;

[0010] - a stacking geometry as disclosed in US patent applications 2008 / 060189, US 2008 / 0057392, and US patent 7335448.

[0011] The electrolyte component 1 may be in solid, liquid or gel form. In the latter form, the component may comprise a polymer, ceramic or microporous composite separator soaked in organic or ionic liquid electrolyte(s) which allows the movement of the Lithium ion from the cathode to the anode for charging and vice versa for discharging, which generates the current. The electrolyte is generally a mixture of organic solvents, for example carbonates to which a lithium salt, typically LiPF6, is added.

[0012] The positive electrode or cathode 2 is made of Lithium cation insertion materials which are generally composite, such as LiFePO4, LiCoO2, LiNi0.33Mn0.33Co0.33O 2-

[0013] The negative electrode or anode 3 is very often made of graphite carbon or Li4TiO5O12 (titanate material), possibly also based on silicon or a composite formed from silicon.

[0014] The current collector 4 connected to the positive electrode is generally made of aluminum.

[0015] The current collector 5 connected to the negative electrode is generally made of copper, nickel-plated copper or aluminum.

[0016] A lithium-ion battery or accumulator can obviously comprise a plurality of electrochemical cells which are stacked on top of each other.

[0017] Traditionally, a Li-ion battery or accumulator uses a pair of materials at the anode and the cathode allowing it to operate at a high voltage level, typically equal to 3.6 Volts.

[0018] Depending on the type of application targeted, the aim is to produce either a thin and flexible lithium-ion accumulator or a rigid accumulator: the packaging is then either flexible or rigid and in the latter case constitutes a sort of case.

[0019] Flexible packaging is usually manufactured from a multi-layer composite material, consisting of a stack of aluminum layers covered by one or more polymer films laminated by bonding.

[0020] Rigid packaging is used when the intended applications are restrictive where a long service life is required, for example with much higher pressures to be supported and a stricter required level of sealing, typically less than 10 8mbar.l / s, or in environments with high constraints such as the aeronautical or space sector.

[0021] Also, to date a rigid packaging used consists of a metal case, ty stainless steel (316L or 304 stainless steel) or aluminum (Al 1050 or Al 3003), or titanium. In addition, aluminum is generally preferred for its high thermal conductivity coefficient as explained below.

[0022] The geometry of most rigid Li-ion battery packaging cases is cylindrical, because most electrochemical cells in batteries are wound by winding in a cylindrical geometry around a cylindrical mandrel. Prismatic shapes of cases have also already been made by winding around a prismatic mandrel.

[0023] Patent application FR3004292 describes the use of the interior of the mandrel as an air blade to cool the core of a wound cell of a metal-ion accumulator.

[0024] One of the types of rigid cylindrical shaped case, usually manufactured for a high capacity Li-ion accumulator, is illustrated in [Fig.3].

[0025] A rigid prismatic shaped case is also shown in [Fig.4].

[0026] The housing 6 comprises a cylindrical side casing 7, a base 8 at one end, a cover 9 at the other end, the base 8 and the cover 9 being assembled to the casing 7. The cover 9 supports the current output poles or terminals 4, 5. One of the output terminals (poles), for example the positive terminal 4 is welded to the cover 9 while the other output terminal, for example the negative terminal 5, passes through the cover 9 with the interposition of a seal not shown which electrically insulates the negative terminal 5 from the cover.

[0027] The widely manufactured rigid housing type also consists of a stamped cup and a cover, welded together on their periphery. On the other hand, current collectors include a bushing with a portion projecting from the top of the housing and which forms a terminal also called the exposed pole of the battery.

[0028] A battery pack P is made up of a variable number of accumulators which can reach several thousand which are electrically connected in series or in parallel with each other and generally by connection bars, usually called busbars.

[0029] An example of a battery pack P is shown in [Fig.5]. This pack consists of two modules M1, M2 of identical Li-ion accumulators A connected together in series, each module M1, M2 consisting of four rows of accumulators connected in parallel, each row consisting of a number equal to six Li-ion accumulators.

[0030] As shown, the mechanical and electrical connection between two Li-ion accumulators of the same row is made by screwing busbars B1, advantageously made of copper, each connecting a positive terminal 4 to a negative terminal 5. The connection between two rows of accumulators in parallel within the same module M1 or M2 is ensured by a busbar B2, also advantageously made of copper. The connection between the two modules M1, M2 is ensured by a busbar B2, also advantageously in copper.

[0031] In the development and manufacture of lithium-ion batteries, for each profile / new demand, regardless of the market players, this requires precise sizing (series / parallel electrical architectures, mechanical, thermal, etc.) to optimally design a high-performance and safe battery pack.

[0032] A lithium electrochemical system, whether at the cell, module or pack scale, produces exothermic reactions regardless of the given cycling profile. Thus, at the scale of a unit accumulator, depending on the chemistries considered, the optimal operation of lithium ion accumulators is limited within a certain temperature range.

[0033] An electrochemical accumulator must operate within a defined temperature range, typically generally less than 70°C at its outer casing surface, otherwise its performance will be degraded, or even physically degraded to the point of destruction. Examples include lithium iron-phosphate accumulators which have an operating range generally between -20°C and +60 °C. Above 60 °C, materials can suffer significant degradation, reducing cell performance. Beyond a so-called thermal runaway temperature, which can be between 70 °C and 110 °C, exothermic internal chemical reactions begin. When the battery is no longer able to dissipate enough heat, the cell temperature increases until it is destroyed, a phenomenon commonly referred to as thermal runaway. This runaway can be followed by gas generation and explosion and / or fire.

[0034] Also, maintaining a temperature below 70°C makes it possible to increase the lifespan of an accumulator, because the higher the operating temperature of an accumulator, the more its lifespan will be reduced.

[0035] Furthermore, certain accumulator chemistries require an operating temperature well above ambient temperature and consequently, it is necessary to regulate their temperature level by initial preheating of the accumulators, or even by permanently maintaining the temperature of the accumulators.

[0036] In a battery, or battery pack with several Li-ion accumulators, placing more or less different accumulators in series or in parallel can have consequences on the resulting performance of the pack.

[0037] It is thus recognized that in a battery pack, for example of an electric vehicle, the aging dispersions can be high depending for example on the position of the accumulators, following aging asymmetries between the accumulators. emulators or differences in use (thermal variations between the core and the edges of the pack, current gradient, etc.)

[0038] Also, in order to limit premature aging of the pack, it is necessary to optimize the operating temperature and the temperature dispersion from one accumulator to another. An accumulator (or accumulators) that ages (age) faster than the others can have a direct impact on the electrical performance of the complete battery pack.

[0039] At the module and pack level, typically below 0°C for example, it may be necessary to use a BMS, in order to limit the power required from the pack in order to avoid degradation of the accumulators.

[0040] It is recalled here that the BMS (English acronym for “Battery Management System”) is used to protect the elements from factors increasing their danger, such as excessively high currents, unsuitable potentials (too high or too low), limit temperatures and therefore has the particular function of stopping current applications as soon as threshold voltage values ​​are reached, i.e. a difference in potentials between the two active insertion materials. The BMS therefore stops current applications (charging, discharging) as soon as threshold voltages are reached.

[0041] Beyond a higher temperature, typically of the order of 70°C, it is also necessary to be vigilant because electrochemical reactions can lead to the destruction of the unit accumulators and cause propagation of an internal fault in the accumulator, generally an internal short circuit, which can lead in the extreme to the explosion of the pack. In this case, it is also necessary to use the BMS, in order to protect the accumulators.

[0042] The difficulty arises in ensuring temperature uniformity within a battery pack.

[0043] Consequently, these thermal considerations generally require regulation of the temperature of the accumulators of a battery pack.

[0044] In the literature, the solutions disclosed to attempt to ensure temperature homogeneity within a battery pack can be essentially classified into three categories.

[0045] The first category concerns the use of cold plates.

[0046] Patent US8609268 thus discloses a cold plate system inside which a refrigerant fluid flows, making it possible to drain the heat from accumulators in contact with the cold plate.

[0047] Patent application WO2011 / 013997 proposes cooling fins arranged inside a stack of flat cells to drain heat from the cells to a fluid circulating at the bottom of the stack.

[0048] The second category concerns cooling by material with a change of phase.

[0049] The third category concerns solutions where a heat transfer fluid (gaseous or liquid) is circulated within a battery pack.

[0050] Patent US5320190 thus proposes air circulation for cooling a vehicle battery pack, either by directly using the air impacting the vehicle during driving, or by using a fan for the parking phases or just after driving.

[0051] Patent CN202259596U proposes a battery pack which integrates air distributors.

[0052] In patent application WO2012 / 165781, a system of air guide plates is proposed which a priori makes it possible to reduce the temperature difference between battery modules.

[0053] A coolant can be used instead of air. Indeed, the notions of cost, size and additional mass can be predominant factors depending on the application considered.

[0054] For example, air cooling is the least expensive solution since, as indicated, it consists of forced air ventilation between the accumulators. On the other hand, the thermal performance of air cooling is of poor quality due to the low exchange coefficient and the low thermal inertia. Thus, in this type of cooling, the first accumulator will heat up despite everything in contact with the air and the air temperature will increase. When the second accumulator passes, the air is hotter and the accumulator is hotter than the first. In the end, we can therefore obtain accumulators whose temperature is inhomogeneous.

[0055] Liquid cooling solutions are therefore significantly more efficient in terms of heat exchange: they consist of direct cooling by thermal conduction using a dielectric liquid.

[0056] For example, patent application WO2008 / 156737 and patent US2013196184 propose a system of channels which each follow a part of the periphery of several cylindrical accumulators parallel to each other. A heat transfer liquid flows inside these channels to drain the heat.

[0057] Patent US8877366 relates to a liquid cooling solution flowing through external tubes which cool by thermal conduction, fins inserted between accumulators.

[0058] Patent FR3010834 discloses a device for thermal regulation of a battery pack, comprising a tube heat exchanger in contact with the accumulators at the bottom of the casing (envelope) of the battery pack.

[0059] In addition to the thermal considerations mentioned, the accumulators must sometimes be put into mechanical compression, in order to maximize their lifespan.

[0060] Indeed, it is known that good control of a compression force applied to the electrochemical cells provides a gain in the lifespan of the cells and the related battery pack.

[0061] It is also known that there is an optimal compression force for cells to maximize their lifespan. If the compression force is reduced by 50% compared to the optimal compression force, there is generally an increase in aging of the order of 50% as well.

[0062] In fact, compression applied to the largest active surface of an electrochemical cell limits the delamination of its internal layers (electrodes, separator, active layers) and therefore allows a significant gain in performance in terms of lifespan and nominal operation.

[0063] Depending on the shape of the accumulators, the compression to be applied is more or less important. In the case of accumulators with prismatic geometry with flexible or rigid packaging, it turns out that compression is essential.

[0064] Figures 6A and 6B schematically show the compression forces F as they must be implemented on the main faces of a flexible or rigid packaging 6 of a Li-ion accumulator A in order to compress the different layers of the electrochemical cell C.

[0065] Usually, compression is applied by means of mechanical tie rods which are arranged around the different accumulators stacked in groups. The tightening torque applied to the tie rods determines the compressive force applied to the cells.

[0066] A major disadvantage of mechanical tie rods is that it is very difficult to control the compression force during operation of the accumulators of the battery pack, as well as during their aging. Indeed, during the charging and discharging of the accumulators, and during their aging, these tend to undergo variations in volume by swelling / deflation of the cells.

[0067] This phenomenon of cell swelling increases with their aging, which leads to a sharp increase in the compression force to be applied. In fact, this degrades the lifespan of the cells and induces mechanical and active material oversizing.

[0068] Thus, generally speaking, usually, to compensate for this predictable degradation, manufacturers design an oversized battery pack to ensure a determined performance at the end of its life. For example, for an end-of-life performance of a 10Ah pack, typically after 10 years, designers initially size a 12Ah battery pack. In other words, they provide a margin of around 20% to compensate for the aging of the cells.

[0069] Patent US7858224B2 discloses an alternative mechanical solution to mechanical tie rods: compression is ensured by tightening two straps arranged on compression plates on either side of a stack of cells arranged side by side and separated by spacer plates inside which a heat transfer fluid circulates. This tightening device using straps and compression plates has the same major drawback as a device with mechanical tie rods.

[0070] Patent US6372377B1 discloses a battery pack of nickel-metal hydride Ni-MH type accumulators, in the form of a stacked accumulator module which implements metal bars positioned along the four sides of the module and are welded to the four corners of the module where the bars meet, thus forming a continuous band around the module. This clamping device using welded metal bars does not allow compression management over time. In addition, the compression applied is uniaxial.

[0071] In addition to the aforementioned drawbacks, all the mechanical solutions proposed to date necessarily involve the addition of mechanical parts (compression plates, tie rods, straps, bars, etc.), which makes the battery packs heavier and can complicate their assembly.

[0072] There is a need to improve the compression solutions for the accumulators of a battery pack, in particular in order to increase the lifespan of the latter, while making it possible to define a modular pack optimized in terms of power, size and weight.

[0073] Furthermore, the improvement must not be made to the detriment of the need to control the operational safety of each accumulator.

[0074] The aim of the invention is to respond at least in part to this(these) need(s). Statement of the invention

[0075] To do this, the invention relates, in one of its aspects, to a battery pack comprising:

[0076] - a waterproof envelope, delimiting a reservoir;

[0077] - a plurality of electrochemical accumulators each comprising at least one electrochemical cell C formed of a cathode, an anode and an electrolyte interposed between the cathode and the anode, and a packaging arranged to contain the electrochemical cell in a sealed manner;

[0078] - a fluid circuit, configured to circulate a heat transfer dielectric fluid to inside the tank, including in the spaces between the accumulators, the fluid circuit comprising at least one means for raising the pressure level of the fluid within the tank, so as to compress each electrochemical accumulator cell by the fluid.

[0079] Thus, the invention consists of proposing simultaneous compression of all the electrochemical accumulator cells of a battery pack directly by through the dielectric fluid whose primary function is the thermal management of the pack.

[0080] In other words, the dielectric fluid provides a dual function, i.e. thermal management of the battery pack and compression by fluid pressure of the electrochemical cells that compose it.

[0081] In other words, thanks to the invention, the thermal and compression of the accumulators of a complete battery pack can be controlled simultaneously in a precise and homogeneous manner, by means of the same pressure regulation means applied by the heat transfer fluid within the sealed tank in which the battery pack is immersed. Indeed, the dielectric heat transfer fluid applies an identical pressure, whatever its value, at any point of all the electrochemical cells of the battery pack.

[0082] In the context of the invention, the dielectric fluid may be a dielectric gas or a dielectric liquid.

[0083] Two alternatives can be considered for compressing the electrochemical cells of the pack.

[0084] The first consists of carrying out a variable compression which is passive, that is to say not controlled, by using one or more materials which expand according to different conditions within the reservoir and / or the accumulators. Advantageously, it is possible to use, as dielectric fluid or not, a material which swells within the volume of the reservoir according to the temperature and which would compress the cells at high temperature, preferably a thermo-expandable wax which can be integrated either as a dielectric fluid or as a solid within another dielectric fluid.

[0085] The second alternative consists of carrying out active compression of the dielectric fluid.

[0086] Thus, according to this alternative, the pressure of the dielectric fluid can advantageously be measured using a pressure sensor, and controlled by an electronic pressure control unit.

[0087] Active control of the fluid pressure within the reservoir makes it possible to manage the compression of the electrochemical cells, advantageously as a function of the state of charge (SOC) and / or at least one aging state (state of health (SOH) and / or state of power (SOP)) of the plurality of accumulators, and / or the mode of use of the battery pack (temperature, current, etc.). This makes it possible to envisage active management to optimize the lifetime of the electrochemical cells and therefore of the battery pack.

[0088] By "state of aging", is meant here and within the framework of the invention, the state of health, usually designated by its English acronym SOH for "State Of Health" and / or the state of power usually designated by its English acronym SOP for “State of Power”.

[0089] The state of health SOH is the ratio of the available capacity of the accumulator to the initial capacity of the accumulator. The SOH is representative of an energy state of the accumulator and can be estimated by different methods known from the state of the art.

[0090] The power state SOP is dependent on the SOH and the internal resistance of the accumulator. The SOP is determined by evaluating the available capacity and the internal resistance of the accumulator.

[0091] As an advantageous example, it is possible to implement pressure management adapted during cycling of the accumulators according to the level of their SOC: the pressure variations can be of the order of a few millibars.

[0092] Compression control can be achieved directly by controlling a dedicated compression pump.

[0093] To determine the optimal pressure to apply, those skilled in the art may carry out lifetime tests on electrochemical cells compressed using a dielectric fluid, such as an oil, by varying the pressure, typically between 1 and 20 bars.

[0094] The invention provides numerous advantages, among which we can cite:

[0095] - a gain in the lifespan of a battery pack; - a gain in mass of a battery pack which results from the gain in lifespan. In Indeed, generally speaking, to date, battery pack designers have a tradition of oversizing packs to ensure a specific performance at the end of their life. For example, for a battery pack performance with an end-of-life capacity, typically after 10 years of service, of the order of 100Ah, designers originally design a battery pack with a capacity of 12Ah, i.e. an oversizing margin equal to 20% margin to compensate for the aging of the electrochemical cells. Removing all or part of the usual oversizing therefore makes it possible to obtain, at a lower weight, performances of the same order at the end of the pack's life. - a gain in mass by eliminating the mechanical parts usually ensuring the compression of electrochemical cells by mechanical tie rods or other means (compression plates, straps, etc.) - a gain in operating safety of the battery pack. Indeed, on the one hand, the external pressurization of the cells by the dielectric fluid makes it possible to delay the moment of opening the vent of each of the accumulators in the event of thermal runaway of the latter. This therefore leaves more time for incident management. On the other hand, the measurement of a global variation in fluid pressure makes it possible to anticipate the detection of a fault in an electrochemical cell, more quickly than by a conventional measurement by temperature linked to inertia. Furthermore, the implementation of the invention allows a possible increase in the flow rate of the dielectric fluid over time via the control of the circulation pump of the fluid circuit, which has the effect of minimizing the effects of unwanted events, such as thermal runaway of an electrochemical cell, and the effects of an internal short circuit in a cell by thermal propagation in the oil; a gain in mass by reducing the thickness of the accumulator casing. Indeed, by directly compressing the electrochemical cells by the dielectric fluid of the cooling circuit, the casing no longer has to ensure, as according to the state of the art, the mechanical recovery of internal forces. Its thickness can therefore be reduced. Typically, to date a Li-ion accumulator casing is made of aluminum, nickel-plated steel of approximately 0.5 mm thickness.

[0096] According to the second alternative, according to an advantageous embodiment, the pressure raising means comprises (comprise) a compression pump or means of pressurizing by mechanical wave(s), adapted to pressurize the dielectric fluid in the tank and thus compress each electrochemical accumulator cell.

[0097] According to a first advantageous variant embodiment, the sealed envelope is flexible, the fluid circuit comprises:

[0098] a pump for circulating the dielectric fluid inside the tank, the compression pump or the means of pressurizing by mechanical wave(s), connected to the tank, and adapted to pressurize the fluid in the tank and thus compress each electrochemical accumulator cell.

[0099] According to a second advantageous embodiment variant, the sealed envelope is rigid, the fluid circuit comprises:

[0100] a dielectric fluid circulation pump, an expansion tank, connected to the reservoir and adapted to compensate for the increase in volume of the fluid which expands when it heats up, the compression pump or the means of pressurizing by mechanical wave(s), connected to the expansion tank, and adapted to pressurize the fluid in the expansion tank and thereby in the reservoir and thus compress each electrochemical accumulator cell.

[0101] Advantageously, the compression pump or the means of pressurizing by mechanical wave(s) is (are) connected:

[0102] - to the gaseous sky of the expansion tank, so as to pressurize the gas in the sky and thus the fluid into the expansion tank and through it into the reservoir, or

[0103] - in a closed circuit to the expansion tank and the reservoir, so as to pressurize directly the dielectric fluid into the expansion tank and thereby into the reservoir.

[0104] Preferably, in the latter case, the battery pack comprises an expansion valve arranged on a fluid supply line between the reservoir and the expansion tank, the compression pump or the means for pressurizing by mechanical wave(s) being arranged on a fluid return line between the expansion tank and the reservoir.

[0105] Advantageously, the circulation pump and / or the compression pump is (are) arranged inside the tank.

[0106] Advantageously, the means of pressurizing by mechanical wave(s) are implemented by piezoelectric actuation.

[0107] According to another advantageous embodiment, the battery pack comprises:

[0108] - at least one pressure sensor adapted to measure the pressure of the fluid within of the tank;

[0109] - an electronic pressure control unit, connected to the pressure sensor and suitable for controlling the pressure raising means(s) as a function of the pressure measured by the sensor.

[0110] Preferably, the electronic pressure control unit is integrated into a battery management system (BMS), in particular in an engine control unit (ECU), and / or an energy management system (EMS).

[0111] The accumulators may be electrically connected to each other and kept spaced apart from each other inside the tank.

[0112] According to another advantageous embodiment variant, the fluid circuit comprises a gas / gas or liquid / gas or liquid / liquid type heat exchanger, one circuit of which constitutes the dielectric fluid circuit. Preferably, the heat exchanger is integrated at least partially through the wall of the sealed casing.

[0113] According to another embodiment variant, the battery pack comprises a safety vent integrated into the wall of the sealed casing, so as to evacuate the fluids in the event of unwanted overpressure.

[0114] The packaging of the accumulators can be flexible packaging or boxes.

[0115] For application to a Li-ion battery pack, each accumulator is a Li-ion accumulator in which:

[0116] - the negative electrode(s) material is chosen from the group comprising graphite, lithium, titanate oxide Li4TiO5Oi2; - the positive electrode(s) material is chosen from the group comprising LiFePO4, LiCoO2, LiNi0.33Mn0.33Co0.33CL.

[0117] The invention also relates to a method of operating a battery pack described above, comprising the following steps:

[0118] a / circulation of a heat transfer dielectric fluid inside the tank, including in the spaces between the accumulators;

[0119] b / continuous measurement of the pressure of the fluid inside the tank;

[0120] c / if the pressure value measured in b / is lower than a predetermined threshold pressure value, then raising the pressure level of the fluid within the tank so as to compress each electrochemical accumulator cell by the fluid.

[0121] Preferably, the threshold pressure value is predetermined as a function of the state of charge (SOC) and / or at least one aging state (state of health (SOH) and / or state of power (SOP)) of the plurality of accumulators, and / or the mode of use of the battery pack (temperature, current, etc.).

[0122] Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0123] [Fig.l] is an exploded perspective schematic view showing the various elements of a lithium-ion accumulator.

[0124] [Fig.2] is a front view showing a lithium-ion battery with its packaging flexible according to the state of the art.

[0125] [Fig.3] is a perspective view of a state-of-the-art lithium-ion battery with its rigid packaging consisting of a cylindrical box.

[0126] [Fig.4] is a perspective view of a lithium-ion accumulator according to the state of art with its rigid packaging consisting of a prismatic-shaped box.

[0127] [Fig.5] is a perspective view of an assembly using busbars state-of-the-art lithium-ion accumulators, forming a battery pack

[0128] [Fig.6A] is a schematic view illustrating the compressive forces that must be applied to the main faces of a prismatic format Li-ion battery.

[0129] [Fig.6B] is another schematic view illustrating the compressive forces that must be applied to the main faces of a prismatic format Li-ion battery.

[0130] [Fig.7] is a longitudinal sectional view of a battery pack according to the invention with a fluid circuit that is both heat transfer fluid and compresses the pack's accumulators.

[0131] [Fig.7A] is a cross-sectional view of a battery pack according to the invention with a fluid circuit that is both heat transfer fluid and compresses the pack's accumulators.

[0132] [Fig-8] is a schematic diagram showing an embodiment of a heat transfer fluid circuit within a battery pack according to the invention with an expansion tank and a dedicated compression pump.

[0133] [Fig.9] is a schematic diagram of another embodiment of a heat transfer fluid circuit within a battery pack according to the invention with an expansion tank and a dedicated compression pump. Detailed description

[0134] Figures 1 to 6B relate to different examples of Li-ion accumulators, flexible packaging and accumulator cases as well as a battery pack according to the state of the art. These figures 1 to 6B have already been commented on in the preamble and are therefore not commented on further below.

[0135] For the sake of clarity, the same references designating the same elements according to the state of the art and according to the invention are used for all figures 1 to 9.

[0136] Throughout the present application, the terms "lower", "upper", "bottom", "top", "below" and "above" are to be understood by reference to a sealed casing of a battery pack according to the invention horizontally with the Li-ion accumulator cases positioned vertically.

[0137] Figures 7 and 7A show an example of a battery pack P of Li-ion, Al, A2,...,A8 accumulators, according to an alternative of the invention.

[0138] The illustrated A1-A8 accumulators have prismatic format cases.

[0139] The battery pack P firstly comprises a rigid sealed envelope 10 delimiting a reservoir filled with a heat transfer dielectric fluid (L) circulating in a fluid circuit 20.

[0140] The rigid sealed casing 10 is sized to withstand pressure, typically up to 7 bars. The casing 10 may have a cylindrical or parallelepipedal overall geometry. As an example of casing 10, it is possible to envisage, to withstand a maximum pressure of the order of 7 bars, a generally cylindrical tank, made of S355 steel, 2.5 mm thick for the closing end parts of a cylindrical part 1 mm thick.

[0141] A plurality of accumulators A1-A8 is held inside the tank by a mechanical holding structure 11. This structure 11 makes it possible to keep the accumulators A1-A8 regularly spaced apart from each other inside the tank. The separation spaces E between two adjacent accumulators can therefore be traversed by the dielectric fluid (L) circulating in the circuit 20.

[0142] Furthermore, the accumulators A1-A8 are electrically connected to each other in particular by means of electrical connection bars or busbars B, inside the casing 10 and connected by wires 12 to an external connection 13, 14, mounted in a sealed crossing through the wall of the casing 10.

[0143] The accumulators can be electrically connected in series and / or in parallel in groups.

[0144] The fluid circuit 20 makes it possible to cool / heat the plurality of accumulators A1-A8 within the tank by circulating the dielectric fluid L, including in the spaces E separating the accumulators.

[0145] To do this, the fluid circuit 20 comprises a circulation pump 21 adapted to circulate the fluid L, at constant pressure, in the reservoir 10. For example, the flow rate of a circulating dielectric oil can be of the order of 2600 l / h. As shown in FIGS. 7 and 7A, the circulation pump is preferably integrated inside the reservoir 10, which makes it possible to eliminate the use of piping.

[0146] To ensure the cooling / heating of the dielectric fluid L and therefore of the accumulators A1-A8 of the pack P, the fluid circuit 20 constitutes one of the two circuits of a gas / liquid or liquid / liquid exchanger. As shown in FIGS. 7 and 7A, the exchanger 22 is preferably integrated in the lower part of the tank by passing through the wall of the sealed casing 10.

[0147] A safety vent 15 may be integrated into the wall of the sealed envelope, so as to evacuate the fluids in the event of unwanted overpressure inside the sealed envelope 10.

[0148] According to the invention, the fluid circuit 20 comprises at least one means for raising the pressure level applied by the fluid within the reservoir, so as to directly compress each electrochemical accumulator cell by the fluid L.

[0149] Thus, first of all, a compression pump 23 is provided, adapted to pressurize the dielectric fluid in the reservoir 10 and thus compress each electrochemical accumulator cell A1-A8.

[0150] For example, in the case of prismatic format accumulators, manufacturers recommend applying a compression force of 200 daN on the large side faces.

[0151] For an accumulator whose side face dimensions are equal to 173mm x 125mm, the application of 1 bar ensures a force of 216 daN of stable / regulated compression over time, which is in accordance with the manufacturers' recommendations.

[0152] The compression pump 23 must therefore guarantee a dielectric fluid pressure L of the order of 1 bar.

[0153] Preferably, like the circulation pump 21, the compression pump 23 is arranged inside the sealed envelope 10, which makes it possible to avoid the use of pipes.

[0154] Thus, in the fluid circuit 20 according to the invention, the control of the distribution of the fluid is carried out by means of two independent pumps 21, 23, one dedicated to circulation allowing the flow rate of the fluid to be managed and the other dedicated to the compression of the fluid. This guarantees simple and precise control.

[0155] An expansion tank 24 is connected to the reservoir 10. This expansion tank 24 makes it possible to compensate for the increase in volume of the fluid in the reservoir 10 which expands when it heats up, and thus guarantees a constant pressure in the latter.

[0156] The gases generated in the heat transfer circuit are therefore recovered in the ceiling 25 of the expansion tank 24.

[0157] Thus, in the event of gas overpressure, the gaseous ceiling 25 fills and beyond a predetermined overpressure threshold, this can automatically open the electrical circuit at the scale of the battery pack P and thus guarantee permanent operational safety.

[0158] According to a first embodiment, illustrated in [Fig.8], the compression pump 23 is connected to the gaseous headspace 25 of the expansion tank 24. Thus, when desired, the compression pump 23 comes to pressurize the gas in the headspace and thus the dielectric liquid L in the expansion tank 24 and therefore, in the tank 10 which is connected.

[0159] According to a second embodiment, illustrated in [Fig.9], the compression pump 23 is connected in a closed circuit to the expansion tank 24 and to the reservoir 10, so as to directly pressurize the dielectric liquid L in the expansion tank 24.

[0160] In this second mode, an expansion valve 26 is arranged on a fluid supply line 27 between the reservoir 10 and the expansion tank 24.

[0161] The compression pump 23 is then arranged on a fluid return line 28 between the expansion tank 24 and the reservoir 10.

[0162] For the active control of the pressurization of the dielectric fluid in the reservoir 10, a pressure sensor 29 is firstly provided, adapted to measure the pressure of the fluid within the reservoir 10.

[0163] As illustrated in Figures 7, 7A and 8, the pressure sensor 29 can directly measure the pressure of the gases in the ceiling 25 of the expansion tank, which makes it possible to return to the pressure value of the dielectric liquid L in the tank 10.

[0164] Alternatively, as illustrated in [Fig.9], the pressure sensor 29 can directly measure the pressure of the fluid L within the circuit 20.

[0165] An electronic pressure control unit 30 connected to the pressure sensor 29 makes it possible to directly control the compression pump 23 as a function of the pressure measured by the sensor 29. For on-board applications, preferably, the pressure control unit 30 can constitute or be integrated into an engine control unit. (ECU English acronym for “Engine Control Unit”).

[0166] Thus, the compression control that is sought on the electrochemical cells of the battery pack P is advantageously carried out directly by controlling the compression pump 23.

[0167] The invention is not limited to the examples which have just been described; it is possible in particular to combine characteristics of the examples illustrated within non-illustrated variants.

[0168] Other variants and improvements may be envisaged without departing from the scope of the invention.

[0169] If in all of the detailed examples, the accumulators A1-A8 illustrated are of prismatic format, it is possible to envisage achieving all of the characteristics of the invention with accumulators of cylindrical format.

[0170] If in all of the examples illustrated, the packaging of the accumulators in accordance with the invention are rigid packaging (cases), the invention of course applies to all accumulators with flexible packaging.

Claims

Claims

1. Battery pack (P) comprising: - a sealed envelope (10), delimiting a reservoir; - a plurality of electrochemical accumulators (A1, A2...A8) each comprising at least one electrochemical cell C formed of a cathode (2), an anode (3) and an electrolyte interposed between the cathode and the anode, and a packaging (6) arranged to contain the electrochemical cell in a sealed manner; - a fluid circuit (20), configured to circulate a heat transfer dielectric fluid inside the reservoir, including in the spaces between the accumulators, the fluid circuit comprising at least one means for raising the pressure level (23) applied by the fluid within the reservoir, so as to compress each electrochemical accumulator cell by the fluid.the lifting means being the dielectric fluid based on one or more materials which expand according to predetermined conditions inside the tank and / or the accumulators.

2. Battery pack according to claim 1, the dielectric fluid being based on a thermo-expandable wax.

3. Battery pack according to one of the preceding claims, comprising: - at least one pressure sensor (29) adapted to measure the pressure of the fluid within the reservoir; - an electronic pressure control unit (30), connected to the pressure sensor and adapted to control the pressure raising means(s) as a function of the pressure measured by the sensor.

4. Battery pack according to claim 3, the electronic pressure control unit being integrated into a battery management system (BMS), in particular in an engine control unit (ECU), and / or an energy management system (EMS).

5. Battery pack according to one of the preceding claims, the accumulators being electrically connected to each other and kept spaced apart from each other inside the tank.

6. Battery pack according to one of the preceding claims, the fluid circuit comprising a gas / gas or liquid / gas or liquid / liquid type heat exchanger, one circuit of which constitutes the dielectric fluid circuit.

7. Battery pack according to claim 6, the heat exchanger (22) being integrated at least partially through the wall of the waterproof envelope.

8. Battery pack according to one of the preceding claims, comprising a safety vent (15) integrated into the wall of the sealed envelope, so as to evacuate the fluids in the event of unwanted overpressure.

9. Battery pack according to one of the preceding claims, the packaging of the accumulators being flexible packaging or boxes.

10. Battery pack according to one of the preceding claims, each accumulator being a Li-ion accumulator in which: - the negative electrode(s) material is chosen from the group comprising graphite, lithium, titanate oxide Li4TiO5Oi2; the positive electrode(s) material is chosen from the group comprising LiFePO4, LiCoO2, LiNi0.33Mn0.33Co0.33O2.

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