Patent Publication Number: US-2021188130-A1

Title: Fuel cell hybrid power system

Description:
FIELD OF THE INVENTION 
     The present invention relates to fuel cell hybrid power systems. 
     BACKGROUND 
     Fuel cell hybrid electric Unmanned Aerial Vehicles (UAVs) have gained considerable interest in the aviation industry, for example, due to their low noise, reduced emissions and potential for replacing satellites in certain applications. 
     However, a limitation of UAVs is the maximum flight duration or endurance capability of the propulsion system. 
     SUMMARY OF THE INVENTION 
     The present inventor has realised that improving power management in a power system of a fuel cell hybrid electric UAV would tend to improve UAV flight endurance. 
     The present inventors have realised it would be beneficial to provide a power management algorithm that optimises use of a fuel cell, a battery, and a supercapacitor. The present inventors have realised that it would be beneficial to constrain the fuel cell, battery, and/or supercapacitor to operate in regions of lower losses by sharing the load power demand. 
     In a first aspect, the present invention provides a power system comprising: an electrical power bus; one or more loads electrically coupled to the electrical power bus; a fuel cell electrically coupled to the electrical power bus; a battery; a first switch electrically coupled between the battery and the electrical power bus; a supercapacitor; a second switch electrically coupled between the supercapacitor and the electrical power bus; a resistor coupled between the electrical power bus and the supercapacitor via the second switch; and a power system manager configured to control operations of the first switch and the second switch. The first switch is switchable between a first state and a second state. The first state of the first switch is such that the battery is directly connected to the electrical power bus such that the battery provides electrical power directly to the electrical power bus. The second state of the first switch is such that the battery is disconnected from the electrical power bus. The second switch is switchable between a first state, a second state, and a third state. The first state of the second switch is such that the supercapacitor is directly connected to the electrical power bus such that the supercapacitor provides electrical power directly to the electrical power bus. The second state of the second switch is such that the supercapacitor is disconnected from the electrical power bus. The third state of the second switch is such that the supercapacitor is connected to the electrical power bus via the resistor such that the supercapacitor receives electrical power from the electrical power bus via the resistor. 
     The supercapacitor arranged to be charged through the resistor advantageously tends to limit inrush current to the supercapacitor from the electrical power bus. This limitation of inrush current tends to prevent large drops in bus voltage, thereby improving the reliability of power supply to the one or more loads. Furthermore, this tends to reduce a power demand (e.g. an instantaneous demand) placed on the fuel cell, when the supercapacitor is charging. Thus, energy losses of the fuel cell tend to be reduced. Furthermore, the likelihood of the battery being caused to operate in a high loss region when the supercapacitor is charging tend to be reduced. Thus, detrimental effects on the battery state of health tend to be avoided. 
     A resistance of the resistor may be substantially equal to a voltage of the electrical power bus divided by a charging current for the supercapacitor. 
     The power system may further comprise a measurement device configured measure fuel level for the fuel cell, and to send the measured fuel level for the fuel cell to the power system manager. The power system may further comprise a first state of charge measurement device configured measure a state of charge of the battery, and to send the measured state of charge of the battery to the power system manager. The power system may further comprise a second state of charge measurement device configured to measure a state of charge of the supercapacitor, and to send the measured state of charge of the supercapacitor to the power system manager. 
     The power system manager may be configured to control operation of either or both of the first and second switches depending upon one or more parameters selected from the group of parameters consisting of: a state of charge of the battery, a state of charge of the supercapacitor, a fuel level for the fuel cell, and a load demand of the one or more loads. 
     The battery may be a rechargeable battery. The power system may further comprise a battery charger coupled between the electrical power bus and the battery via the first switch. The first switch may be switchable between its first state, its second state, and a third state. The third state of the first switch may be such that the battery is connected to the electrical power bus via the battery charger such that the battery charger charges the battery with electrical power from the electrical power bus. 
     The power system may further comprise a DC-DC converter electrically coupled between the fuel cell and the electrical power bus. The power system may comprise only a single DC-DC converter. The DC-DC converter may be configured to convert a power supply received from the fuel cell into a constant voltage power supply having a voltage substantially equal to a predefined voltage level, and to output the constant voltage power supply to the electrical power bus. The battery may be configured to provide, to the electrical power bus, an electrical power supply having a voltage substantially equal to the predefined voltage level. 
     The power system may further comprise a third switch coupled between the DC-DC converter and the electrical power bus. The power system manager is configured to control operation of the third switch depending upon one or more parameters selected from the group of parameters consisting of: a state of charge of the battery, a state of charge of the supercapacitor; a fuel level for the fuel cell, and a load demand of the one or more loads. The third switch may be switchable between a first state and a second state. The first state of the third switch may be such that the fuel cell is electrically connected to the electrical power bus via the DC-DC converter such that the fuel cell provides electrical power directly to the electrical power bus. The second state of the third switch may be such that the DC-DC converter and the fuel cell are disconnected from the electrical power bus. 
     In a further aspect, the present invention provides a vehicle comprising a power system according to any preceding aspect. The vehicle may be an unmanned air vehicle. 
     In a further aspect, the present invention provides a method of providing a power system. The method comprises: electrically coupling one or more loads to an electrical power bus; electrically coupling a fuel cell to the electrical power bus; providing a battery; electrically coupling a first switch between the battery and the electrical power bus; providing a supercapacitor; electrically coupling a second switch between the supercapacitor and the electrical power bus; electrically coupling a resistor between the electrical power bus and the supercapacitor via the second switch; and operatively coupling a power system manager to the first switch and the second switch, the power system manager configured to control operations of the first switch and the second switch. The first switch is switchable between a first state and a second state. The first state of the first switch is such that the battery is directly connected to the electrical power bus such that the battery provides electrical power directly to the electrical power bus. The second state of the first switch is such that the battery is disconnected from the electrical power bus. The second switch is switchable between a first state, a second state, and a third state. The first state of the second switch is such that the supercapacitor is directly connected to the electrical power bus such that the supercapacitor provides electrical power directly to the electrical power bus. The second state of the second switch is such that the supercapacitor is disconnected from the electrical power bus. The third state of the second switch is such that the supercapacitor is connected to the electrical power bus via the resistor such that the supercapacitor receives electrical power from the electrical power bus via the resistor. 
     The method may further comprise switching, by the power system manager, the second switch into its third state, thereby causing the supercapacitor to receive electrical power from the electrical power bus (i.e. be charged) via the resistor. 
     In a further aspect, the present invention provides a power system comprising: an electrical power bus; one or more loads electrically coupled to the electrical power bus; a fuel cell; a DC-DC converter electrically coupled between the fuel cell and the electrical power bus; a battery; a switch electrically coupled between the battery and the electrical power bus; and a power system manager configured to control operation of the switch. The switch is switchable between a first state and a second state. The first state of the switch is such that the battery is directly connected to the electrical power bus such that the battery provides electrical power directly to the electrical power bus. The second state of the switch is such that the battery is disconnected from the electrical power bus. 
     The power system may comprise only a single DC-DC converter. 
     The power system may further comprise a measurement device configured measure fuel level for the fuel cell, and to send the measured fuel level for the fuel cell to the power system manager. 
     The power system may further comprise a first state of charge measurement device configured measure a state of charge of the battery, and to send the measured state of charge of the battery to the power system manager. 
     The power system manager may be configured to control operation of the switch depending upon one or more parameters selected from the group of parameters consisting of: a state of charge of the battery, a state of charge of a supercapacitor, a fuel level for the fuel cell, and a load demand of the one or more loads. 
     The battery may be a rechargeable battery. The power system may further comprise a battery charger coupled between the electrical power bus and the battery via the switch. The switch may be switchable between its first state, its second state, and a third state. The third state of the switch may be such that the battery is connected to the electrical power bus via the battery charger such that the battery charger charges the battery with electrical power from the electrical power bus. 
     The power system may further comprise a supercapacitor and a further switch electrically coupled between the supercapacitor and the electrical power bus. The power system manager may be configured to control operation of the further switch. The further switch may be switchable between a first state and a second state. The first state of the further switch may be such that the supercapacitor is directly connected to the electrical power bus such that the supercapacitor provides electrical power directly to the electrical power bus. The second state of the further switch may be such that the supercapacitor is disconnected from the electrical power bus. 
     The power system may further comprise a second state of charge measurement device configured measure a state of charge of the supercapacitor, and to send the measured state of charge of the supercapacitor to the power system manager. 
     The power system manager may be configured to control operation of the further switch depending upon one or more parameters selected from the group of parameters consisting of: a state of charge of the battery, a state of charge of a supercapacitor, a fuel level for the fuel cell, and a load demand of the one or more loads. 
     The power system may further comprise a resistor coupled between the electrical power bus and the super capacitor via the further switch. The further switch may be switchable between its first state, its second state, and a third state. The third state of the further switch may be such that the supercapacitor is connected to the electrical power bus via the resistor such that the supercapacitor receives electrical power from the electrical power bus via the resistor. 
     The DC-DC converter may be configured to convert a power supply received from the fuel cell into a constant voltage power supply having a voltage substantially equal to a predefined voltage level, and to output the constant voltage power supply to the electrical power bus. The battery may be configured to provide, to the electrical power bus, an electrical power supply having a voltage substantially equal to the predefined voltage level. 
     The power system may further comprise a second further switch coupled between the DC-DC converter and the electrical power bus. The power system manager may be configured to control operation of the second further switch depending upon one or more parameters selected from the group of parameters consisting of: a state of charge of the battery, a state of charge of a supercapacitor; a fuel level for the fuel cell, and a load demand of the one or more loads. The second further switch may be switchable between a first state and a second state. The first state of the second further switch may be such that the fuel cell is electrically connected to the electrical power bus via the DC-DC converter such that the fuel cell provides electrical power directly to the electrical power bus. The second state of the further switch may be such that the DC-DC converter and the fuel cell are disconnected from the electrical power bus. 
     In a further aspect, the present invention provides a vehicle comprising a power system according to any preceding aspect. The vehicle may be an unmanned air vehicle. 
     In a further aspect, the present invention provides a method of providing a power system. The method comprises: electrically coupling one or more loads to an electrical power bus; providing a fuel cell; electrically coupling a DC-DC converter between the fuel cell and the electrical power bus; providing a battery; electrically coupling a switch between the battery and the electrical power bus; and operatively coupling a power system manager to the switch, the power system manager configured to control operation of the switch. The switch is switchable between a first state and a second state. The first state of the switch is such that the battery is directly connected to the electrical power bus such that the battery provides electrical power directly to the electrical power bus. The second state of the switch is such that the battery is disconnected from the electrical power bus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration (not to scale) of an unmanned air vehicle; 
         FIG. 2  is a schematic illustration (not to scale) showing an embodiment of a power system of the unmanned air vehicle; 
         FIG. 3  is a schematic illustration (not to scale) showing further details of a battery module of the power system; 
         FIG. 4  is a schematic illustration (not to scale) showing further details of a supercapacitor module of the power system; and 
         FIG. 5  is a process flow chart showing certain steps of an embodiment of a method of operation of the power system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration (not to scale) of an unmanned air vehicle (UAV)  2 . The UAV  2  comprises a power system  4 . 
       FIG. 2  is a schematic illustration (not to scale) showing an embodiment of the power system  4 . 
     In this embodiment, the power system  4  comprises a fuel cell  10 , a fuel cell controller  12 , a converter  14 , a battery module  16 , a supercapacitor module  18 , a plurality of loads (which are collectively represented in  FIG. 2  by a single box and the reference numeral  20 ), and a power system manager  22 . 
     The fuel cell  10  is a Proton Exchange Membrane (PEM) fuel cell configured to convert chemical energy in the form of hydrogen fuel into direct current (DC) electricity through a chemical reaction of positively charged hydrogen ions with oxygen from a received air supply. A supply of hydrogen fuel (not shown in the Figures) is located on board the UAV  2 . The fuel cell  10  is coupled to the fuel cell controller  12  such that operation of the fuel cell  10  may be controlled by the fuel cell controller  12 . The fuel cell  10  is further coupled to the converter  14  via a diode  24  such that DC electricity output by the fuel cell  10  is supplied to the converter  14 . The fuel cell  10  is further coupled to power system manager  22  such that information may be sent from the fuel cell  10  to power system manager  22 , as described in more detail later below. 
     The fuel cell controller  12  is a controller coupled to the fuel cell  10  and configured to control operation of the fuel cell  10 . Control of the fuel cell  10  by the fuel cell controller  12  is described in more detail later below. The fuel cell controller  12  is coupled to the power system manager  22  such that information may be sent between the power system manager  22  and the fuel cell controller  12 . 
     In this embodiment, the converter  14 , the battery module  16 , the supercapacitor module  18 , and the plurality of loads  20  are coupled in parallel between a first electrical wire  26  and a second electrical wire  28 . The first wire  26  and the second wire  28  form a DC bus. A voltage between the first wire  26  and the second wire  28  is a DC bus voltage. The second electrical wire  28  is grounded. Further systems on board the UAV  2  that utilise electrical power may be coupled to the DC bus between the first and second wires  26 ,  28 . 
     The converter  14  is a DC-DC converter. The converter  14  is coupled to the fuel cell  10  via the diode  24  such that, in operation, the converter  14  receives DC electrical power from the fuel cell  10 . The converter  14  is configured to, in operation, convert DC electrical power received from the fuel cell  10  from a first voltage level to a second voltage level. The converter  14  is configured to output DC electrical power at the second voltage level to the DC bus. In other words, the converter  14  is configured to establish a DC bus voltage having the second voltage level between the first wire  26  and the second wire  28 . For example, the converter  14  may receive a constant current, variable voltage power supply from the fuel cell (e.g. a power supply having constant current and a voltage that varies in the range 18V to 72V), and may convert this received power supply to a variable current, constant voltage supply (e.g. a power supply having variable current and a constant 24V voltage). 
     In this embodiment, the converter  14  is coupled to the DC bus (in particular, to the first wire  26 ) via a first switch  30 . The first switch  30  is a solid state switch. The first switch  30  is coupled to the power system manager  22  such that the power system manager  22  may control the first switch  30  to connect or disconnect the converter  14  from the first wire  26 . 
     The battery module  16  is described in more detail later below with reference to  FIG. 3 . The battery module  16  is electrically coupled to the DC bus between the first wire  26  and the second wire  28 . The battery module  16  is configured to either receive electrical power from the DC bus, or to supply DC electrical power to the DC bus. The battery module  16  is further coupled to the power system manager  22  such that signals may be sent between the battery module  16  and the power system manager  22 . Control and operation of the battery module  16  is described in more detail later below, for example with reference to  FIG. 5 . 
     The supercapacitor module  18  is described in more detail later below with reference to  FIG. 4 . The supercapacitor module  18  is electrically coupled to the DC bus between the first wire  26  and the second wire  28 . The supercapacitor module  18  is configured to either receive electrical power from the DC bus, or to supply DC electrical power to the DC bus. The supercapacitor module  18  is further coupled to the power system manager  22  such that signals may be sent between the supercapacitor module  18  and the power system manager  22 . Control and operation of the supercapacitor module  18  is described in more detail later below, for example with reference to  FIG. 5 . 
     The plurality of loads  20  may include any appropriate aircraft subsystems located on board the UAV  2 . The loads  20  may include, but are not limited to, a mission system, a navigation system, an avionics systems, a fuel system, and an environmental control system. The plurality of loads  20  is electrically coupled to the DC bus between the first wire  26  and the second wire  28 , and may receive electrical power from the DC bus. The plurality of loads  20  is further coupled to the power system manager  22  such that signals may be sent between the supercapacitor module  18  and the power system manager  22 . 
     The power system manager  22  may include one or more processors configured to control the operation of the power system  4 . Control of the power system  4  by the power system manager  22  is described in more detail later below, for example with reference to  FIG. 5 . 
     Apparatus, including the power system manager  22 , for implementing the above arrangement, and performing the method steps to be described later below, may be provided by configuring or adapting any suitable apparatus, for example one or more computers or other processing apparatus or processors, and/or providing additional modules. The apparatus may comprise a computer, a network of computers, or one or more processors, for implementing instructions and using data, including instructions and data in the form of a computer program or plurality of computer programs stored in or on a machine readable storage medium such as computer memory, a computer disk, ROM, PROM etc., or any combination of these or other storage media. 
       FIG. 3  is a schematic illustration (not to scale) showing further details of the battery module  16 . 
     In this embodiment, the battery module  16  comprises a battery  34 , a first state of charge module  36 , a second switch  38 , and a battery charger  40 . 
     The battery  34  may be any appropriate type of rechargeable battery, preferably a lithium polymer battery. In this embodiment, the battery  34  is configured to output a DC power supply having a constant voltage appropriate for the application, e.g. 24V. The battery  34  is coupled to the first state of charge module  36  such that the first state of charge module  36  may determine a state of charge of the battery  34 . The battery  34  is further coupled to the DC bus between the first wire  26  and the second wire  28  via the second switch  38 . 
     The first state of charge module  36  is configured to detect a state of charge of the battery  34 . In addition to being coupled to the battery  34 , the first state of charge module  36  is coupled to the power system manager  22  such that the battery state of charge information may be sent from the first state of charge module  36  to the power system manager  22 . The first state of charge module  36  may comprise a state of charge circuit integrated with the rechargeable battery  34 . 
     In this embodiment, the second switch  38  is single pole, triple throw (SPTT) switch (or equivalently, a single pole double throw (SPDT) switch having a stable off position, e.g. in the centre between its two throws). The second switch  38  is switchable between three states, each of which corresponds to a respective wiring path. In its first state, the second switch  38  directly electrically connects the battery  34  to the DC bus, i.e. between the first wire  26  and the second wire  28 . When the second switch  38  is in this first state, the battery  34  discharges, thereby providing DC electrical power to the DC bus. In its second state, the second switch  38  electrically connects the battery  34  to the DC bus via the battery charger  40 . When the second switch  38  is in this second state, the battery charger  40  charges the battery  34  using electrical power drawn from the DC bus. In its third state, the second switch  38  is open, and the battery  34  is disconnected from the DC bus. When the second switch  38  is in this third state, the battery  34  tends to substantially retain its charge, neither drawing power from nor supplying power to the DC bus. 
     The second switch  38  is a solid state switch. The second switch  38  is coupled to the power system manager  22  such that the power system manager  22  may control the second switch  38 . 
     The battery charger  40  may be any appropriate type of battery charger or recharger. The battery charger  40  is coupled to the DC bus between the first wire  26  and the second wire  28  such that the battery charger  40  may draw electrical power from the DC bus to charge the battery  34 . The battery charger  40  is configured to charge the battery  34  when the battery charger  40  is electrically coupled to the battery  34 , i.e. when the second switch  38  is in its second state. The battery charger  40  advantageously tends to avoid having to remove the battery  34  from the UAV  2  for charging. 
       FIG. 4  is a schematic illustration (not to scale) showing further details of the supercapacitor module  18 . 
     In this embodiment, the supercapacitor module  18  comprises a supercapacitor  44 , a second state of charge module  46 , a third switch  48 , and a resistor  50 . 
     The supercapacitor  44  (sometimes called an ultracapcitor) is a high-capacity electrochemical capacitor. The supercapacitor  44  is coupled to the second state of charge module  46  such that the second state of charge module  46  may determine a state of charge of the supercapacitor  44 . The supercapacitor  44  is further coupled to the DC bus between the first wire  26  and the second wire  28  via the third switch  48 . 
     The second state of charge module  46  is configured to detect a state of charge of the supercapacitor  44 . In addition to being coupled to the supercapacitor  44 , the second state of charge module  46  is coupled to the power system manager  22  such that supercapacitor state of charge information may be sent between from second state of charge module  46  to the power system manager  22 . 
     In this embodiment, the third switch  48  is single pole, triple throw (SPTT) switch (or equivalently, a single pole double throw (SPDT) switch having a stable off position, e.g. in the centre between its two throws). The third switch  48  is switchable between three states, each of which corresponds to a respective wiring path. In its first state, the third switch  48  directly electrically connects the supercapacitor  44  to the DC bus, i.e. between the first wire  26  and the second wire  28 . When the third switch  48  is in this first state, the supercapacitor  44  discharges, thereby providing DC electrical power to the DC bus. In its second state, the third switch  48  electrically connects the supercapacitor  44  to the DC bus via the resistor  50 . When the third switch  48  is in this second state, the supercapacitor  44  is charged from the DC bus via the resistor  50 . In its third state, the third switch  48  is open, and the supercapacitor  44  is disconnected from the DC bus. When the third switch  48  is in this third state, the supercapacitor  44  tends to substantially retain its charge, neither drawing power from nor supplying power to the DC bus. 
     The third switch  48  is a solid state switch. The third switch  48  is coupled to the power system manager  22  such that the power system manager may control the third switch  48 . 
     The resistor  50  may be any appropriate type of resistor having any appropriate resistance. The resistance of the resistor  50  may be selected based on a received charging current to be received by the supercapacitor  44 . Preferably, this charging current is within the optimum nominal discharge regions of the battery  34  and the fuel cell  10 . In this embodiment, the resistance of the resistor  50  is equal to the DC bus voltage divided by the charging current for the supercapacitor  44 . For example, if the charging current within the high nominal discharge region of the battery  34  and an optimum region of the fuel cell  10  is 10 A, and the DC bus voltage is 24V, the resistance of the resistor  50  is 24/10=2.4Ω. 
     In this embodiment, as described in more detail later below, the power management algorithm switches from the resistor charge path based on the supercapacitor state of charge. 
     In this embodiment, the resistor  50  is a heat dissipating ceramic resistor with a heat sink. 
     Advantageously, the resistor  50  tends facilitate a power demand of the supercapacitor  44  being met by the fuel cells  10  and/or battery  34 . For example, if the resistor  50  was not used when charging the supercapacitor  44 , the supercapacitor  44  would tend to experience an initial high charge current, for example, of 170 A. It is likely that the fuel cell  10  would not be able to meet this high power demand. Also, this will tend to cause the battery  34  to operate in a high loss region, which will tend to result in high battery capacity charge loss. Also, this will tend to have a detrimental effect on the battery state of health and also on the fuel cell  10 . These disadvantages tend to be at least partially overcome by use of the resistor  50 . 
     The resistor  50  is coupled between the DC bus and the supercapacitor  44  such that, when the third switch  48  is in its second state, the supercapacitor  44  charges through the resistor  50 . The resistor  50  advantageously tends to limit inrush current to the supercapacitor  44  from the DC bus when the supercapacitor  44  is charging. This limitation of the inrush current advantageously tends to prevent or oppose large drops in DC bus voltage, thereby improving the reliability of the power supply to the plurality of loads  20 . Also, the limitation of the inrush current advantageously tends to limit the current received by the supercapacitor  44  from the fuel cell  10  and/or the battery  34 . This tends to provide for reduced energy losses of the fuel cell  10  and/or battery  34 , and tends to prevent damage to the fuel cell  10  and/or battery  34 . 
     Thus, an embodiment of the power system  4  for the UAV  2  is provided. 
     What will now be described is the controlling of the various components and systems of the power system  4  by the power system manager  22 . 
     In some embodiments, the fuel cell  10  is treated as the primary power source in the power system  4 . In some embodiments in which the fuel cell  10  is the UAV&#39;s primary power source, the battery  34  is only used when current demand exceeds the maximum current that may be provided by the fuel cell  10 , and/or during high transient load. The fuel cell current limit may be set for high hydrogen utilisation. Also, the fuel cell controller  12  is configured for high performance and hydrogen utilisation. In some embodiments in which the fuel cell  10  is the UAV&#39;s primary power source, the supercapacitor  44  is connected in parallel with the fuel cell  10  to filter transient load power surges and to prevent degradation of fuel cell performance. Advantageously, it tends to be possible to land the UAV  2  using power from only the battery  34 , for example, if the fuel cell  10  were to fail. Preferably, the battery  34  is maintained at high state of charge in embodiments in which the fuel cell  10  is the UAV&#39;s primary power source. 
     In some embodiments, the battery  34  is treated as the primary power source in the power system  4 . In some embodiments in which the battery  34  is the UAV&#39;s primary power source, the fuel cell  10  is used only to recharge the battery  34  during light loads when the battery state of charge is below a threshold level (e.g. 60%), or when current demand exceeds the battery current nominal limit. Advantageously, this power supply strategy tends to provide for highly efficient charging of the battery  34  to prevent power losses during charging the battery  34  from the fuel cell  10 . This tends to be a particularly cost effective power supply strategy. Furthermore, lithium polymer batteries tend to occupy smaller volumes compared to fuel cells having the same power rating. Thus, space on board the UAV  2  is saved. Also, a relatively small fuel cell  10  can be used to directly charge the battery  34   
     In some embodiments, a load to source matching optimised strategy is implemented to control the power system  4 . In such embodiments, each power source on the UAV  2  is selected to supply a load demand that is specific to that type of power source. For example, the fuel cell  10  may be selected to supply steady loads, and the battery  34  may be selected to supply transient loads. In some such embodiments, the power system  4  may be controlled to maintain the battery state of charge is above a threshold level (e.g. 60%). In some embodiments in which the load to source matching optimised strategy is implemented, the supercapacitor  44  may be selected to supply peak transient loads. 
     In some embodiments, the power system manager  22  controls the power system  4  by implementing a plurality of rules. 
     In some embodiments, the power system manager  22  acquires, for example from the loads  20 , a value of the electrical current demanded by the loads  20  (hereinafter denoted I L ). In some embodiments, I L  may be expressed as a percentage of the maximum current that may be demanded by the loads  20 . The power system manager  22  may also acquire, for example from the fuel cell  10 , a value of the maximum current that the fuel cell  10  is able to supply to the DC bus (hereinafter denoted I F ). The power system manager  22  may also acquire, for example from the battery module  16 , a value of the maximum current that the battery  34  is able to supply to the DC bus (hereinafter denoted I B ). In some embodiments, the power system manager  22  controls the battery module  16  using the following rules: 
     i) If I L &lt;I F  then the power system manager  22  controls the second switch  38  such that the battery  34  is charged by the battery charger  40 , i.e. the second switch  38  is switched into its second state. In this embodiment, if I L &lt;I F  then the load state of the power system  4  is said to be “LIGHT”. 
     ii) If I F ≤I L &lt;I B  then the power system manager  22  controls the second switch  38  such that the battery  34  discharges to the DC bus, i.e. the second switch  38  is switched into its first state. In this embodiment, if I F ≤I L &lt;I B  then the load state of the power system  4  is said to be “NORMAL”. 
     iii) If I B ≤I L &lt;I F +I B  then the power system manager  22  controls the second switch  38  such that the battery  34  discharges to the DC bus, i.e. the second switch  38  is switched into its first state. In this embodiment, if I B ≤I L &lt;I F +I B  then the load state of the power system  4  is said to be “HEAVY”. 
     iv) If I L ≤I F +I B  then the power system manager  22  controls the second switch  38  such that the battery  34  discharges to the DC bus, i.e. the second switch  38  is switched into its first state. In this embodiment, if I L ≤I F +I B  then the load state of the power system  4  is said to be “PEAK”. 
     The load management strategy advantageously tends to charge the battery  34  during LIGHT load. Advantageously, the power system manager  22  may manage the load demand of the system by load shedding during peak loads e.g. by terminating the charging process. 
     In some embodiments, the power system manager  22  acquires a measurement of the state of charge of the battery  34  from the first state of charge module  36 . The battery state of charge measurement taken by the first state of charge module  36  is hereinafter denoted a S B . The battery state of charge measurement may be a percentage charge of the battery  34 . The power system manager  22  may control the battery module  16  using the battery state of charge measurement using the following rules: 
     v) If S B ≥90% then the power system manager  22  controls the second switch  38  such that the battery  34  discharges to the DC bus, i.e. the second switch  38  is switched into its first state. 
     Alternatively, in some embodiments, if S B ≥90% then the power system manager  22  controls the second switch  38  such that either the battery  34  discharges to the DC bus or is disconnected from the DC bus. In some embodiments, if S B ≥90% and the load state of the power system  4  is not LIGHT, the battery  34  discharges to the DC bus (i.e. the second switch  38  is switched to its first state). In some embodiments, if S B ≥90% and the load state of the power system  4  is LIGHT, the battery  34  is disconnected from the DC bus (i.e. the second switch  38  is switched to its third state). 
     In this embodiment, if S B ≥90% then the state of charge of the battery  34  is said to be “MAXIMUM”. 
     vi) If 80%≤S B &lt;90% then the power system manager  22  controls the second switch  38  such that either the battery  34  discharges to the DC bus or the battery  34  is disconnected from the DC bus i.e. the second switch  38  is switched into either its first state or third state. 
     In some embodiments, if 80%≤S B &lt;90% and the load state of the power system  4  is not LIGHT, the battery  34  discharges to the DC bus (i.e. the second switch  38  is switched to its first state). In some embodiments, if 80%≤S B &lt;90% and the load state of the power system  4  is LIGHT, the battery  34  is disconnected from the DC bus (i.e. the second switch  38  is switched to its third state). 
     In this embodiment, if 80%≤S B &lt;90% then the state of charge of the battery  34  is said to be “HIGH”. 
     vii) If 60%≤S B &lt;80% then the power system manager  22  controls the second switch  38  such that either the battery  34  discharges to the DC bus or the battery  34  is charged by the battery charger  40 , i.e. the second switch  38  is switched into either its first state or second state. 
     In some embodiments, if 60%≤S B &lt;80% and the load state of the power system  4  is not LIGHT, the battery  34  discharges to the DC bus (i.e. the second switch  38  is switched to its first state). In some embodiments, if 60%≤S B &lt;80% and the load state of the power system  4  is LIGHT, the battery  34  is charged by the battery charger  40  (i.e. the second switch  38  is switched to its second state). 
     In this embodiment, if 60%≤S B &lt;80% then the state of charge of the battery  34  is said to be “NORMAL”. 
     viii) If 50%≤S B &lt;60% then the power system manager  22  controls the second switch  38  such that either the battery  34  discharges to the DC bus or the battery  34  is charged by the battery charger  40 , i.e. the second switch  38  is switched into either its first state or second state. 
     In some embodiments, if 50%≤S B &lt;60% and the load state of the power system  4  is HEAVY or PEAK, the battery  34  discharges to the DC bus (i.e. the second switch  38  is switched to its first state). In some embodiments, if 50%≤S B &lt;60% and the load state of the power system  4  is LIGHT or NORMAL, the battery  34  is charged by the battery charger  40  (i.e. the second switch  38  is switched to its second state). 
     In this embodiment, if 50%≤S B &lt;60% then the state of charge of the battery  34  is said to be “LOW”. 
     ix) If S B &lt;50% then the power system manager  22  controls the second switch  38  such that either the battery  34  is disconnected from the DC bus or the battery  34  is charged by the battery charger  40 , i.e. the second switch  38  is switched into either its third state or second state. 
     In some embodiments, if S B &lt;50% and the load state of the power system  4  is LIGHT, the battery  34  is charged by the battery charger  40  (i.e. the second switch  38  is switched to its second state). In some embodiments, if S B &lt;50% and the load state of the power system  4  is not LIGHT, the power system manager  22  sheds loads from the DC bus, e.g. by turning off one or more of the loads  20  accordingly to a priority hierarchy. If after load shedding the load state of the power system  4  become LIGHT, the battery  34  is charged by the battery charger  40 . However, if the load state of the power system  4  remains not LIGHT, the battery  34  is disconnected from the DC bus (i.e. the second switch  38  is switched to its third state). 
     In this embodiment, if S B &lt;50% then the state of charge of the battery  34  is said to be “MINIMUM”. 
     Preferably, the power system manager  22  controls the battery module  16  such that the state of charge of the battery  34 , S B , does not fall below a threshold minimum level. 
     Preferably, the power system manager  22  controls the battery module  16  such that a battery maximum current limit is not exceeded. 
     In some embodiments, the power system manager  22  acquires a measurement of the state of charge of the supercapacitor  44  from the second state of charge module  46 . The supercapacitor state of charge measurement taken by the second state of charge module  46  is hereinafter denoted a S S . The supercapacitor state of charge measurement may be a percentage charge of the supercapacitor  44 . The power system manager  22  may control the supercapacitor module  18  using the supercapacitor state of charge measurement using the following rules: 
     x) If S S≥ 90% then the power system manager  22  controls the third switch  48  such that the supercapacitor  44  discharges to the DC bus, i.e. the third switch  38  is switched into its first state. 
     In some embodiments, if S S≥ 90% then the power system manager  22  controls the third switch  48  such that either the supercapacitor  44  discharges to the DC bus or is disconnected from the DC bus. In some embodiments, if S S ≥90% and I L ≥30 A (or some other threshold value which may be equal to the current limit of the fuel cell  10 ), the supercapacitor  44  discharges to the DC bus (i.e. the third switch  48  is switched to its first state). In some embodiments, if S S ≥90% and I L &lt;30 A (or some other threshold value), the supercapacitor  44  is disconnected from the DC bus (i.e. the third switch  48  is switched to its third state). 
     In this embodiment, if S S≥ 90% then the state of charge of the supercapacitor  44  is said to be “MAXIMUM”. 
     xi) If 70%≤S S &lt;90% then the power system manager  22  controls the third switch  48  such that either the supercapacitor  44  discharges to the DC bus or the supercapacitor  44  is charged from the DC bus via the resistor  50 , i.e. the third switch  48  is switched into either its first state or second state. In some embodiments, if 70%≤S S &lt;90% and I L ≤30 A (or some other threshold value), the supercapacitor  44  discharges to the DC bus (i.e. the third switch  38  is switched to its first state). In some embodiments, if 70%≤S S &lt;90% and I L ≤30 A (or some other threshold value), the supercapacitor  44  is charged from the DC bus (i.e. the third switch  38  is switched to its second state). In this embodiment, if 70%≤S S &lt;90% then the state of charge of the supercapacitor  44  is said to be “NORMAL”. 
     xii) If 60%≤S S &lt;70% then the power system manager  22  controls the third switch  48  such that either the supercapacitor  44  discharges to the DC bus or the supercapacitor  44  is charged from the DC bus via the resistor  50 , i.e. the third switch  48  is switched into either its first state or second state. In some embodiments, if 60%≤S S &lt;70% and I L ≤30 A (or some other threshold value), the supercapacitor  44  is charged from the DC bus (i.e. the third switch  38  is switched to its second state). In some embodiments, if 60%≤S S &lt;70% and S B  is NORMAL or above and I L  is lower than HEAVY, the supercapacitor  44  is charged from the DC bus via the resistor  50  (i.e. the third switch  38  is switched to its second state). In some embodiments, if 70%≤S S &lt;90% and I L &lt;30 A (or some other threshold value), the supercapacitor  44  is discharged to the DC bus (i.e. the third switch  38  is switched to its first state). In this embodiment, if 60% S S &lt;70% then the state of charge of the supercapacitor  44  is said to be “LOW”. 
     xiii) If S S &lt;60% then the power system manager  22  controls the third switch  48  such that either the supercapacitor  44  is either disconnected from the DC bus or the supercapacitor  44  is charged from the DC bus via the resistor  50 , i.e. the third switch  48  is switched into either its third state or second state. In some embodiments, if S S &lt;60% and I L ≥30 A (or some other threshold value), or I L  is above HEAVY, the supercapacitor  44  is disconnected from the DC bus (i.e. the third switch  38  is switched to its third state). In some embodiments, if S S &lt;60% and S B  is NORMAL or above and I L  is lower than HEAVY, the supercapacitor  44  is charged from the DC bus via the resistor  50  (i.e. the third switch  38  is switched to its second state). In some embodiments, if S S &lt;60% and I L &lt;30 A (or some other threshold value), the supercapacitor  44  is charged from the DC bus via the resistor  50  (i.e. the third switch  38  is switched to its second state). In this embodiment, if S S &lt;60% then the state of charge of the supercapacitor  44  is said to be “MINIMUM”. 
     In some embodiments, the power system manager  22  acquires a measurement of the fuel level of the fuel cell  10 , for example from the fuel cell  10  or the fuel cell controller  12 . The fuel cell fuel level may be a measurement of the amount of hydrogen fuel remaining for the fuel cell  10 . The fuel cell fuel level may be expressed as a percentage of the total fuel cell fuel capacity on the UAV  2 . The fuel cell fuel level may be through of as a “state of charge” of the fuel cell  10  and is hereinafter denoted as S F . The power system manager  22  may control, via the fuel cell controller  12 , operation of the fuel cell  10  using the following rules: 
     xiv) If S F ≥90% then the power system manager  22  controls fuel cell  10  to be either ON (and supplying power to the DC bus) or OFF (and not supplying power to the DC bus). In some embodiments, if S F ≥90% and I L &gt;0 A (or some other threshold value), the fuel cell  10  is turned ON. In some embodiments, if S F ≥90% and I L ≤0 A (or some other threshold value), the fuel cell  10  is turned OFF. In this embodiment, if S F ≥90% then the fuel level for the fuel cell  10  is said to be “MAXIMUM”. 
     xv) If 40%≤S F &lt;90% then the power system manager  22  controls the fuel cell  10  to be either ON or OFF. In some embodiments, if 40%≤S F &lt;90% and I L &gt;0 A (or some other threshold value), the fuel cell  10  is turned ON. In some embodiments, if 40%≤S F &lt;90% and I L  0 A (or some other threshold value), the fuel cell  10  is turned OFF. In this embodiment, if 40%≤S F &lt;90% then the fuel level for the fuel cell  10  is said to be “NORMAL”. 
     xvi) If 20%≤S F &lt;40% then the power system manager  22  controls the fuel cell  10  to be either ON or OFF. In some embodiments, if 20%≤S F &lt;40% and I L &gt;0 A (or some other threshold value), the fuel cell  10  is turned ON. In some embodiments, if 20%≤S F &lt;40% and I L ≤0 A (or some other threshold value), the fuel cell  10  is turned OFF. In this embodiment, if 20%≤S F &lt;40% then the fuel level for the fuel cell  10  is said to be “LOW”. 
     xvii) If S F &lt;20% then the power system manager  22  controls the fuel cell  10  to be either ON or OFF. In some embodiments, if S F &lt;20% and I L &gt;60 A (or some other threshold value), the fuel cell  10  is turned ON. In some embodiments, if S F &lt;20% and I L ≤60 A (or some other threshold value), the fuel cell  10  is turned OFF. In this embodiment, if S F &lt;20% then the fuel level for the fuel cell  10  is said to be “MINIMUM”. 
     In some embodiments, if the battery  34  is being charged by the battery charger  40 , the power system manager  22  controls the fuel cell  10  via the controller  12  such that the fuel cell is ON and supplying power to the DC bus. 
     In some embodiments, if the fuel level for the fuel cell  10  is below LOW, i.e. is MINIMUM, the power system manager  22  controls the fuel cell  10  via the controller  12  such that the fuel cell is OFF and not supplying power to the DC bus. 
     Preferably, the power system manager  22  controls the fuel cell  10  such that the fuel cell current maximum limit is not exceeded. 
       FIG. 5  is a process flow chart showing certain steps of an embodiment of a method of operation of the power system  4 . 
     It should be noted that certain of the process steps depicted in the flowchart of  FIG. 5  and described below may be omitted or such process steps may be performed in differing order to that presented above and shown in  FIG. 5 . Furthermore, although all the process steps have, for convenience and ease of understanding, been depicted as discrete temporally-sequential steps, nevertheless some of the process steps may in fact be performed simultaneously or at least overlapping to some extent temporally. 
     Some or all of the above mentioned rules may be implemented in the embodiment illustrated in  FIG. 5  and described in more detail below. Similarly, process steps depicted in the flowchart of  FIG. 5  and described below may be used to implement some or all of the above described rules. 
     At step s 2 , the power system manager  22  acquires a measurement of I L , for example, from the plurality of loads  20  and/or the DC bus. 
     At step s 4 , the power system manager  22  acquires a measurement of S F , for example, from the fuel cell  10  or the fuel cell controller  12 . 
     At step s 6 , the power system manager  22  acquires a measurement of S B , for example, from the first state of charge module  36 . 
     At step s 8 , the power system manager  22  acquires a measurement of S S , for example, from the second state of charge module  46 . 
     At step s 10 , the power system manager  22  determines whether or not I L &gt;0. 
     If at step s 10 , it is determined that I L &lt;0, the method of  FIG. 5  ends. The method may be restarted at a later time. 
     However, if at step s 10 , it is determined that I L &gt;0, the method proceeds to step s 12 . 
     At step s 12 , the power system manager  22  determines whether or not I L ≤I F . 
     If at step s 12 , it is determined that I L ≤I F , the method proceeds to step s 14 . 
     However, if at step s 12 , it is determined that I L &gt;I F , the method proceeds to step s 34 . Step s 34  will be described in more detail later below after a description of steps s 14  to s 32 . 
     At step s 14 , the power system manager  22  determines whether or not S F &gt;20%. 
     If at step s 14 , it is determined that S F &gt;20%, the method proceeds to both step s 16  and s 20 . 
     However, if at step s 14 , it is determined that S F ≤20%, the method proceeds to both steps s 18  and s 20 . Step s 20  will be described in more detail later below after a description of steps s 16  and s 18 . 
     At step s 16 , responsive to determining that S F &gt;20% at step s 14 , the power system manager  22  controls the power system  4  to operate in “fuel cell mode”. In this embodiment, in fuel cell mode, the power system manager  22  controls the fuel cell  10  via the fuel cell controller  12  to be ON and supplying power to the DC bus. In some embodiments, in fuel cell mode, the battery  34  and/or the supercapacitor  44  may be disconnected from the DC bus or being charged. In fuel cell mode, at least the fuel cell  10  tends to satisfy the current demands of the loads  20 . After step s 16 , the method proceeds back to step s 2 . 
     At step s 18 , responsive to determining that S F ≤20% at step s 14 , the power system manager  22  generates a warning that the fuel level of the fuel cell  10  is low. This warning may be stored or logged on board the UAV  2  and/or may be transmitted to an entity remote from the UAV  2 , for example a human controller or owner of the UAV  2 . 
     At step s 20 , the power system manager  22  determines whether or not S B &lt;50%. 
     If at step s 20 , it is determined that S B &lt;50%, the method proceeds to both step s 22  and s 24 . 
     However, if at step s 20 , it is determined that S B ≥50%, the method proceeds to both steps s 26  and s 28 . Steps s 26  and s 28  will be described in more detail later below after a description of steps s 22  and s 24 . 
     At step s 22 , responsive to determining that S B &lt;50% at step s 20 , the power system manager  22  generates a warning that the battery state of charge is low. This warning may be stored or logged on board the UAV  2  and/or may be transmitted to an entity remote from the UAV  2 , for example a human controller or owner of the UAV  2 . 
     At step s 24 , responsive to determining that S B &lt;50% at step s 20 , the power system manager  22  controls the battery module  16  such that the battery  34  is charged by the battery charger  40 . In this embodiment, the power system manager  22  controls the second switch  38  to switch to its second state such that the battery charger  40  charges the battery  34 . 
     Returning now to the case where, at step s 20 , it is determined that S B  50%, at step s 26  the power system manager  22  controls the power system  4  to operate in “battery mode”. In this embodiment, in battery mode, the power system manager  22  controls the battery module  16  such that the battery  34  supplies power to the DC bus. In this embodiment, the power system manager  22  controls the second switch  38  to switch to its first state such that the battery  34  discharges to the DC bus. In some embodiments, in battery mode, the supercapacitor  44  may be disconnected from the DC bus or being charged. In battery mode, at least the battery  34  (and in some embodiments the fuel cell  10 ) tends to satisfy the current demands of the loads  20 . In some embodiments, in battery mode, the fuel cell  10  may be disconnected from the DC bus e.g. via the first switch  30 , and/or the fuel cell  10  may be turned off. After step s 26 , the method proceeds back to step s 2 . 
     At step s 28 , responsive to determining that S B ≥50% at step s 20 , the power system manager  22  determines whether or not S S &lt;60%. 
     If at step s 28 , it is determined that S S &lt;60%, the method proceeds to step s 30 . 
     However, if at step s 28 , it is determined that S S ≥60%, the method proceeds to step s 32 . 
     At step s 30 , responsive to determining that S S &lt;60% at step s 28 , the power system manager  22  controls the supercapacitor module  18  such that the supercapacitor  44  is charged from the DC bus via the resistor  50 . In this embodiment, the power system manager  22  controls the third switch  48  to switch to its second state such that the supercapacitor  44  is charged. In some embodiments, the power system manager  22  may also generate a warning that the state of charge of the supercapacitor  44  is low. This warning may be stored or logged on board the UAV  2  and/or may be transmitted to an entity remote from the UAV  2 , for example a human controller or owner of the UAV  2 . After step s 30 , the method proceeds back to step s 2 . 
     At step s 32 , responsive to determining that S S ≥60% at step s 28 , the power system manager  22  controls the power system  4  to operate in “supercapacitor mode”. In this embodiment, in supercapacitor mode, the power system manager  22  controls the supercapacitor module  18  such that the supercapacitor  44  supplies power to the DC bus. In this embodiment, the power system manager  22  controls the third switch  48  to switch to its first state such that the supercapacitor  44  discharges to the DC bus. In some embodiments, in supercapacitor mode, the battery  34  may be disconnected from the DC bus or being charged. In supercapacitor mode, at least the supercapacitor  44  (and in some embodiments the fuel cell  10 ) tends to satisfy the current demands of the loads  20 . In some embodiments, in supercapacitor mode, the fuel cell  10  may be disconnected from the DC bus e.g. via the first switch  30 , and/or the fuel cell  10  may be turned off. After step s 32 , the method proceeds back to step s 2 . 
     Returning now to the case where at step s 12  it is determined that I L &gt;I F , at step s 34  the power system manager  22  determines whether or not I F &lt;I L ≤I B . 
     If at step s 34 , it is determined that I F &lt;I L ≤I B , the method proceeds to step s 36 . 
     However, if at step s 34 , it is determined that I L &gt;I B , the method proceeds to step s 44 . Step s 44  will be described in more detail later below after a description of steps s 36  to s 42 . 
     At step s 36 , the power system manager  22  determines whether or not S B &lt;50%. 
     If at step s 36 , it is determined that S B ≥50%, the method proceeds to step s 26 , which is described in more detail earlier above. 
     However, if at step s 36 , it is determined that S B &lt;50%, the method proceeds to both step s 38  and s 40 . 
     At step s 38 , the power system manager  22  sheds loads from the DC bus, e.g. by turning off one or more of the loads  20  accordingly to a priority hierarchy. In this embodiment, non-essential loads are shed. After step s 38 , the method proceeds back to step s 2 . 
     Responsive to determined that S B &lt;50% at step s 36 , the method proceeds to both step s 26  and s 40 . 
     Step s 26  is described in more detail earlier above. 
     At step s 40 , the power system manager  22  determines whether or not S S &lt;60%. 
     If at step s 40 , it is determined that S S ≥60%, the method proceeds to step s 32 . Step s 32  is described in more detail earlier above. 
     However, if at step s 40 , it is determined that S S &lt;60%, the method proceeds to step s 42 . 
     At step s 42 , responsive to determining that S S &lt;60% at step s 40 , the power system manager  22  controls the supercapacitor module  18  such that the supercapacitor  44  is disconnected from the DC bus. In this embodiment, the power system manager  22  controls the third switch  48  to switch to its third state thereby disconnecting the supercapacitor  44  from the DC bus. After step s 42 , the method proceeds back to step s 2 . 
     Returning now to the case where at step s 34  it is determined that I L &gt;I B , at step s 44  the power system manager  22  determines whether or not I B &lt;I L ≤I B +I F . 
     If at step s 44 , it is determined that I B &lt;I L ≤I B +I F , the method proceeds to step s 46 . 
     However, if at step s 44 , it is determined that the condition that I B &lt;I L ≤I B +I F  is not satisfied, the method proceeds to step s 58 . Step s 58  will be described in more detail later below after a description of steps s 46  to s 56 . 
     At step s 46 , the power system manager  22  determines whether or not S B &gt;50%. 
     If at step s 46 , it is determined that S B &gt;50%, the method proceeds to both steps s 48  and s 50 . 
     However, if at step s 46 , it is determined that S B ≤50%, the method proceeds to step s 56 . Step s 56  will be described in more detail later below after a description of steps s 48  to s 54 . 
     At step s 48 , responsive to determining that S B &gt;50% at step s 46 , the power system manager  22  controls the power system  4  to operate in “fuel cell and battery mode”. In this embodiment, in fuel cell and battery mode, the power system manager  22  controls the fuel cell  10  via the fuel cell controller  12  to be ON and supplying power to the DC bus, and also the power system manager  22  controls the battery module  16  such that the battery  34  supplies power to the DC bus. In this embodiment, the power system manager  22  controls the second switch  38  to switch to its first state such that the battery  34  discharges to the DC bus. In some embodiments, in fuel cell and battery mode, the supercapacitor  44  may be disconnected from the DC bus or being charged. In fuel cell mode, at least the fuel cell  10  and the battery  34  tend to satisfy the current demands of the loads  20 . After step s 48 , the method proceeds back to step s 2 . 
     At step s 50 , responsive to determining that S B &gt;50% at step s 46 , the power system manager  22  determines whether or not S S &lt;60%. 
     If at step s 50 , it is determined that S S ≥60%, the method proceeds to step s 52 . 
     However, if at step s 50 , it is determined that S S &lt;60%, the method proceeds to step s 54 . 
     At step s 52 , responsive to determining that S S ≥60% at step s 50 , the power system manager  22  controls the power system  4  to operate in “supercapacitor mode”. In this embodiment, in supercapacitor mode, the power system manager  22  controls the supercapacitor module  18  such that the supercapacitor  44  supplies power to the DC bus. In this embodiment, the power system manager  22  controls the third switch  48  to switch to its first state such that the supercapacitor  44  discharges to the DC bus. In some embodiments, in supercapacitor mode, the battery  34  may be disconnected from the DC bus or being charged. In supercapacitor mode, at least the supercapacitor  44  (and in some embodiments the fuel cell  10 ) tends to satisfy the current demands of the loads  20 . After step s 52 , the method proceeds back to step s 2 . 
     Advantageously, connecting the supercapacitor  44  to the DC bus tends to filter transient or spikes in the DC bus that may be caused by switching or other transient loads. The relatively high power density characteristics of the supercapacitor  44  tend to be effectively utilised for this effect. This advantageously tends to avoid wasting capacitor charge. 
     At step s 54 , responsive to determining that S S &lt;60% at step s 50 , the power system manager  22  controls the supercapacitor module  18  such that the supercapacitor  44  is disconnected from the DC bus. In this embodiment, the power system manager  22  controls the third switch  48  to switch to its third state thereby disconnecting the supercapacitor  44  from the DC bus. After step s 54 , the method proceeds back to step s 2 . 
     At this step the load on the DC bus is in the HEAVY state. Also, the supercapacitor state of charge is MINIMUM. This means that the voltage across the supercapacitor  44  tends to be below the DC bus voltage. Thus, by disconnecting the super capacitor  44  from the DC bus, the supercapacitor  44  drawing more current from the battery  34  and the fuel cell  10  tends to be avoided. This tends to avoid the battery  34  and the fuel cell  10  operating in regions of high losses. 
     Returning to the case where at step s 46  it is determined that S B ≤50%, at step s 56 , the power system manager  22  sheds loads from the DC bus, e.g. by turning off one or more of the loads  20  accordingly to a priority hierarchy. In this embodiment, non-essential loads are shed. After step s 56 , the method proceeds back to step s 2 . 
     Returning to the case where at step s 44  it is determined that the condition that I B &lt;I L ≤I B +I F  is not satisfied, at step s 58  the power system manager  22  determines whether or not I L &gt;I B +I F . 
     If at step s 58 , it is determined that I L &gt;I B +I F , the method proceeds to step s 56 . Step s 56  is described in more detail earlier above. 
     However, if at step s 58 , it is determined that the condition that I L &gt;I B +I F  is not satisfied, the method proceeds back to step s 2 . 
     Thus, an embodiment of a method of operation of the power system  4  is provided. 
     The above described system and methods tend to improve UAV flight endurance. 
     The above described system and methods tend to optimise use of a fuel cell, a battery, and a supercapacitor. The fuel cell, battery, and/or supercapacitor tend to be constrained to operate in regions of lower losses by sharing the load power demand. 
     Advantageously, the above described system and methods tend to ensure that the power supply unit on the UAV (i.e. the fuel cell, the battery, and the supercapacitor) meets the load demand from the loads, and that the load does not exceed the overall system limit. 
     Typically, the rate of charge and discharge of a lithium, polymer battery is characterised by a “C rating”. Discharging the battery at high C rating tends to reduce the normal operation region of the battery due to exponential losses and capacity losses. Advantageously, the above described battery management processes tend to prevent the battery from operating in regions of high energy losses. The state of charge of the battery is monitored to determine the battery state of operation based on the state of load. 
     Advantageously, the above described methods and apparatus tend to ensure that battery state of heath is maintained by preventing over charging and over discharging of the battery, for example, beyond manufacture recommendations. The above described constraints tend to prevent battery capacity losses due to high current discharge rate. Also, the above constraints tend to prevent or oppose the fuel cell operating in high power loss regions. 
     Advantageously, the above described system and method may be implemented with either a constant load or a variable load profile. 
     Advantageously, the above described system and method tends to takes into account the states of all power sources and loads on the UAV. 
     Many conventional power management systems and algorithms for a fuel cell hybrid system require the use of multiple DC/DC converters. For example, some conventional systems couple a respective DC-DC converter to each of a fuel cell, a battery, and/or a supercapacitor. 
     Advantageously, the above described system and method tends to avoid the use of multiple DC-DC converters. Thus, the weight of the UAV tends to be reduced. In the above described system, only a single DC-DC converter is used to maintain DC bus voltage, by preventing voltage fluctuations caused by fuel cell voltage output variation due to variation in load current demand. The battery and the supercapacitor modules have substantially the same voltage ratings as the DC-DC converter voltage output. 
     Many conventional power management systems and algorithms use multiple DC-DC converters to execute controller commands. In contrast, the above described methods and apparatus use solid state switches to execute the power management control commands. Inputs to the power system manager for controlling the power system include the battery state of charge, the supercapacitor state of charge, the fuel cell state of charge, and a current load demand. 
     Advantageously, the above described system and methods tend to constrain the fuel cell to operate in regions of higher efficiency to reduce or prevent regions of high Ohmic losses and avoid concentration losses. 
     Furthermore, current spike demands from both the battery and the fuel cell tend to be limited to reduce losses. Both of these power sources can share the load to ensure that they both operate in high optimal regions when load current demand is higher than the optimal operating region of either of these power sources. 
     Advantageously, the supercapacitor tends to filter “voltage ripples” during switching between the fuel cell and the battery. Also, the supercapacitor tends to protect the fuel cell from surges in current demand. The supercapacitor also tends to reduce voltage ripples caused by operation of the DC/DC converter. 
     Advantageously, the states of charge of the battery and supercapacitor may be considered by the control algorithm to prevent battery over discharge and battery overcharge. Also, the state of charge of the supercapacitor tends to be maintained within nominal working voltage to prevent high inrush current during supercapacitor charging. A high inrush current could cause the battery or fuel cell to operate out of the optimal operating regions. 
     In the above embodiments, the power system is implemented on board a UAV. However, in other embodiments, the power system is implemented on a different entity other than a UAV, for example, a different type of vehicle such as a manned aircraft, a land-based vehicle, or a water-based vehicle. 
     In the above embodiments, the power system comprises a single fuel cell, a single battery, and a single supercapacitor. However, in other embodiments, the power system comprises multiple fuel cells. In some embodiments, the power system comprises multiple batteries (e.g. multiple batteries that are substantially identical to each other). In some embodiments, the power system comprises multiple supercapacitors. In some embodiments, one or more of the supercapacitors is replaced by a further battery. 
     In the above embodiments, the fuel cell is a PEM fuel cell. However, in other embodiments, the fuel cell is a different type of fuel cell, for example a phosphoric acid fuel cell. 
     In the above embodiments, the fuel cell is arranged to provide electrical power to the DC bus. However, in some embodiments, the fuel cell is arranged only to charge the one or more batteries and/or the one or more supercapacitors. 
     In the above embodiments, the converter is a DC-DC converter configured to output to the DC bus a constant voltage power supply having a voltage of 24V. However, in other embodiments, the converter may be a different type of converter and may be configured to provide a different output to the DC bus, e.g. a constant voltage power supply having a voltage other than 24V. In the above embodiments, the battery is a lithium polymer battery. However, in other embodiments, the battery is a different type of battery, or other rechargeable store of electrical power. 
     In the above embodiments, the battery is configured to provide constant current output of 24V. However, in other embodiments, the battery is configured to produce a different output. 
     In the above embodiments, the second and third switch are solid state SPTT switches. Also, the first switch is a solid state switch. However, in other embodiments, one or more of the switches may be a different type of switch that provides the above described functionality. For example, in some embodiments, one or more of the switches is a switch system comprising multiple individual switches. 
     In some above embodiments, the power system manager controls the power system by implementing some or all of the above described rules. However, in other embodiments, one or more different rules may be implemented instead of or in addition to one or more of the above described rules. 
     In some above embodiments, the rules implemented by the power system manager to control the power system include the above described Boolean conditions. However, in other embodiments, the Boolean condition of one or more of the rules is different to that described above. For example, in the above embodiments, certain rules include a Boolean condition specifying a threshold for a state of charge of the battery. However, in other embodiments, a different threshold for the state of charge of the battery may be applied.