Battery pack module

Abstract

A battery pack module 700 includes a plurality of battery cells 501 arranged in a battery cell array; and a circuit board 704 having a first face (708, Figure 8) facing the battery array and a second

Description

TECHNICAL FIELD The disclosure relates to a battery pack module with temperature sensors. BACKGROUND Interest in aircraft with electric and hybrid electric propulsion systems is increasing because of the need to reduce carbon emissions and pollution, and because of developments in the facilitating electrical technologies. Hybrid electric propulsion systems include both internal combustion engines, for example gas turbines or diesel engines, and energy storage, typically in the form of batteries. Purely electric propulsion systems completely dispense with internal combustion engines and use only batteries or, in some instances, fuel cells, as an energy source for their propulsors. The terms Advanced Air Mobility (AAM) and Urban Air Mobility (UAM) refer to the use of aircraft - typically electric and hybrid electric aircraft - to transport passengers relatively short distances, for example tens or perhaps hundreds of kilometres. Most proposed AAM platforms have Vertical Take-Off and Landing (VTOL) or Short Take-Off and Landing (STOL) capabilities so that the aircraft can take-off and land at locations convenient for passengers, for example at so-called ‘vertiports’ close to or in urban environments. It is expected that the number of passengers carried by AAM platforms will be relatively small - likely fewer than twenty and typically of the order of five to ten. Various configurations have been proposed for AAM platforms, and one example is illustrated in Figure 4. The aircraft 400 includes a fuselage 411 and a distributed propulsion system which includes a number, in this case six, of propulsors 412 in the form of open rotors driven by rotary electric machines. The propulsors, which in this case are attached to wings 413 or other flight surfaces 414, are tiltable between a VTOL configuration and a horizontal flight configuration to facilitate both modes of flight. Other proposed configurations are helicopter-like configurations, for example multi-copters (e.g., quadcopters with ducted fans) and the like. Battery packs for electric or hybrid vehicles, including aircraft, require a battery monitoring system, which is responsible for monitoring the key parameters of the battery pack that are relevant to safety and performance. One such key parameter is temperature. To comply with standards for airborne lithium-ion battery systems, including RTCA DO-311 A, a monitoring system must be able to determine whether any individual battery cell in a battery pack is outside of a manufacturer specified operating temperature range. This can require a temperature sensor for each battery cell or group of battery cells in a battery pack. Typical battery packs may contain around hundreds of battery cells, with a vehicle typically requiring multiple such packs depending on power requirements. Since a single battery cell failure may lead to failure of an entire battery pack, it is important that close monitoring of temperatures is achieved, with preferably a temperature sensor for each individual battery cell. Current monitoring systems may employ surface mount temperature sensors on a circuit board which is itself mounted to a module of battery cells. An advantage of this approach is that it allows the designer to deploy many sensors without requiring many corresponding connecting wires, as the wiring is integrated within the circuit board. However, this approach limits where the sensors can be physically placed and connected to the battery cells to measure their temperatures. One current approach is to place the sensors on the bottom side of the circuit board, so they are oriented face down onto the battery cells. A thermal connection between each temperature sensor and battery is then made using a thermal paste or a compressible thermal pad. Although such connections may be suitable for prototyping, it is difficult to verify that a connection has been made successfully during the assembly process because line of sight is not easily available due to the placement of the sensors. Also, the use of adhesives is not desirable in a high vibration environment such as in an aircraft application. SUMMARY According to a first aspect there is provided a battery pack module comprising: a plurality of battery cells arranged in a battery cell array; a circuit board having first and second opposing faces, a first face of the circuit board facing the battery cell array, the circuit board comprising a plurality of openings corresponding to the respective plurality of battery cells; a plurality of temperature sensors, each temperature sensor mounted to the second face of the circuit board; and a thermally conductive connection connecting each temperature sensor to a respective one of the plurality of battery cells through one of the respective openings. Each temperature sensor may be mounted to a metallic area surrounding the temperature sensor on the second face of the circuit board, the thermally conductive connection connected to the metallic area. The thermally conductive connection may comprise a metallic wire having a first end bonded to the battery cell and a second end bonded to the metallic area. The first end of the metallic wire may be bonded to an outer casing of the battery cell. Each temperature sensor may be electrically isolated from the metallic area. The temperature sensor may be a surface mount component. Each battery cell may be cylindrical, with the battery cells arranged in a parallel battery cell array. According to a second aspect there is provided a battery pack comprising: a plurality of battery pack modules, each battery pack module according to the first aspect; and a battery management module connected to receive a temperature signal from each of the temperature sensors on the plurality of battery pack modules. According to a third aspect there is provided a vehicle comprising a battery pack according to the second aspect. The vehicle may be an aircraft. The skilled person will appreciate that except where mutually exclusive, a feature described in relation to any one of the above aspects may be applied mutatis mutandis to any other aspect. Furthermore except where mutually exclusive any feature described herein may be applied to any aspect and / or combined with any other feature described herein. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which: Figure 1 is a schematic illustration of a hybrid electric aircraft propulsion system; Figure 2 is a schematic illustration of a purely electric aircraft propulsion system; Figure 3 is a perspective view of an example electric aircraft; Figure 4 is a perspective view of an electric aircraft in a VTOL configuration; Figure 5 is a schematic sectional side view diagram illustrating an example battery cell connected to a temperature sensor via a thermally conductive connection; Figure 6 is a schematic plan view of an example battery cell connected to a temperature sensor via a thermally conductive connection; Figure 7 is a schematic diagram in plan view of an array of temperature sensors on a circuit board connected to an array of battery cells; Figure 8 is a schematic diagram in sectional view of the array of temperature sensors on a circuit board connected to an array of battery cells; and Figure 9 is a schematic diagram of an example battery pack comprising a plurality of battery modules. DETAILED DESCRIPTION Figure 1 Referring to Figure 1, the propulsion system of a hybrid electric aircraft is generally indicated at 100 and incorporates both an engine 110, such as a gas turbine engine, and a battery pack 130. Both the engine 110 and the battery pack 130 are used as energy sources to power a motor-driven propeller 116, as well as ancillary electrical systems (not shown). The propulsion system 100 of the hybrid electric aircraft will typically further comprise a generator 111, an AC / DC converter 112, a high voltage DC (HVDC) distribution bus 113, a DC / AC converter 114, an electric motor 115 that drives the propeller 116, and a DC / DC converter 117. A shaft of the engine 110 is coupled to and drives the rotation of a shaft of the generator 111 which thereby produces alternating current. The AC / DC converter 112, which faces the generator 111, converts the alternating current into direct current which is fed to various electrical systems via the HVDC distribution bus 113. These electrical systems include the motor 115 that drives the propeller 116. The motor 115 will typically be a synchronous motor that interfaces with the HVDC distribution bus 113 via the DC / AC converter 114. The battery pack 130, which may be made up of a number of lithium-ion battery modules connected in series and / or parallel, is connected to the HVDC distribution bus 113 via the DC / DC converter 117. The DC / DC converter 117 converts between a voltage of the battery pack 130 and a voltage of the HVDC distribution bus 113. In this way, the battery pack 130 can replace or supplement the power provided by the engine 110 (by discharging and thereby feeding the HVDC distribution bus 113) or can be charged using the power provided by the engine 110 (by being fed by the HVDC distribution bus 113). Figure 2 A battery pack will also appear in the propulsion system of a purely electric aircraft, generally indicated as 200 in Figure 2. The battery pack 230 feeds a HVDC distribution bus 213, optionally via DC / DC converter (not shown), which delivers power to one or more synchronous motors 215 via a DC / AC converter 214. The one or more motors 215 drive the one or more propellers 216 that propel the aircraft. Figure 3 Figure 3 shows a purely electric aircraft 300. A battery pack 330 is indicated. Figure 4 Figure 4 illustrates an electric aircraft 400 which may be used for AAM applications. The aircraft includes a fuselage 411, which incorporates a cabin for occupants, and a distributed propulsion system which in this case has six open rotor propulsors 412 driven by rotary electric machines. Also visible in Figure 1 is a retractable undercarriage 415 in which a landing platform, in this case having wheels, can be stowed during flight. The size of the fuselage 411 and the cabin will depend on the application requirements. In this example the cabin and fuselage are sized for five occupants, including a pilot. It is however envisaged that some AAM platforms will not require a pilot and will instead be flown under the control of an autopilot system. Four of the propulsors 412 are attached to the wings 413 of the aircraft 400, and the remaining two propulsors 412 are attached to a separate flight control surface 414 located towards the rear of the aircraft 400. In this embodiment the wings 413 and the rear control surface 414 are tiltable between a VTOL configuration in which the axes of the rotors point upward and a horizontal flight configuration in which the axes of the rotors point forward. The horizontal flight configuration, whilst principally used for horizontal flight, may also be used for taxiing and possibly STOL operation if supported. The electrical systems, including the electric machines that power the aircraft 400, receive electrical power from one or more battery packs (not shown) located within the aircraft 400. The battery packs may be located in any suitable part or parts of the aircraft, including the fuselage 411, the wings 413 and the propulsors 412. Whilst the illustrated aircraft 400 is an electric VTOL (eVTOL) aircraft, it will be appreciated that AAM platforms could also be of the STOL type and could also be hybrid electric aircraft that include both engines and batteries. Hybrid electric platforms may utilize similar distributed propulsion system configurations, but the underlying power system may be a series hybrid, parallel hybrid, turboelectric or other type of hybrid power system. Figure 5 Figure 5 is a schematic diagram of an example battery cell 501 thermally connected to a temperature sensor 502. The battery cell 501 may be cylindrical in form, for example having a circular section. Other form factors of the battery cell 501 may also be possible. The temperature sensor 502 is mounted to a thermal island 503 on a circuit board 504. The thermal island 503 is metallic region, typically copper, that is thermally connected to the battery 501 by a thermal contact 505. The thermal contact 505 may be formed by a wire bond between the thermal island 503 and the casing of the battery cell 501. The casing of the battery cell 501 will typically form the negative terminal 507 of the cell 501, while the positive terminal 506 is exposed at an end of the cell 501. The thermal island 503 surrounds the temperature sensor 502 so that heat can be transferred from the battery cell 501 to the thermal island 503 to be measured by the sensor 502. The sensor 502 is electrically isolated from the thermal island to prevent the battery cell 501 short circuiting through the sensor 502. The relative orientation of the temperature sensor 502 and battery cell 501 allows the sensor and thermal contact 502 to the battery cell 501 to be made in an accessible location. Wire bonding or ribbon bonding the thermal island 503 to the battery cell allows a thermal connection to be made that ensures an accurate temperature measurement of the battery cell 501 is made. Existing manufacturing processes may be used to create robust and verifiable thermal connections between multiple battery cells and temperature sensors, which can be automated and used at scale for arrays of battery cells, an example of which is described below. The thermal contact 505 may alternatively be made to other points on the battery cell 501, such as the positive terminal. In the arrangement shown in Figure 5, the thermal contact is made at the end of the cell 501 because this is likely to be where the cell 501 is hottest in use if a base cooling arrangement is used. Other cooling arrangements may require different connection positions. The length of the thermal contact 505 is preferably kept as short as possible to minimise a temperature differential between the battery cell 501 and the thermal island 503. Figure 6 Figure 6 illustrates the example battery cell 501 and temperature sensor 502 arrangement in plan view, with the circuit board 504 having a cut-out allowing access to the battery cell 501. Figures 7 & 8 Figure 7 illustrates schematically an example battery pack module 700 comprising a plurality of battery cells 501 arranged in a battery array 711. Figure 8 illustrates a section across the battery pack module 700. Each battery cell 501 is cylindrical in form and arranged in a parallel battery cell array 711, i.e. with a longitudinal axis 710 of each cell 501 aligned in parallel. The array 711 is shown in the form of a hexagonal array, which permits the maximum packing density for circular cylindrical cells. The module 700 comprises a circuit board 704 having first and second opposing faces 708, 709, the second face 709 being shown uppermost in Figure 7. The opposing first face 708 of the circuit board 704 faces the battery array 711. The circuit board 704 has a plurality of openings 712 corresponding to the respective plurality of battery cells 501. The module 700 comprises a plurality of temperature sensors 502, each of which is mounted to the second face 708 of the circuit board 704. A thermally conductive connection 505 connects each temperature sensor 502 to a respective one of the plurality of battery cells 501 through one of the respective openings 712 in the circuit board 704. In the example module 700 illustrated in Figures 7 and 8, each temperature sensor 502 is mounted to a thermal island in the form of a metallic area 503 surrounding the temperature sensor 502 on the second face 709 of the circuit board 704. Each thermally conductive connection 505 is connected to a respective metallic area 503. The thermally conductive connection 505 may be in the form of a metallic wire having a first end 713 bonded to the battery cell 501 and a second end 714 bonded to the metallic area 503. The first end 713 may be bonded to an outer casing of the battery cell 501. As described above, each temperature sensor 502 is preferably electrically isolated from the metallic area 503. This may be achieved by providing an electrically insulating layer between the metallic area 503 and the temperature sensor 502. Each temperature sensor 502 may for example be a surface mount component. The battery module 700 will also comprise other connections and components that are not illustrated in Figures 7 and 8, such as connections between each of the battery cells 501 to electrically connect the cells together. Electrical traces on the circuit board will also be required to connect the temperature sensors 502 to an output interface to enable temperature signals to be received by a battery management module. Figure 9 Figure 9 illustrates schematically an example battery pack 900 comprising a plurality of battery pack modules 700a-d. Each battery pack module 700a-d comprises the features of the battery pack module 700 described above in relation to Figures 7 and 8. The battery pack 900 comprises a battery management module 901, which is connected to receive a temperature signal from each of the temperature sensors on the 5 plurality of battery pack modules 700a-d. The battery packs 700a-d may be arranged in series or parallel, or a combination of series and parallel, according to current and voltage requirements. A vehicle, such as the aircraft 300, 400 described above, may comprise the battery pack 900. The battery pack 900 may form the battery 10 pack 130, 230 of Figures 1 and 2 as part of an aircraft propulsion system. Other embodiments are within the scope of the invention, which is defined by the appended claims.

Claims

1. A battery pack module (700) comprising:a plurality of cylindrical battery cells (501) arranged in a parallel battery cell array (711);a circuit board (704) having first and second opposing faces (708, 709), a first face (708) of the circuit board (704) facing the battery cell array (711), the circuit board (704) comprising a plurality of openings (712) corresponding to the respective plurality of battery cells (501);a plurality of temperature sensors (502), each temperature sensor (502) mounted to the second face (709) of the circuit board (704);for each temperature sensor of the plurality of temperature sensors, a metallic area (503) surrounding the temperature sensor (502) on the second face (709) of the circuit board (704), the temperature sensor being electrically isolated from the metallic area (503); andfor each temperature sensor of the plurality of temperature sensors, a thermally conductive connection (505) connecting the metallic area (503) to an outer casing of a respective one of the plurality of battery cells (501) through one of the respective openings (712), the thermally conductive connection comprising a metallic wire or ribbon (505) having a first end (713) bonded to the outer casing of the cylindrical battery cell (501) and a second end (714) bonded to the metallic area (503).

2. The battery pack module (700) of any preceding claim, wherein the temperature sensor (502) is a surface mount component.

3. A battery pack (900) comprising:a plurality of battery pack modules (700a-d), each battery pack module (700a-d) according to claim 1 or claim 2; anda battery management module (901) connected to receive a temperature signal from each of the temperature sensors (502) on the plurality of battery pack modules (700a-d).CMCM4. A vehicle (300, 400) comprising a battery pack (900) according to claim 3.

5. The vehicle according to claim 4, wherein the vehicle is an aircraft (300, 400).5 6. The vehicle according to claim 5, wherein the aircraft (300, 400) isan electric or hybrid-electric aircraft.

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