FIELD OF THE INVENTION

[0001] The present invention relates to a battery arrangement and a method of operating a battery arrangement for a solar powered aerial vehicle.

BACKGROUND OF THE INVENTION

[0002] A solar powered aerial vehicle uses solar panels to charge its battery or batteries during daylight hours. The batteries power motors operating the vehicle's propellers, as well as other functional equipment such as cameras, receivers, transmitters, navigational systems, antennas etc. Efficient management of the solar power system enables long duration flights to be undertaken.

[0003] The aerial vehicle may be manned or unmanned. For long duration, endurance flights where independent flight for extended time periods spanning potentially weeks or months is required, the vehicle is typically unmanned and operating at high altitude, e.g. in the stratosphere. Flight at stratospheric altitudes has the advantage that the stratosphere exhibits very stable atmospheric conditions, with wind strengths and turbulence levels at a minimum between altitudes of approximately 18 to 30 kilometres.

[0004] Temperature in the stratosphere can vary according to the time of day or night and also according to the flight altitude. The stratosphere extends generally from around 10 kilometres to around 50 kilometres above mean sea level, and the temperature near 50 kilometres altitudes is higher than the temperature at lower stratospheric altitudes. The temperature due to altitude variation ranges from around minus three degrees Celsius (-3°C) to around minus sixty degrees Celsius (-60°C). The minimum temperature given night time lows and altitude variation can be around minus eighty to minus ninety degrees Celsius (-80 to -90°C).

[0005] The charge and discharge performance of battery technologies is significantly reduced as the temperature drops below zero degrees Celsius (0°C). At minus twenty degrees Celsius (-20°C) most battery types stop functioning. Some battery technologies, for example nickel-cadmium (NiCd) or speciality lithium-ion (Li-ion), can operate at a reduced discharge rate down to a temperature of around minus forty degrees Celsius (-40°C), but charging at such low temperatures is not practical. Permanent damage can occur for example if the electrolyte freezes, and/or leaves plating deposits on the electrodes which are not removed over the charge-discharge cycle.

[0006] When operating batteries in an environment where temperatures are below 0°C it is known to heat batteries to maintain effective charge and discharge rates, and to prevent damage occurring.

SUMMARY OF THE INVENTION

[0007] A first aspect of the invention provides a method of operating a battery arrangement for a solar powered aerial vehicle, the battery arrangement comprising a plurality of batteries arranged in a plurality of groups, each group including one or more batteries, the groups including a first battery group, a second battery group and a third battery group, the method comprising the steps of discharging the first battery group via an electrical heating device, the first battery group being at an operative temperature range, the operative temperature range being at or above zero degrees Celsius, warming the second battery group from a non-operative temperature range, the non-operative temperature range being below zero degrees Celsius, to the operative temperature range using heat energy from the electrical heating device, whilst the first battery group is being discharged and the second battery group is being warmed, the third battery group remains at the non-operative temperature range.

[0008] A further aspect of the invention provides a battery arrangement for a solar powered aerial vehicle comprising a plurality of batteries arranged in a plurality of groups, each group including one or more batteries, the groups including a first battery group, a second battery group and a third battery group, and a controller, the controller being configured to control at least a temperature range and a charge status of each battery, and the first battery group is adapted to discharge via an electrical heating device, the first battery group being at an operative temperature range, the operative temperature range being at or above zero degrees Celsius, the second battery group is adapted to be warmed by the heating device from a non-operative temperature range, the non-operative temperature range being below zero degrees Celsius, to the operative temperature, wherein whilst the first battery group is being discharged and the second battery group is being warmed, the third battery group is arranged to remain at the non-operative temperature range.

[0009] Advantageously, batteries not required imminently for use are not heated.

Shortly before use only those batteries to be used are heated to within an acceptable operating temperature range. Given low ambient temperatures, the temperature of those batteries not required for use is allowed to drop to below an operative temperature. The inventor has made the insight that, rather than heating all batteries to an operating temperature and then continuously providing top up heat energy to maintain an operative battery temperature, it is surprisingly more energy efficient to allow unnecessary batteries to go cold and heat only those batteries required from a low and inoperative temperature to an operative temperature range just prior to use.

[0010] A battery, cell or accumulator is defined as a container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power. A solar powered aerial vehicle is any vehicle capable of flight at any altitude and powered by one or more solar energy collecting cells or panels.

[0011] The operative temperature range is a temperature range at which the battery arrangement is capable of efficiently being charged and discharged. Efficiency is a measure of the rate of charge and/or discharge of the battery. Charge and discharge should not result in damage to the battery. Similarly, the non-operative temperature range is a temperature range at which the battery arrangement is not capable of charging or discharging efficiently. As stated above, below zero degrees Celsius most battery types become ineffective and hence for most practical purposes inoperable. Equally, charge and discharge performance is affected by higher temperatures, with charge acceptance significantly reduced above forty five degrees Celsius (45°C) for most battery types (such as Li-ion, NiCd, Nickel-metal hydride and lead acid). Battery longevity is also reduced. An operative temperature range is therefore selected to be between zero and forty five degrees, preferably between zero and ten degrees Celsius.

[0012] The battery arrangement may be located on a solar powered aerial vehicle. The first battery group may discharge by providing power to equipment on the solar powered aerial vehicle. Equipment may be any of a range of equipment carried by the aerial vehicle, such as the motors driving the propellers, but also any functional or ancillary equipment, for example cameras, receivers, transmitters, navigational systems, antennas etc.

[0013] The battery arrangement may therefore provide power for the aerial vehicle to continue flight and other functions through the night when no solar energy gain is available. Batteries may be cycled between the first group, second group and third group according to power requirement. The power requirement for warming batteries using the battery arrangement of the first aspect is minimised by the third battery group remaining unheated at the inoperative temperature range.

[0014] Minimising power requirement enables the required capacity of the battery arrangement to be reduced, and therefore enables less weight to be carried by the aerial vehicle during flight. The battery capacity may also be matched to the amount of solar gain generally available during the day so as to enable minimal or no active power management function to be carried by the aerial vehicle. Minimising equipment and hence weight carried in this way may allow the aerial vehicle to continue flight for an extended period of time.

[0015] The solar powered aerial vehicle may operate in the stratosphere. The operating altitude may be between 10 to 50 kilometres, preferably between 18 to 30 kilometres. Operating the aerial vehicle at an altitude range where atmospheric conditions are at their most stable further allows the power requirement to be minimised.

[0016] The electrical heating device is powered via charge supplied by the battery or batteries in the first battery group. The ambient temperature range may be low enough that the first group of batteries require ongoing warming to maintain batteries at the operative temperature range. The electrical heating device may therefore be proximate the first battery group in order to maintain the first battery group at the operative temperature range. Since unnecessary heating of the batteries is to be avoided, the electrical heating device connected to the first battery group may be switched off once the first battery group is fully discharged. Spent batteries have insufficient charge remaining to usefully operate equipment, and so have no purpose unless solar gain or another charging source is available to recharge batteries at the operative temperature range. Therefore, battery charge may be conserved by not heating batteries in this state.

[0017] Fully discharged batteries may have charge remaining, but not sufficient charge to provide the power required by the aerial vehicle at that time. These third group, discharged batteries, if located on a solar powered aerial vehicle, await recharging by the solar panels during the daytime. Equally, a fully charged battery may hold a majority of charge rather than being literally fully 100% charged.

[0018] Batteries in the first battery group may be used in a number of ways. The first battery group may discharge via the heating device whilst also discharging by providing charge to equipment on the solar powered aerial vehicle. If solar gain or an alternative battery charging source is available, the first battery group may be being charged whilst also discharging.

[0019] In order to control a charge status and a temperature of each battery group, a controller may include a timing device and/or a temperature sensor. The controller may include a timing device, the second battery group being warmed via the electrical heating device for a time specified by the controller and monitored by the timing device. The second battery group may be warmed for a predetermined time in order to reach the operative temperature range. A sensor proximate each battery group may provide temperature information, the electrical heating device being operated according to the temperature information. A sensor proximate each battery group may provide temperature information to the controller, the controller operating the electrical heating device according to the temperature information. The timing device may include a clock. The clock may receive a time signal from an external source, such as a GPS device. Alternatively the clock may be calibrated according to an interval in a solar day. The interval in the solar day may be defined by, e.g. sunrise, sunset, peak solar gain, etc. which may be determined from a sunlight sensor or from the solar cells for example. [0020] When at least one of the battery groups is charged and not being discharged, it may be allowed to cool to the non-operative temperature range whilst another of the battery groups is being discharged.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0022] Figure 1 is a perspective view of an unmanned aerial vehicle (UAV) according to an embodiment of the invention,

[0023] Figure 2 is a cross sectional view though the wing of the UAV of Figure 1, showing the leading edge region and part of a rib,

[0024] Figure 3 is the cross sectional view though the wing of the UAV as shown in Figure 2, showing an arrangement of a temperature sensor close to the battery array,

[0025] Figure 4 is a cross sectional view through the battery array of Figures 2 and 3, showing individual batteries in a cluster, the heating element and insulation,

[0026] Figure 5 is a flow diagram showing how the charge status and temperature of a battery or group of batteries varies over time,

[0027] Figure 6 shows an alternative embodiment for operative first group batteries, including the solar recharging stage, and

[0028] Figure 7 is a schematic diagram showing how battery status changes over time, throughout the day and night over a 24 hour period.

DETAILED DESCRIPTION OF EMBODIMENT(S)

[0029] In an embodiment, the aerial vehicle is a UAV 100 having two wings 102, a fuselage 104, and a tailplane 106, as shown in Figure 1. In alternative embodiments, the aerial vehicle may be manned and may take any form suitable for the flight conditions at the planned altitude. The UAV 100 illustrated is configured to be lifted to the stratosphere by a lighter than air carrier where it is released for long duration flight. The UAV 100 is exclusively solar powered, i.e. it carries no other fuel source for powered flight. [0030] The wings 102 extend either side of the fuselage 104, and are elongate in a spanwise direction. Each wing 102 has a nine metre first section that extends generally perpendicularly from the fuselage i.e. the wing section is level or straight, and approximately parallel with the ground. A dihedrally angled wingtip extends a further from the outboard end of the first wing section. In alternative embodiments, the UAV 100 may have a total wingspan of around 20 to 60 metres. Equally, the wing configuration could be tapered in the outboard direction, and the wings may be horizontal or have a dihedral or an anhedral angle from the point the wing meets the fuselage 104, or from any point along the wing 102.

[0031] Each of the wings 102 carry a motor driven propeller 112 powered by rechargeable batteries carried within the UAV structure. The batteries are recharged during flight via solar energy collecting cells 114 located on the external surface of the aerial vehicle. Each propeller 112 is lightweight, in an embodiment the propellers 112 each weigh less than one kilogram and are greater than 2 metres in length. Figure 1 shows each wing carrying a single propeller, however in alternative embodiments multiple propellers may be provided on each wing. The propellers 112 are shaped for high altitude, low speed flight. The payload of the vehicle is also carried mainly within the wing structure, but could alternatively be distributed within any part of the UAV, depending on size and weight balance requirements.

[0032] Figure 1 shows a detail of the wing surface in one place showing an exemplary solar cell 114. Solar cells 114 cover generally the majority of the top surface of each wing 102, including each dihedral wingtip 116. The solar cells 114 are an integral part of the upper wing structure, such that the upper wing surface is flush with the leading edge. The solar cells form the upper aerodynamic surface of each wing. The solar cells may be arranged as a single unit covering the span of the wing or may be several separately located cells at different points along the wing. In alternative embodiments, solar cells may cover only part of each wing, be located on only one wing 102 or be located additionally or alternatively elsewhere on the UAV 100, such as on the tailplane or the fuselage 104.

[0033] In this embodiment the fuselage 104 is a minimal structure, comprising simply a lightweight tube, with the wings 102 and tailplane 106 attached to the tube. The tube is of carbon fibre construction, having a diameter in the range of 60 to 120mm. In alternative embodiments, the fuselage 104 may be constructed of any lightweight material, for example wood, plastic or fibre reinforced composite, and may be hollow or solid, and of any shape suitable for having wings and tailplane attached. The shape and dimensions of the fuselage 104 may vary along the length of the fuselage, for example to provide weight balance, and may be elliptical or tapered. The nose 108 of the fuselage 104 extends forwards of the wings and acts to counter balance the weight of the tailplane 106. The nose 108 also provides optional payload storage capacity.

[0034] The tailplane 106 has cruciform vertical and horizontal stabilising surfaces attached to the fuselage 104. The trailing portion of the stabiliser has an active movable rudder 110 located at the upper and lower portion of the vertical stabilising surface.

[0035] The battery arrangement 118 is located mainly in the leading edge 120 of the wing 102. Figure 1 shows the battery arrangement 118 extending across the span of each wing including the fuselage. In this embodiment the battery arrangement is located as a single assembly or array across the majority of both wings. The battery arrangement may alternatively comprise a plurality of battery arrays arranged as a single unit covering the span of the wing or may be several separately located arrays at different points along the wing. In alternative embodiments, the battery arrays may cover only part of each wing, be located on only one wing 102 or be located additionally or alternatively elsewhere on the UAV 100, such as on the tailplane or the fuselage 104.

[0036] Each wing 102 comprises a plurality of chordwise extending ribs and spanwise extending spars. Figure 2 is a cross section through an embodiment of the wing, showing the leading edge section 120 together with part of a rib 122. The leading edge section 120 has a cut out 124 shaped to accept and locate a battery array 125 comprising one or more groups of batteries. The position of the battery array 125 within the leading edge 120 is adjustable within the cut out 124 to enable weight balancing of the batteries with other equipment and payload. The cut out 124 in Figures 2 and 3 is shown as an oval shape, however in alternative embodiments the slot may be L-shaped, U- or C-shaped, Z- or T-shaped or any other shape required to enable the location of the batteries to be adjusted within the leading edge.

[0037] A controller 126 is connected to the solar panels 114 and to each battery either individually or via the battery array 125. The controller 126 is shown in Figures 2 and 3 as being located within the wing section adjacent the leading edge 120, however the controller 126 could be located within the leading edge or anywhere within the UAV convenient for space or weight balancing. The controller 126 controls which batteries are in use and what equipment each battery or battery array 125 is being used to provide power for. The controller 126 also controls the temperature of each battery or battery array via an electrical heating device. In this embodiment the heating device is a heating element arranged around the battery array 125. Alternatively, each battery may be located individually proximate a heating element, or a plurality of heating elements may be present, serving one or more battery arrays 125. The heating device may alternatively be a thermal blanket or other warming apparatus.

[0038] Operation of the heating element is controlled by a timing device 127 located within the controller 126. The time required to heat the battery or batteries within the battery array 125 to an acceptable operating temperature given the ambient environmental temperatures is known and used by the controller 126 to ensure that batteries required to be in use are pre-heated in time. Alternatively, and as shown in Figure 3, the battery array 125 may include one or more temperature sensors 128, which provide the temperature of the battery or batteries directly to the controller 126, enabling the controller 126 to activate the heating element(s) as required to maintain a desired temperature range.

[0039] As shown in the cross sectional view through the battery array 125 in Figure 4, each battery 130 is arranged in a cluster of batteries. In this embodiment four batteries 130 are arranged radially around a central point and are in contact with each other. Further batteries are located in a similar arrangement extending in front of and behind the batteries shown in Figure 4 to form an array 125 of batteries. In alternative embodiments, the batteries 130 may well not be circular in shape. Fewer or a greater number of batteries 130 can be located together in a radial or other shaped arrangement. There is no requirement that the batteries 130 are touching. In this embodiment multiple separate battery arrays 125 extend in a spanwise direction along the wing of the UAV, and are taped together into units that can be conveniently assembled into the wing. Alternatively, each battery array 125 could be located within a container such as a tube to provide structural support and protection as well as insulation.

[0040] The batteries 130 in this embodiment are based on rechargeable lithium- ion (Li-ion) technology. Alternative suitable battery technologies are available, for example lithium sulphur cells offer a lightweight solution delivering a high energy density - a high Watt hour per kilogram performance enables the UAV to carry less battery weight for the same battery performance, hence extending potential flight duration.

[0041] Since the ambient temperature is below zero degrees Celsius (0°C) and could reach minus eighty or minus ninety degrees Celsius (-80 to -90°C), the battery arrays 125 are also insulated. Insulation limits the minimum temperature of each battery to around minus thirty five degrees Celsius (-35°C). In this embodiment, insulation is provided by foam 134 surrounding the battery clusters, as shown in Figure 3. In other embodiments, insulation could be provided by any appropriate lightweight material capable of operating at the extremes of temperature encountered.

[0042] A heating element 132 shown in Figure 4 surrounds the battery cluster and extends along the battery array 125. In this embodiment, the battery cluster is taped in order to hold the batteries together and ensure there is a protective barrier between the heating element 132 and the batteries 130. The insulation 134 is placed around the battery cluster and heating element 132. The heating element 132 is a nickel- chromium alloy (nichrome) heating wire coiled around the batteries 130 in the cluster, and extending along the battery array 118. The heating element 132 could equally be made of copper or any other conducting material. The heating element 132 could be a ribbon or strip rather than a wire. The heating element 132 could be placed along the length or across the width of the battery cluster 125 rather than being coiled around the cluster.

[0043] Operation of the heating element 132 is controlled by the controller 126 as described above. The heating element 132 is powered by one or more of the batteries 130 or battery arrays 125. The heating element 132 thereby serves to warm one or more arrays of charged batteries to a usable operating temperature (or operative temperature range). Once at the operative temperature range, the warmed batteries power the motors driving the propeller 112 and/or other equipment on the UAV 100, as directed by the controller 126. In this embodiment, the target temperature is around five degrees Celsius (5°C), and a range of between around zero to ten degrees Celsius (0-10°C) is considered optimal. Each battery array 125 is connected to the controller 126, and the controller 126 manages the state of charge and temperature of each battery array 125. In an alternative embodiment, thermal control may be provided for each individual battery 130, i.e. each battery may have a dedicated heating element and/or surrounding insulation. The controller 126 may control each battery 130 individually rather than controlling a cluster of batteries as a single unit or array, thereby providing the maximum flexibility in terms of adjusting the battery power available to the UAV and its functions at any given time.

[0044] The battery array 125 (or assembly of battery arrays) comprising the tubular assembly including the insulation 134, heating element 132 and batteries 130 is then inserted through the cut out 124 in the leading edge 120 section of each rib 122. The location of the battery array 125 within the cut out is adjustable in order to optimise the weight balance of the battery arrangement 118 within the wing 102 and UAV 100.

[0045] Figure 5 provides a simplified explanation of the different stages of operating of the battery arrangement 118 of the current embodiment. There are generally three stages, categories, states or groups that a battery array might be in at any one time. For ease of reference, the state of the battery arrays will be termed 'group' henceforth, and batteries and battery arrays are to be understood to be interchangeable terms for the one or more battery located in an array. In each group there may be one or a plurality of batteries, which in this embodiment are located together into a tubular array with a common heating element. There may be one or more battery arrays 125 in each group. Batteries or battery arrays in each group may not be physically located adjacent each other.

[0046] Shortly before a battery array 125 is required to be put into operation, it is warmed via the heating element 132 to a minimum operative temperature. Batteries in the first group 1 are at an operative temperature range and have sufficient charge to power the UAV and whatever equipment is required to be operational at that point in time. First group 1 batteries are also being used to warm batteries in the second group. Potentially first group batteries are also used to heat themselves should the ambient temperature be low enough that the first group batteries require ongoing warming to maintain an operative temperature range, this is shown by the dotted return line against the first group 1 in Figure 5. Batteries in the second group 2 are fully charged and being warmed by the first group 1 batteries in preparation for use to power equipment on the UAV 100. First group 1 batteries are in the process of being discharged via powering the UAV 100, its motors and propellers or any other ancillary equipment in use at the time. Battery arrays in the third group 3 are being stored at ambient temperature. The temperature at the battery may be above the ambient temperature due to the insulation and any remnant heat in the array assembly, but these batteries are not being warmed. Given the low external environmental temperature, third group 3 batteries are unable to be operated, and are left dormant. Third group 3 batteries could be fully charged but not required imminently for use, or they could be fully discharged batteries. Fully discharged batteries may have charge remaining, but not sufficient charge to provide the power required by the UAV at that time. These third group 3, discharged batteries await recharging by the solar panels 114 during the daytime, as shown by Figures 6 and 7. Equally fully charged may mean holding a majority of charge rather than being literally fully 100% charged. 47] Figure 6 provides an alternative embodiment, in that different batteries within the first group 1 operative batteries have different tasks. Providing charge to power the UAV is carried out by one or more first group 1 batteries or arrays, whilst warming other batteries is the task of a different first group 1 battery or battery array. The other batteries requiring warming may be first group 1 or second group 2 batteries. When the ambient temperature is at its lowest, at lower stratospheric altitudes and at night time, the temperature of operative first group 1 batteries is at a temperature where ongoing warming is required to maintain an operative temperature range. At around - 80°C to -90°C, the temperature at the battery is around -35°C, and even with a slight temperature rise caused by internal cell resistance during charge or discharge, the battery temperature does not reach the operative temperature range. Figure 6 shows how spent third group 3 batteries are recharged by the solar cells 114 during the daytime, when solar gain is available.

[0048] Figure 7 provides a schematic overview of how the charge status and temperature of the different battery groups varies during the course of the day and night. As the sun rises, the UAV 100 has been operating through the night on batteries charged by the solar panel arrangement 114 the previous day. At this point in the cycle, minimum charge remains in the battery arrangement. Since the aerial vehicle in this embodiment is a UAV 100 designed for minimum weight, potentially there is only one battery or battery array with charge remaining. This is represented by the battery symbol at line a and column I (a- 1) in Figure 7. It is to be noted that this battery could be a single or multiple batteries arranged in one or more arrays. This battery is at its warmed operating temperature, has some charge remaining and is being used to power the UAV 100 and its equipment. The remaining batteries are discharged and not being warmed, i.e. they are at a non-operative temperature range.

[0049] As the sun rises and the solar panel or panels begin to collect solar energy, the first group batteries at the operative temperature range are carrying out multiple functions. First group batteries are receiving charge from the solar cells; discharging by operating the UAV, and also operating the heating element, so as to warm the next set of battery arrays to an operative temperature range (b-I in Figure 7). Once at the operative temperature range, the batteries are able to receive charge provided by solar gain (b-II). All other batteries (b-III) remain unheated. Once the first group batteries are once more fully charged, the second group batteries become first group batteries and take over providing charge to power the UAV and its equipment (c-I to c-III). The fully charged batteries at c-I are left to go cold and become dormant, third group batteries. The batteries now at the operative temperature range then operate the heating element so as to warm the next set of batteries requiring charging at c-III in Figure 7. This process continues until all batteries are recharged (dl-III). The battery or battery array or arrays operating the UAV 100 continue to be kept at a target operating temperature range of between zero and ten degrees Celsius (0-10°C). The remaining batteries are not warmed and remain at ambient temperature. [0050] At this point, any solar gain still available is not wasted, but is used to warm all the batteries. The ongoing solar energy collected is directed to the heating element or elements and all batteries are warmed, even though this may raise their temperature to above the target operating temperature range. Battery temperature may reach forty five degrees Celsius (45°C) or above, as shown by line e in Figure 7. Heating the batteries in this way ensures that no available solar gain is wasted, and allows all batteries to be warmed in readiness for the following night at low temperatures. The batteries are warmed beyond the operative temperature range, on the basis that the warmer battery temperature will offset the temperature drop to an extent as night time falls, and battery temperature can be kept higher for longer. By warming the batteries in this way, the power requirement to warm batteries to the operative temperature range during periods of low temperature can be minimised or at least offset. Equally, this is an effective use of excess solar energy and the system or arrangement can be balanced so that power management is either not required or is kept to a minimum. Additionally, or alternatively, any solar gain available once the batteries are charged may be used to power the UAV 100 to climb to a higher altitude.

[0051] As the sun sets and solar gain is lost, the batteries are generally all warm and at a temperature range where the batteries are operative. As night falls and solar gain is lost completely, the batteries cool again, such that those not being warmed are fully charged but reach a non-operative temperature range and are dormant third group batteries (f-I).

[0052] The battery array in column III of Figure 7 continues to be used to power the UAV. Once fully discharged and since night has fallen, this battery array (f-III) is unable to be recharged without solar energy available. The UAV is therefore powered by alternative battery arrays until they also are discharged so as to ineffective. As each battery array is spent in this way, so another array is brought online, as shown by lines f and g in Figure 7. The first group, operative battery array also charges the heating element in order to warm the next battery array in readiness for use. Until at dawn the following day, there is one array remaining with charge left to power the UAV (shown by the battery symbol in column I in line h of Figure 7). The cycle then begins again at dawn. This battery array then warms the battery array next in line for operation, and is itself recharged, as soon as solar gain becomes available.

[0053] Since weight is critical to the UAV's ability to remain airborne for long durations, the number of batteries carried on each flight or mission is minimised, however in alternative embodiments of the aerial vehicle there may be any number of batteries remaining with charge at the end of the night.

[0054] Batteries do not operate effectively at extremes of either high or low temperature. Warming cold batteries left at ambient temperatures below zero degrees Celsius addresses the extremes of low temperature. Excessive heating of the fully charged batteries during periods of strong solar gain may also pose a problem. For example, above around forty five degrees Celsius (45°C) Lithium-ion batteries have a reduced charge acceptance, and prolonged exposure to heat reduces longevity. In the current embodiment, the solar panel power generation capacity is closely matched to the power requirements of the UAV so that no significant power management functionality is required and minimal excess solar energy results. Short periods where battery temperatures may be above forty five degrees Celsius (45°C) are accepted. In this case, there is a balance to be struck between ensuring the UAV will be provided with sufficient power to operate through the night until sun rise provides solar gain to recharge the batteries, and also ensuring that the batteries are not subjected to excessively high temperatures whilst being charged or for long durations. Alternatively, in order to guard against the batteries reaching temperatures above which permanent damage may occur, an overheat protection may be provided. The overheat protection may take the form of a heat sink to safely dissipate excess heat energy, to which the controller directs the solar gain once all batteries are fully charged and have reached a maximum acceptable temperature.

[0055] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.