Patent Description:
Future aerospace applications including more electric, hybrid electric and purely electric aircraft will likely use high voltage battery packs made up of a number of series- and/or parallel- connected battery modules. These battery packs may, for example, be used to power one or more motors used to propel the aircraft and/or to provide power to one or more ancillary systems.

Heating of battery cells beyond their normal operating range can occur for various reasons. For example, a fault may develop in one or more of the modules, or the modules could be exposed to an external source of heat such as a fire, direct sunshine or exhaust gas from a gas turbine engine. Heat from one battery module can spread to others, which could lead to thermal runaway of an entire battery pack. Excessive heating and thermal runaway creates a risk of fire and with it damage to the battery pack, damage to the surrounding structure of the aircraft and risk of electric shock, for example by causing damage to electrical insulation and structures that support the cells and bus bars.

It is desirable to monitor temperatures arising in the battery pack to check its operation and to protect against the consequences of excessive cell heating. Some systems may monitor battery temperatures at a module level. Future aerospace applications may however require temperature monitoring at a more granular level.

European Patent Application publication <CIT> relates to a battery pack with thermistors on flexible layers for temperature measurement.

United States Patent Application publication <CIT> relates to a battery module with a PCB sensing assembly.

United States Patent Application publication <CIT> relates to a spacer for a battery pack.

The present disclosure provides a battery assembly that uses flexible PCBs to incorporate temperature sensing capability at a more granular level. Methods of assembling a battery assembly are also provided.

According to a first aspect there is provided a battery assembly comprising: an array of cylindrical battery cells, the array comprising plural rows, each row comprising plural cells, the plural rows including at least a first row, a second row adjacent to the first row and, and a third row adjacent to the second row, each cylindrical cell extending between opposed first and second ends in a direction perpendicular to a plane of the array; a flexible printed circuit board, PCB, provided between two adjacent rows of the plurality of rows of cells; a plurality of sensor carriers separate to the flexible PCBs, each sensor carrier comprising an aperture for receiving a temperature sensor in the direction perpendicular to the plane of the array, each sensor carrier being located in a gap formed between cells of two adjacent rows; and a plurality of temperature sensors, each temperature sensor received within the aperture of one of the sensor carriers, each temperature sensor electrically connected to one of the plurality of flexible PCBs, each temperature sensor being operable to sense a temperature of one or more cells of the two adjacent rows.

The gap may be a central gap between four cells, the four cells consisting of two cells from a first of the two rows and two cells from a second of the two rows.

The sensor carrier is separate to, but may be held in contact by, the flexible PCBs. The carrier may be shaped to cooperate with shapes of the cells surrounding the gap. The carrier comprises one or more apertures, for example central apertures, for receiving one or more temperature sensors. The carrier may comprise arcuate extensions for cooperating with the shape of circular cells, for example three or four arcuate extensions.

The cells of adjacent rows of the array may be offset with respect to each other, and each temperature sensor may be provided in a gap formed between a group of three cells, the three cells consisting of first and second cells belonging to one of the two rows and a third cell belonging to the other of the two rows and adjacent to the first and second cells.

The cells of adjacent rows may be offset with respect to each other by half a cell width, which may conveniently mean that each gap is of equivalent size. Alternatively they may be offset by more or less than half a cell or may not be offset at all.

The flexible PCB may be attached, for example adhesively attached, to side walls of the cells of one or both of the two adjacent rows. Other attachments means such as fasteners may also be used.

The battery assembly may further comprise an additional flexible PCB affixed to an outer facing surface or side wall of the cells of an outermost row of the array. The additional flexible PCB may be electrically coupled to a plurality of additional temperature sensors, each of the plurality of additional temperature sensors operable to sense a temperature of one or more cells of the outermost row. Each additional temperature sensor may be in physical contact with the side wall of one of the cells, and may be held in place by the flexible PCB. There may be one temperature sensor per cell for the outermost row of cells.

Each cell may have a side wall extending perpendicular to a plane of the array. Each temperature sensor may be positioned at the same distance along the side wall, as measured from one end of the cell.

The or each flexible PCB may be electrically connected to an onward electrical path at one end of the row or rows of cells to which it is adjacent. Electrical signals generated by a temperature sensor may be transmitted to the onward electrical path via the flexible PCB to which the temperature sensor is electrically connected.

The cells of each respective row of cells may be electrically connected together in parallel.

The battery assembly may be one of a plurality of electrically connected modules or sub-modules of a battery pack.

According to a second aspect, there is provided an aircraft propulsion system comprising a battery assembly according to the first aspect.

According to a third aspect there is provided an aircraft comprising a battery assembly according to the first aspect or an aircraft propulsion system according to the second aspect. The aircraft may be an electric or hybrid electric aircraft.

The terms "battery assembly", "battery module", "battery channel" and "battery pack" are used herein in a general sense to refer to electrical energy storage units that include various arrangements of electrically connected battery cells. The term "battery pack" is generally used to refer to an arrangement comprising one or more independent battery channels. The term "battery channel" is generally used to refer to an arrangement comprising one or more electrically connected battery modules. The term "battery module" is generally used to refer to an arrangement comprising one or more electrically connected battery assemblies. The term "battery assembly" is generally used to refer to any arrangement comprising one or more electrically connected battery cells. The battery cells may be cylindrical cells or another type of cell, for example pouch cells or prismatic cells.

With reference to <FIG>, a gas turbine engine is generally indicated at <NUM>, having a principal and rotational axis <NUM>. The engine <NUM> comprises, in axial flow series, an air intake <NUM>, a propulsive fan <NUM>, an intermediate pressure compressor <NUM>, a high-pressure compressor <NUM>, combustion equipment <NUM>, a high-pressure turbine <NUM>, an intermediate pressure turbine <NUM>, a low-pressure turbine <NUM> and an exhaust nozzle <NUM>. A nacelle <NUM> generally surrounds the engine <NUM> and defines both the intake <NUM> and the exhaust nozzle <NUM>.

The gas turbine engine <NUM> works in the conventional manner so that air entering the intake <NUM> is accelerated by the fan <NUM> to produce two air flows: a first air flow into the intermediate pressure compressor <NUM> and a second air flow which passes through a bypass duct <NUM> to provide propulsive thrust. The intermediate pressure compressor <NUM> compresses the air flow directed into it before delivering that air to the high pressure compressor <NUM> where further compression takes place.

The compressed air exhausted from the high-pressure compressor <NUM> is directed into the combustion equipment <NUM> where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines <NUM>, <NUM>, <NUM> before being exhausted through the nozzle <NUM> to provide additional propulsive thrust. The high <NUM>, intermediate <NUM> and low <NUM> pressure turbines drive respectively the high pressure compressor <NUM>, intermediate pressure compressor <NUM> and fan <NUM>, each by suitable interconnecting shaft.

Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g., two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. The gas turbine engine may also incorporate or interface with one or more batteries, for example to power ancillary electrical systems and/or to cooperate with one or more electric machines involved in the transfer of mechanical power to and from one or more of the interconnecting shafts.

Now referring to <FIG>, the propulsion system of a hybrid electric aircraft is generally indicated <NUM> and incorporates both an engine <NUM>, such as the gas turbine engine <NUM> described above with reference to <FIG>, and a battery pack <NUM>. Both the engine <NUM> and the battery pack <NUM> are used as energy sources to power a motor-driven propeller <NUM>, as well as ancillary electrical systems (not shown). The propulsion system <NUM> of the hybrid electric aircraft will typically further comprise a generator <NUM>, an AC/DC converter <NUM>, a high voltage DC (HVDC) distribution bus <NUM>, a DC/AC converter <NUM>, a motor <NUM> that drives the propeller <NUM>, and a DC/DC converter <NUM>.

A shaft of the engine <NUM> is coupled to and drives the rotation of a shaft of the generator <NUM> which thereby produces alternating current. The AC/DC converter <NUM>, which faces the generator <NUM>, converts the alternating current into direct current which is fed to various electrical systems via the HVDC distribution bus <NUM>. These electrical systems include the motor <NUM> that drives the propeller <NUM>. The motor <NUM> will typically be a synchronous motor that interfaces with the HVDC distribution bus <NUM> via the DC/AC converter <NUM>.

The battery pack <NUM>, 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 <NUM> via the DC/DC converter <NUM>. The DC/DC converter <NUM> converts between a voltage of the battery pack <NUM> and a voltage of the HVDC distribution bus <NUM>. In this way, the battery pack <NUM> can replace or supplement the power provided by the engine <NUM> (by discharging and thereby feeding the HVDC distribution bus <NUM>) or can be charged using the power provided by the engine <NUM> (by being fed by the HVDC distribution bus <NUM>).

A battery pack will also appear in the propulsion system of a purely electric aircraft, generally indicated as <NUM> in <FIG>. The battery pack <NUM> feeds a HVDC distribution bus <NUM>, possibly via DC/DC converter (not shown), which delivers power to one or more synchronous motors <NUM> via a DC/AC converter <NUM>. The one or more motors <NUM> drive the one or more propellers <NUM> that propel that aircraft.

Battery packs used in these applications may have high terminal voltages, for example 500V to 3kV. The use of high voltages advantageously allows for a reduction in the weight of the power distribution cabling, but it does create risk factors. For example, a fault within one or more of the battery modules could lead to cell overheating, which in turn creates a risk of thermal runaway, fire and electric shock.

In order to mitigate the risk of cell overheating, it is desirable to measure the temperatures arising in the battery pack. However, making direct temperature measurements can prove difficult, especially if it is desired to measure temperatures at a granular level, for example at the level of one or several cells. This difficulty is, in part, because of the typically dense packing of cells in battery packs used in these and other applications. Dense cell packing is generally required in view of space constraints and the need to supply battery packs with a high power density.

To illustrate this problem, <FIG> shows the construction of an exemplary battery pack. It is to be understood that the design shown in <FIG> is not intended to limit the present disclosure, but rather to illustrate how a battery pack may be constructed and may include a large number of densely packed cells.

Referring first to the top row of images, the left-most image shows a single battery cell, in this case a <NUM>. 2V <NUM> cylindrical cell having positive and negative terminals located at the same top end of the cell. In the next image, twelve of the cells are provided in a row and are connected in parallel, via a collector in the form of a bus bar adjacent to the row, to form a super-cell. Five super-cell rows are then connected together in series to form a five-row array of super-cells having a total voltage of about 20V and including sixty tightly packed cylindrical cells. In the next image, three of these arrays (totalling fifteen super-cells) are connected together in series to form a module-half having a terminal voltage of about 60V. In the final image of the top row, two of the module-halves are connected together in series to form a battery module having a terminal voltage of approximately 120V. It will be appreciated that, in this example, the battery module includes <NUM> of the cylindrical cells.

Now referring to the bottom row of images, in the left hand image a battery channel is formed by connecting six of the battery modules together in series. The channel thus has a terminal voltage of about 720V and consists of <NUM>,<NUM> cells. Finally, in the right hand image, a battery pack includes a battery case that houses three of the battery channels. That is, the battery pack includes three independent 720V power supplies each having <NUM>,<NUM> battery cells, for a total of <NUM>,<NUM> cells. It will of course be appreciated that the battery pack could have any number of channels, including one channel.

It can be appreciated that the battery pack is densely packed with cells. What little space remains in the battery pack is mostly taken up by carrier frame material, which is included to provide adequate structure and protection. Further space is taken up by the electrical connections that connect the cells together and to onward electrical paths, and by various cooling systems for carrying heat away from the cells.

This dense packing results in very little space for the provision of additional components such as temperature sensors. Whilst one or two sensors per module or module-half may be included, finding room to provide additional temperature sensors becomes increasingly difficult. Furthermore, each temperature sensor must be connected to an onward electrical path, and the number of sensor connections can quickly become overwhelming given the number of cells in the pack, in this case <NUM>,<NUM> cells.

To address these and other problems, the present disclosure utilizes flexible PCBs, which can be sandwiched between adjacent rows of cells, to provide electrical connections to and from temperature sensors positioned in the core of a battery assembly.

<FIG> illustrates a partially assembled array <NUM> of battery cells <NUM> that is or forms part of a battery assembly <NUM>. The term "battery assembly" <NUM> as used herein is a general term that encompasses entire battery packs; battery modules that when connected to other battery modules form a battery pack; and any other battery assembly that includes one or more arrays of battery cells.

The battery assembly <NUM> includes a carrier frame <NUM> that includes an approximately rectangular array <NUM> of apertures <NUM> that are sized and shaped to receive battery cells <NUM>. In this example the cells <NUM> are cylindrical cells, and so the apertures <NUM> are circular in cross-section. The carrier frame <NUM> provides the battery assembly <NUM> with structural rigidity while the apertures <NUM> accurately locate the cells <NUM> in their intended positions.

The array <NUM> includes a plurality of rows; in this case five rows labelled A, B, C, D and E (A-E). Each row A-E includes a plurality of apertures <NUM> for a corresponding plurality of cells <NUM>; in this case thirty-six apertures <NUM> for thirty-six cells <NUM>. The array <NUM> lies in a plane, with each row extending along one direction in the plane. Without loss of generality this direction will be designated the x-direction. Adjacent rows are offset in a direction perpendicular to the x-direction, which will be described as the y-direction. The cells <NUM> themselves extend perpendicular to the plane of the array <NUM>, in the z-direction.

In <FIG> the battery assembly <NUM> is only partially assembled, with the cells <NUM> of only one of the rows, row A, received within the respective apertures <NUM>. In accordance with the present disclosure, before inserting the cells of the adjacent row B within their apertures, a thin and flexible printed circuit board (PCB) <NUM> is provided over the inside facing side walls <NUM> of the cells <NUM> of the first row A. In this way, when the cells <NUM> of the adjacent row B are inserted into their apertures, the flexible PCB <NUM> will be sandwiched between the two adjacent rows A, B.

The flexible PCB <NUM> extends the entire length of the row A and incorporates electrical conduits for carrying electrical signals. In this way, the flexible PCB <NUM> is able to provide electrical connections to and from temperature sensors <NUM> (not visible in <FIG>) that are provided between the adjacent rows A, B. The location of the temperature sensors <NUM> between the rows A, B will be described in more detail below.

Any suitable flexible PCB <NUM> may be used. For example, a PCB made of a thin, electrically insulating substrate (for example a polyamide film) with embedded conductors can be used. The flexibility of the PCB <NUM> allows it to conform to the surfaces of the sidewalls <NUM> of the cells <NUM>. Conveniently, one or both major surfaces of the flexible PCB <NUM> may be covered in an adhesive so that the flexible PCB <NUM> can be affixed to the side walls <NUM> without additional fixings.

In order to monitor the temperatures of the cells <NUM> of the two adjacent rows A, B, temperature sensors <NUM> are provided in spaces between the rows A, B. The preferred locations of the temperatures sensors <NUM> will, to some extent, depend on the chosen geometry of the array <NUM> of the battery assembly <NUM> and the resulting locations of spaces in the array.

In some examples, there may be sufficient space in the y-direction between adjacent rows A, B for temperature sensors <NUM> to be provided adjacent to (for example in direct contact with) the inside facing side walls <NUM> of the cells <NUM>. In other words, a temperature sensor <NUM> may be sandwiched between the flexible PCB <NUM> and a cell <NUM> of one of the two rows A, B. In this way, a temperature sensor <NUM> may be operable to monitor the skin temperature of the cell(s) <NUM> to which it is adjacent. Temperature sensors <NUM> may be provided adjacent to each cell <NUM> or only some of the cells of the two rows A, B.

In other examples, the cells <NUM> of the array <NUM> may be so tightly packed that there is insufficient space in the y-direction for temperature sensors <NUM> to be sandwiched between the flexible PCB <NUM> and the side wall of a cell <NUM>. Thus, in this and other examples, temperature sensors <NUM> may be provided in other available spaces in the array <NUM>. This is illustrated in <FIG>, which show a temperature sensor <NUM> located within a gap <NUM> that is defined between three adjacent battery cells 34i, 34ii, 34iii.

In more detail, it can be seen from <FIG> and <FIG> that adjacent rows of the plurality of rows A-E are offset from each other, in the x-direction, by half the width of a cell <NUM>. That is, the cells <NUM> of each row (e.g., row B) are shifted, relative to the cells <NUM> of the neighbouring row(s) (e.g., A, C), by a distance in the x-direction. Thus the centres of the cells <NUM> of adjacent rows are not aligned in the y-direction. The result is that, when considering the cells of two adjacent rows (e.g., A, B), the cells <NUM> form groups of three cells: two cells 34i, 34ii of one row (A) and a third cell 34iii of the other row (B) that is staggered in the x-direction half way between the first two cells 34i, 34ii. Such a group of three cells 34i, 34ii, 34iii is shown in <FIG>.

Due to the circular cross-section of the cells <NUM>, a gap <NUM> (visible in the semi-transparent view of <FIG>) is formed between the three cells 34i, 34ii, 34iii that can accommodate a temperature sensor <NUM>. The temperature sensor <NUM> located in the gap <NUM> is operable to measure a temperature in its vicinity, for example a temperature representative of one of the three cells 34i, 34ii, 34iii or an average temperature of the three cells 34i, 34ii, 34iii. As can be best seen in <FIG>, the sensor <NUM> can be readily electrically connected to the electrical conduits of the flexible PCB <NUM> that passes between the two rows A, B. The sensor <NUM> may be adhesively attached to the surface of the flexible PCB <NUM> so as to keep the sensor is a substantially constant position relative to the side walls of the cells <NUM>.

<FIG> also illustrate how a temperature sensor <NUM> may be held by or within a sensor carrier <NUM>, which may be formed of a plastic or another suitable material. The sensor carrier <NUM> may include a central slot that is sized and shaped to receive and firmly hold the sensor <NUM> in place. Further, the outer surfaces of the carrier <NUM> may be sized and shaped to conform to the curved sidewalls of the three cells 34i, 34ii, 34iii so that the carrier <NUM> can sit by or be fixed to the side walls of the three cells. In this specific case the carrier <NUM> comprises three arcuate surfaces corresponding to arcuate portions of the cylindrical sidewalls of the three cells 34i, 34ii, 34iii of the group.

Although the example illustrated in <FIG> utilizes cylindrical cells, with adjacent rows staggered by half a cell width, it should be understood that other possibilities are within the scope of present disclosure. For example:.

Any suitable temperature sensor <NUM> can be used. In one example, the temperature sensors <NUM> are standard PT1000 and/or LM50 temperatures sensors.

Now turning to <FIG>, this shows the battery assembly <NUM> of <FIG> but with the cells <NUM> of four rows A-D of the five rows A-E received within the carrier frame <NUM>. Flexible PCBs <NUM> have been provided over the inside facing surfaces of each of the four rows A-D such that, in <FIG>, there are four flexible PBCs <NUM>. Also visible are the ends of the rows of the flexible PCBs <NUM>, which can be seen to terminate in connectors <NUM> by which the electrical conduits of the flexible PCBs <NUM> can be connected to onward electrical paths. These may, ultimately, lead to a battery management system (BMS) or the like that is operable to control the battery assembly <NUM> based at least in part on the temperature measurements received from the sensors <NUM>.

<FIG> shows how an additional flexible PCB <NUM> may be provided over an outside facing surface of an outermost row of cells <NUM> of the array <NUM>. Generally speaking, space is at less of a premium at the outermost rows of cells <NUM>. As such, temperature sensors <NUM> may be provided adjacent to (for example in direct contact with) the side walls <NUM> of the cells <NUM> rather than within the gaps defined between cells. In <FIG> it can be seen that each of the cells <NUM> are equipped with a temperature sensor <NUM> (visible as a cylindrical bulge in the PCB <NUM>) that is sandwiched between flexible PCB <NUM> and the side wall of the cell <NUM>. No sensor carrier <NUM> is utilized in this case, and the flexible PCB <NUM> helps locate the sensors <NUM> in a consistent position along the length of the side walls of the cells <NUM>.

So that the temperatures sensed by the temperature sensors <NUM> can be reliably interpreted, the sensors <NUM> are preferably located in consistent environments. For example, it is generally expected that there will be a thermal gradient across each cell. Thus, to eliminate the effects of the thermal gradients on the interpretation of the sensor readings, each temperature sensor may be positioned at the same vertical distance along the z-axis. Further, where the offset between adjacent rows is greater or less than half a cell width, such that gaps <NUM> are of variable sizes, sensors <NUM> may be placed only in equivalently sized and shaped gaps.

<FIG> shows a more completely assembled battery assembly <NUM>, similar to the battery module shown in <FIG>. For temperature monitoring purposes, flexible PCBs <NUM> are sandwiched between rows of battery cells <NUM>, and additional flexible PCBs <NUM> are provided on outside facing surfaces of the outermost rows. At the row ends, the flexible PCBs <NUM>, <NUM> are connected to their onward electrical paths so that the temperatures sensed by the temperature sensors <NUM> (not visible in <FIG>) can be communicated on for monitoring, for example a BMS.

The number and location of the temperature sensors <NUM> within the battery assembly <NUM> can be varied according to the application requirements and constraints. In some cases there may be one temperature sensor <NUM> for each and every cell <NUM> in the battery assembly <NUM>. In other examples, there may be an average of less than one temperature sensor <NUM> per cell <NUM>, with sensor locations selected so as to provide adequate temperature monitoring across the pack and/or to monitor the temperature of expected 'hotspots'. For instance, one temperature sensor <NUM> may be provided for every two, three, four, five or six cells.

To illustrate this, <FIG> marks exemplary locations of temperature sensors in the core regions of the two halves of the battery assembly of <FIG>. As can be seen in <FIG>, not every gap <NUM> that is defined between a group of three cells 34i, 34ii, 34iii is provided with a temperature sensor <NUM>. Instead, thirty sensors are distributed between the two halves of the assembly, with fifteen sensors per half. Each half has five rows A-E and, in this case, every second row is provided with six temperatures sensors spaced approximately evenly along the length of the row. Each of these temperature sensors is operable to sense the temperature of three adjacent cells 34i, 34ii, 34iii. Such an arrangement may provide temperature monitoring coverage across the majority of the core of the battery assembly <NUM>, without adding to size of the assembly or introducing an overwhelming arrangement of connections.

<FIG> is a flow chart illustrating a method <NUM> of assembling a battery assembly <NUM> in accordance with the present disclosure. It should be understood that, except where the context dictates otherwise, the steps <NUM>-<NUM> need not take place in the order described below.

At <NUM>, a carrier frame <NUM> is obtained. The carrier frame <NUM> includes an array <NUM> of apertures <NUM> that are sized and shaped to receive battery cells <NUM>. The array includes plural rows A-E, and each of the rows includes plural apertures <NUM> for a corresponding plurality of cells <NUM>.

The carrier frame <NUM> may be formed of any suitable material, for example a plastic material or other electrical insulator. The apertures <NUM> may be sized and shaped to receive cells of any suitable geometry, for example cylindrical cells. The array <NUM> can have any suitable geometry. As explained above, the apertures <NUM> of adjacent rows may be staggered in the x-direction so as to form groups of three apertures, each defining a gap <NUM> at the centre of the group.

At <NUM>, battery cells <NUM> are positioned within the apertures of a first row (A) of the array. The first row may be any one of the rows of the array.

At <NUM>, a flexible PCB <NUM> is provided over inside facing side walls <NUM> of the cells <NUM> of the row. The flexible PCB <NUM> is typically a thin, electrically insulating substrate having electrical conduits extending along its length. One or both major surfaces of the flexible PCB <NUM> may include an adhesive allowing it to be readily attached to the side walls <NUM> of the cells <NUM>.

At <NUM>, at least one temperature sensor <NUM> is electrically connected to the flexible PCB <NUM>. The sensors <NUM> may be physically attached to the flexible PCB <NUM>, for example by adhesive. The sensors may be placed in gaps <NUM> between cells <NUM>.

At <NUM>, cells <NUM> of a second row (B) of the array adjacent to the first row (A) are positioned within the apertures of the second row. The cells <NUM> are positioned such that the flexible PCB <NUM> is provided between the adjacent first and second rows of cells.

The temperature sensors of step <NUM> are positioned adjacent to the flexible PCB <NUM> and operable to sense a temperature of one or more cells <NUM> of the two adjacent rows (A, B). Each temperature sensor <NUM> may sense the temperature of one cell, two cells or, in one particular example, an average temperature of three adjacent cells 34i, 34ii, 34iii.

Claim 1:
A battery assembly (<NUM>) comprising:
an array (<NUM>) of cylindrical battery cells (<NUM>), the array comprising plural rows (A-E), each row comprising plural cylindrical cells, the plural rows including at least a first row (A), a second row (B) adjacent to the first row, and a third row (C) adjacent to the second row (B), each cylindrical cell extending between opposed first and second ends in a direction perpendicular to a plane of the array;
a plurality of flexible printed circuit boards, PCBs (<NUM>), each flexible PCB provided between two adjacent rows of the plurality of rows of cells, the plurality of flexible PCBs including a first flexible PCB provided between the adjacent first and second rows (A, B) and a second flexible PCB provided between the adjacent second and third rows (B, C);
a plurality of sensor carriers (<NUM>) separate to the flexible PCBs, each sensor carrier comprising an aperture for receiving a temperature sensor (<NUM>) in the direction perpendicular to the plane of the array, each sensor carrier being located in a gap formed between cells of two adjacent rows; and
a plurality of temperature sensors (<NUM>), each temperature sensor received within the aperture of one of the sensor carriers, each temperature sensor electrically connected to one of the plurality of flexible PCB, each respective temperature sensor being operable to sense a temperature of one or more cells of the respective two adjacent rows.