Abstract:
The disclosed embodiments relate to the design of a fuel cell system which is capable of both providing power to and receiving power from a rechargeable battery in a portable computing device. This eliminates the need for a bulky and heavy battery within the fuel cell system, which can significantly reduce the size, weight and cost of the fuel cell system. This fuel cell system includes a fuel cell stack which converts fuel into electrical power. It also includes a controller which controls operation of the fuel cell system. The fuel cell system additionally includes a power link that transfers electrical power between the fuel cell system and the portable computing device, and a communication link that provides communication between the portable computing device and the controller for the fuel cell system. The controller can regulate both the electrical power provided by the fuel cell system to the portable computing device and the electrical power provided by the rechargeable battery to the fuel cell system.

Description:
RELATED APPLICATION 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/849,558, Attorney Docket Number APL-P9535US1, entitled “Fuel Cell System to Power a Portable Computing Device,” by inventors Bradley L. Spare, Vijay M. Iyer, Jean L. Lee, Gregory L. Tice, Michael D. Hillman and David I. Simon, filed Aug. 3, 2010, and also claims the benefit of U.S. Provisional Application No. 61/355,393, Attorney Docket Number APL-P9535USP1, entitled “Portable Hydrogen Fuel Cell System,” by inventors Bradley L. Spare, Vijay M. Iyer, Jean L. Lee, Gregory L. Tice, Michael D. Hillman and David I. Simon, filed Jun. 16, 2010. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The disclosed embodiments generally relate to systems that use fuel cells to provide electrical power. More specifically, the disclosed embodiments relate to a fuel cell system which is designed to both provide electrical power to and receive electrical power from a portable computing device. 
         [0004]    2. Related Art 
         [0005]    Our country&#39;s continuing reliance on fossil fuels has forced our government to maintain complicated political and military relationships with unstable governments in the Middle East, and has also exposed our coastlines and our citizens to the associated hazards of offshore drilling. These problems have led to an increasing awareness and desire on the part of consumers to promote and use renewable energy sources. For example, the Electronic Product 
         [0006]    Environmental Assessment Tool (EPEAT) is presently used to produce data that helps consumers evaluate the environmental friendliness of electronic products. Moreover, the EPEAT score for an electronic product can be increased by providing a renewable energy source for the product. 
         [0007]    As a consequence of this increased consumer awareness, electronics manufacturers have become very interested in developing renewable energy sources for their products, and they have been exploring a number of promising renewable energy sources such as the hydrogen fuel which is used in hydrogen fuel cells. Hydrogen fuel cells have a number of advantages. Such fuel cells and associated fuels can potentially achieve high volumetric and gravimetric energy densities, which can potentially enable continued operation of portable electronic devices for days or even weeks without refueling. However, it is extremely challenging to design hydrogen fuel cell systems which are sufficiently portable and cost-effective to be used with portable electronic devices. 
       SUMMARY 
       [0008]    The disclosed embodiments relate to the design of a fuel cell system which is capable of both providing power to and receiving power from a rechargeable battery in a portable computing device. This eliminates the need for a bulky and heavy battery within the fuel cell system, which can significantly reduce the size, weight and cost of the fuel cell system. This fuel cell system includes a fuel cell stack which converts fuel into electrical power. It also includes a controller which controls operation of the fuel cell system. The fuel cell system additionally includes a power link that transfers electrical power between the fuel cell system and the portable computing device, and a communication link that provides communication between the portable computing device and the controller for the fuel cell system. The controller can regulate both the electrical power provided by the fuel cell system to the portable computing device and the electrical power provided by the rechargeable battery to the fuel cell system. 
         [0009]    In some embodiments, while regulating the electrical power provided by the rechargeable battery to the fuel cell system, the controller can monitor an operational parameter of the fuel cell stack during a boot-up process of the fuel cell system, and subsequently regulate a discharging current provided by the rechargeable battery to the fuel cell stack based on the value of the operational parameter. 
         [0010]    In some embodiments, while regulating the discharging current based on the value of the operational parameter, the controller determines if the value of the operational parameter is less than a set-point value. If so, the controller maintains the discharging current provided by the rechargeable battery to the fuel cell stack. Otherwise, the controller terminates the discharging current provided by the rechargeable battery to the fuel cell stack. 
         [0011]    In some embodiments, the operational parameter is a hydrogen pressure within the fuel cell stack. 
         [0012]    In some embodiments, the fuel cell system does not have an internal rechargeable battery. 
         [0013]    In some embodiments, while regulating the electrical power provided by the fuel cell system to the portable computing device, the controller can monitor an operational parameter of the fuel cell stack and regulate a charging current provided by the fuel cell stack to the portable computing device based on the value of the operational parameter. 
         [0014]    In some embodiments, while regulating the charging current based on the value of the operational parameter, the controller can communicate with a charging circuit in the portable computing device, wherein the charging circuit is configured to charge the rechargeable battery based on the charging current. 
         [0015]    In some embodiments, while regulating the charging current based on the value of the operational parameter, the controller first determines whether the value of the operational parameter is greater than or smaller than a set-point value. If the value of the operational parameter is greater than the set-point value, the controller regulates the charging current so that the value of the operational parameter decreases. If the value of the operational parameter is smaller than the set-point value, the controller regulates the charging current so that the value of the operational parameter increases. 
         [0016]    In some embodiments, if the value of the operational parameter is greater than the set-point value, the controller can continue regulating the charging current until the value of the operational parameter decreases to the set-point value. On the other hand, if the value of the operational parameter is smaller than the set-point value, the controller can continue regulating the charging current until the value of the operational parameter increases to the set-point value. 
         [0017]    In some embodiments, if the value of the operational parameter is substantially equal to the set-point value, the controller can maintain the charging current so that the value of the operational parameter remains the same. 
         [0018]    In some embodiments, while regulating the charging current to decrease the value of the operational parameter, the controller can transmit a first control signal to the charging circuit in the portable computing device. This first control signal causes the charging circuit to increase the charging speed for the rechargeable battery, which subsequently causes the charging current to increase and the value of the operational parameter to decrease. 
         [0019]    In some embodiments, while regulating the charging current to increase the value of the operational parameter, the controller can transmit a second control signal to the charging circuit in the portable computing device. The second control signal causes the charging circuit to decrease the charging speed for the rechargeable battery, which subsequently causes the charging current to decrease and the value of the operational parameter to increase. 
         [0020]    In some embodiments, the controller transmits the first and the second control signals through the communication link. 
         [0021]    In some embodiments, the controller routes the charging current through the power link. 
         [0022]    In some embodiments, the charging circuit converts the charging current into a charging voltage suitable for charging the rechargeable battery. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0023]      FIG. 1A  illustrates a fuel cell system in accordance with the disclosed embodiments. 
           [0024]      FIG. 1B  illustrates how a fuel cell system can be connected to a portable computing device in accordance with the disclosed embodiments. 
           [0025]      FIG. 2A  illustrates details of the internal structure of a fuel cell system in accordance with the disclosed embodiments. 
           [0026]      FIG. 2B  illustrates a fuel cell system which uses two DC/DC converters in accordance with the disclosed embodiments. 
           [0027]      FIG. 3  presents a flow chart illustrating how a portable computing device can control a fuel cell system in accordance with the disclosed embodiments. 
           [0028]      FIG. 4  illustrates a fuel cell system which uses an external battery to store power in accordance with the disclosed embodiments. 
           [0029]      FIG. 5  presents a flow chart illustrating how a fuel cell system can use an external rechargeable battery on a portable electronic device to maintain hydrogen pressure around a set-point in accordance with the disclosed embodiments. 
           [0030]      FIG. 6  presents a flow chart illustrating a process of increasing the hydrogen pressure in the fuel cell stack from a low pressure to a pressure substantially close to the set-point value in accordance with the disclosed embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    The following description is presented to enable any person skilled in the art to make and use the disclosed embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the disclosed embodiments. Thus, the disclosed embodiments are not limited to the embodiments shown, but are to be accorded the widest scope consistent with the principles and features disclosed herein. 
         [0032]    The data structures and code described in this detailed description are typically stored on a non-transitory computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The non-transitory computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing code and/or data now known or later developed. 
         [0033]    The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a non-transitory computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the non-transitory computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the non-transitory computer-readable storage medium. Furthermore, the methods and processes described below can be included in hardware modules. For example, the hardware modules can include, but are not limited to, application-specific integrated circuit (ASIC) chips, field-programmable gate arrays (FPGAs), and other programmable-logic devices now known or later developed. When the hardware modules are activated, the hardware modules perform the methods and processes included within the hardware modules. 
       Fuel Cell System 
       [0034]      FIG. 1A  provides an external view of a portable fuel cell system  100  in accordance with the disclosed embodiments. This portable fuel cell system  100  includes a fuel cell housing  101 , which contains a power module with a fuel cell stack that is described in more detail below with reference to  FIG. 1B . Fuel cell housing  101  is configured to receive a detachable fuel cartridge  104 , which contains a suitable fuel, such as sodium borohydride (NaBH 4 ). Moreover, fuel cell housing  101  can provide power to a portable electronic device through a special interface  110 . 
         [0035]      FIG. 1B  illustrates some of the internal structures of fuel cell system  100  in accordance with the disclosed embodiments. More specifically, fuel cell system  100  includes a fuel cell stack  102  which produces electrical power by converting a source fuel, such as hydrogen or a hydrocarbon, into an electric current and a waste product. Fuel cell stack  102  comprises a stack of fuel cells, wherein each a fuel cell contains an anode, a cathode, and an electrolyte between the anode and cathode. Electricity may be generated by two chemical reactions within the fuel cell. First, a catalyst at the anode oxidizes the fuel to produce positively charged ions and negatively charged electrons. The electrolyte may allow ions from the oxidation process to pass through to the cathode while blocking passage of the electrons. The electrons may thus be used to drive a load connected to the fuel cell before recombining with the ions and a negatively charged atom (e.g., oxygen) at the cathode to form a waste product such as carbon dioxide and/or water. 
         [0036]    The fuel cells within fuel cell stack  102  may include electrochemical cells that convert a source fuel into electric current and a waste product. For example, the fuel cells may be proton exchange membrane (PEM) fuel cells that use hydrogen as a fuel. The hydrogen may be catalytically split into protons and electrons at the anode of each PEM fuel cell. The protons may pass through an electrically insulating membrane electrode assembly (MEA) to the cathode of the PEM fuel cell, while the electrons may travel through a load to the cathode. The protons and electrons may then react with oxygen atoms at the cathode to form water molecules as a waste product. Alternatively, the fuel cells may correspond to solid oxide fuel cells, molten carbonate fuel cells, direct methanol fuel cells, alkaline fuel cells, and/or other types of fuel cells. 
         [0037]    Because individual fuel cells may generate a voltage (e.g., 0.5-0.7 volts for PEM fuel cells) which is too low to drive some components in a portable electronic device (e.g., processors, peripheral devices, backlights, displays, Universal Serial Bus (USB) ports, etc.), the fuel cells may be electrically connected in a series configuration. For example, a set of 25 PEM fuel cells may be connected in series within fuel cell stack  102  to increase the voltage of fuel cell stack  102  to roughly 12.5-17.5 volts. This increased voltage may then be used to drive components with operating voltages which are at or below the voltage of fuel cell stack  102 . 
         [0038]    Power from fuel cell stack  102  feeds into circuitry  106  that performs control functions and performs direct-current (DC)-to-DC conversion operations before the power feeds through interface  110  to power portable electronic device  120 . The power can also be directed to an internal rechargeable battery  108 , which is configured to store excess power generated by fuel cell stack  102 . Note that internal battery  108  can also be used to power a portable electronic device during a transient period when fuel cell stack  102  is preparing to produce power. Also note that instead of using rechargeable battery  108 , other energy storage methods can be used, such as Super Capacitors or Lithium-Ion Capacitors. In all of these methods, the system simply stores the power in a convenient location to facilitate meeting the subsequent instantaneous power requirements of portable electronic device  120 . 
         [0039]      FIG. 1B  also illustrates how fuel cell system  100  can be connected to a portable electronic device  120  through a special interface  110 . This special interface  110  includes: (1) a power link that provides power  112  to the portable computing device, and (2) a bi-directional communication link that provides bi-directional communication  114  between the portable computing device and the controller for the fuel cell system. This bi-directional communication link enables the portable electronic device to control various aspects of the operation of portable fuel cell system  100  as will be described in more detail below. 
         [0040]    Note that interface  110  may provide physical links for both power and communication. However, in an alternative embodiment, interface  110  may provide a single physical connection for the power link and a wireless bi-directional data link. In other embodiments, the power link may also be wireless. 
         [0041]    Portable electronic device  120  may correspond to a laptop computer, mobile phone, personal digital assistant (PDA), portable media player, digital camera, and/or other type of compact electronic device. For example, portable electronic device  120  may include a processor  126 , a memory  124  and a display  128 , which are all powered by a power source  122 . Power source  122  includes a controller  125  which selectively provides power from an internal rechargeable battery  127 , or from an external source, such as fuel cell system  100 . 
         [0042]    In an alternative configuration, portable fuel cell system  100  can be “daisy-chained” so that it is connected to another fuel cell system which may or may not be connected in turn to another computer system or computing device. Moreover, portable fuel cell system  100  can also operate as a standalone device, wherein it operates to charge up internal battery  108 . 
       Internal Structure of a Fuel Cell System 
       [0043]      FIG. 2A  illustrates details of some of the internal structure of a fuel cell system  100  in accordance with the disclosed embodiments. The illustrated fuel cell system  100  includes a fuel cartridge  104  that plugs into a power module  200 . Power module  200  provides power to a portable computing device through a MagSafe™ connector which is coupled to the end of an interface cable  110 . (Recall that interface cable  110  includes a power link as well as a bi-directional communication link.) Note that the connector on interface cable  110  is not limited to a MagSafe™ connector, and in general can include any type of connector that provides power and bi-directional communication, such as a USB connector or a 30-pin iPod™ connector. Also note that power module  200  is located within a fuel cell housing, such as fuel cell housing  101  illustrated in  FIG. 1A . 
         [0044]    Fuel cartridge  104  is comprised of a number of components, which depend on the nature of the fuel. For example, if the hydrogen is produced by a hydrolysis reaction, the fuel cartridge contains components that may include (in addition to a hydrogen-containing substance) another substance (or substances) that chemically react with the hydrogen-containing substance to release hydrogen. To support a hydrolysis reaction, the fuel cartridge can also include: (1) a feed or pump mechanism to enable the substances to combine to produce hydrogen; (2) a metering mechanism to allow for the correct ratio of substances for optimal hydrogen production; (3) a heat-dissipation mechanism (such as a fan) if the hydrogen-producing reaction is highly exothermic; (4) any filters needed to maintain the requisite purity and/or physical consistency of the reactants; and (5) a mechanism for containing any waste product that may result from the hydrogen-producing reaction. Exemplary fuels that can be used with a hydrolysis reaction can include: Sodium Borohydride, Sodium Silicate, Lithium Hydride, Magnesium Hydride, Lithium Borohydride and Lithium Aluminum Hydride. 
         [0045]    Moreover, if hydrogen is produced by a thermolysis technique, the fuel cartridge may include (in addition to the hydrogen-containing substance) a heater that heats the hydrogen-containing substance to a sufficiently high temperature to liberate hydrogen. It may also contain a structure for thermally insulating the heater, and a structure for containing any waste product that may result. Exemplary fuels that can be used with a thermolysis technique can include: Aluminum Hydride, Amine Borane Complexes (e.g., Ammonia Borane), Hydrocarbons (e.g., Methanol), Lithium Aluminum Hydride, Magnesium Borohydride, and a Magnesium Borohydride-Amine complex. 
         [0046]    The fuel may also take the form of pure hydrogen (e.g., compressed hydrogen gas or liquid hydrogen) in which case the fuel cartridge may contain components such as a metering device (e.g., a valve) and a pressure gauge. Ideally, the fuel has a relatively low life cycle carbon footprint, is not toxic, and generates a waste product that is amenable to being repeatedly re-charged with hydrogen and is not toxic. 
         [0047]    More specifically, fuel cartridge  104  is comprised of a number of components, including fuel and related components  228 . During operation, the fuel and related components  228  create hydrogen gas which passes through a filter  236  before feeding into an H 2  inlet  240  in power module  200 . Operations within fuel cartridge  104  are generally controlled by an EEPROM  234 , which communicates with master control board  212  in power module  200  through an I 2 C bus  258 . A temperature sensor  226  within fuel cartridge  104  determines a temperature of the fuel cartridge  104  and communicates a temperature value T CARTRIDGE    257  to EEPROM  234 . In addition, a cartridge fan  202  within power module  200  pulls a cooling air flow  270  through fuel cartridge  104 . Fuel cartridge  104  also includes a pressure relief device (PRD), such as a valve, which vents hydrogen gas (PRD H 2  out  238 ) if too much hydrogen builds up within fuel cartridge  104 . 
         [0048]    The flow of hydrogen through fuel cell system  100  is illustrated by the dashed lines. Hydrogen gas which is generated by fuel cartridge  104  passes through a pressure sensor  218  in master control board  212  before feeding into fuel cell stack  102 . Fuel cell stack  102  also includes a temperature sensor  216 , which provides an exhaust temperature T EXHAUST  to master control board  212 . Excess hydrogen (along with nitrogen and water) exits fuel cell stack through H 2  outlet  241  and feeds through a pressure sensor  220  in master control board  212  before feeding into a passive purge valve  222 . Passive purge valve  222  vents the excess hydrogen, nitrogen and water through purge output  246 . 
         [0049]    Fuel cell stack  102  generates power from the hydrogen gas. More specifically, voltage outputs V 1 , . . . V N  from individual cells within fuel cell stack  102  feed into master control board  212 , which directs power from these voltage outputs into either: internal battery  108  through V 2S , V S  and GND connections; or into MagSafe™ board  224  though PWR 2S  and GND connections. Internal battery  108  can store the power received from fuel cell stack  102 , whereas MagSafe™ board  224  can direct the power to a portable computing device through interface  110  and MagSafe™ connector  225 . Master control board  212  controls internal battery  108  through a battery management unit (BMU)  206 , which monitors a temperature T B  from internal battery  108 . 
         [0050]    Master control board  212  also independently controls two or more fans, including cartridge fan  202  and stack fan  204 . More specifically, master control board  212  controls cartridge fan  202  by providing power and a pulse-width modulated (PWM) signal  250  to cartridge fan  202 . During this process, control board  212  receives a tachometer signal TACH  252  from cartridge fan  202  which indicates a speed of cartridge fan  202 . As mentioned above, during operation cartridge fan  202  pulls cooling air through fuel cartridge  104 . Similarly, master control board  212  controls stack fan  204  by providing power and a pulse-width modulated (PWM) signal  254  to stack fan  204 . Control board  212  also receives a tachometer signal TACH  256  from stack fan  204  which indicates a speed of stack fan  204 . During operation, stack fan  204  pulls cooling air across master control board  212  and through fuel cell stack  102 . 
         [0051]    Master control board  212  is also coupled to a battery indicator light (BIL) board  214  through an I 2 C link  260 . To determine a state-of-charge of internal battery  108 , a user presses an associated BIL button  213 . In response to this button press, BIL board  214  communicates with BMU  206  within master control board  212  to determine a state-of-charge of internal battery  108 , and then outputs a pattern on BIL LEDs  211 , wherein the pattern indicates the determined state-of-charge. 
       DC/DC Conversion Process 
       [0052]      FIG. 2B  illustrates how the fuel cell system can use two DC/DC converters in accordance with the disclosed embodiments. During operation, master control board  212  receives power from fuel cell stack  102  and converts the power using a first DC/DC converter  280  into a battery voltage which is suitable for charging internal battery  108 . Next, a second DC/DC converter  282  converts the battery voltage into a voltage suitable for powering a portable electronic device, and this voltage is fed into MagSafe™ board  224 , which feeds the power to the portable computing device through interface  110 . 
       System Operation 
       [0053]    Normal operation of the system begins when the fuel cell system  100  is attached to a host, such as portable electronic device  120 . If the state-of-charge of the internal battery in fuel cell system  100  is in a nominal state (between high and low state-of-charge thresholds), power delivery to the host begins. The control system then enters an initialization state and begins requesting fuel from the cartridge. 
         [0054]    The cartridge responds by beginning its fuel generation process. 
         [0055]    During this process, hydrogen may be provided directly from a source of pure hydrogen (such as from a vessel containing compressed hydrogen gas), or it may be generated via thermolysis, hydrolysis, electrolysis, reformation, etc. As hydrogen is generated and transported to the power module, the cells in the fuel cell stack experience a voltage rise to their open circuit voltage (OCV). After the voltages cross a threshold, the controller begins to draw small amounts of current. 
         [0056]    When this current does not excessively result in depression of the cell voltages from OCV, the system transitions into a “run” state. In the run state, hydrogen enters the fuel cell and is converted to current and heat. The oxygen for the reaction is supplied by the stack fan from ambient air, and heat is exchanged by controlling the fan to maintain the stack at a constant temperature. 
         [0057]    Current from the stack is converted to the voltage of the internal battery and stored. The pressure control loop maintains stack outlet pressure at a set-point by controlling the DC/DC input current and charging/discharging the internal battery as necessary. The output of the internal battery charger goes through another DC/DC conversion and then powers the host computer. 
         [0058]    In the case of a fuel cartridge where hydrogen is produced by an exothermic reaction, the controller maintains the cartridge temperature at its set-point using a cartridge fan. 
         [0059]    Many of the above-described operations of fuel cell system  100  can be controlled through communications between fuel cell system  100  and portable electronic device  120  as is described in more detail below with reference to the flow chart in  FIG. 3 . 
         [0000]    Controlling a Fuel Cell System from a Portable Electronic Device 
         [0060]      FIG. 3  presents a flow chart illustrating how a portable computing device can control a fuel cell system in accordance with the disclosed embodiments. The left-hand column of  FIG. 3  lists actions performed by fuel cell system  100  and the right-hand column lists actions performed by portable electronic device  120 . During operation, fuel cell system  100  sends fuel cell state information to portable electronic device  120 , wherein the fuel cell state information is sent through an interface that comprises: a power link that provides power to the portable computing device; and a bi-directional communication link that provides bi-directional communication between the portable computing device and the fuel cell system (step  302 ). For example, the fuel cell state information can specify one or more of the following: how much power is available from the fuel cell system; a state-of-charge of an internal rechargeable battery within the fuel cell system; a temperature of the fuel cell stack; a pressure at an inlet of the fuel cell stack; a pressure at an outlet of the fuel cell stack; cell voltages for individual cells in the fuel cell stack; how much fuel remains in the fuel source; certification information for the fuel cell system; and identification information that facilitates identifying an individual fuel cell unit and/or individual fuel cartridges. 
         [0061]    Next, portable electronic device  120  receives the fuel cell state information (step  304 ), and in response generates fuel cell control information based on the received fuel cell state information (step  306 ). For example, the fuel cell control information can specify one or more of the following: a request for a specified amount of power from the fuel cell system; a reactant rate in the fuel source; a fuel cell stack current to be pulled off the fuel cell stack; a speed of a fan within the fuel stack system; and a command to run diagnostics for the fuel cell system. 
         [0062]    The fuel cell control information can also specify power demand predictions. For example, the fuel cell control information can specify that portable computing device  120  expects to require 60 W of power in ten minutes time. In another example, as the battery within the portable electronic device  120  charges up, the fuel cell control information can specify that the demand for power from the fuel cell system  100  is likely to decrease over time. Providing such power demand predictions enables fuel cell system  100  to optimize its performance differently than if such predictions were not available. 
         [0063]    Portable electronic device  120  then transmits the fuel cell control information to fuel cell system  100  through the bi-directional communication link (step  308 ). Finally, fuel cell system  100  receives the fuel cell control information (step  310 ), and uses the received fuel cell control information to control the fuel cell system (step  312 ). The above-described processes for controlling the fuel cell system can involve using one or more feedback-control loops to actively control one or more operating parameters of the fuel cell system. 
       Fuel Cell System Coupled to an External Power Source 
       [0064]      FIG. 4  illustrates a fuel cell system  400  which uses an external battery to store operating power in accordance with the disclosed embodiments. 
         [0065]    As illustrated in  FIG. 4 , fuel cell system  400  includes many of the same components as fuel cell system  100  in  FIG. 1B , such as a fuel cell stack  402  and a fuel cartridge  404 . Note that all the descriptions for fuel cell stack  102  and fuel cartridge  104  above are applicable to fuel cell stack  402  and fuel cartridge  404 . Power from fuel cell stack  402  feeds into a controller  406  that performs control functions, such as monitoring and regulating a power state of fuel cell stack  402 , monitoring and regulating a hydrogen pressure in fuel cell stack  402 , and regulating a current provided by fuel cell stack  402  to an external electronic device. These controller functions are described in more detail below. 
         [0066]      FIG. 4  also illustrates how fuel cell system  400  can be coupled to a portable electronic device  420  through an interface  410 . Portable electronic device  420  may correspond to a laptop computer, mobile phone, personal digital assistant (PDA), portable media player, digital camera, and/or other type of compact electronic device. For example, portable electronic device  420  may include a processor  422  and a memory  424 , which are all powered by a rechargeable battery (or “battery”)  426 . Portable electronic device  420  also includes a charging circuit  428  coupled to battery  426  which controls both charging and discharging currents of battery  426 . In one embodiment, charging circuit  428  is part of a battery management unit (BMU) (not shown) that controls battery  426  on portable electronic device  420 . 
         [0067]    Note that interface  410  is directly coupled between controller  406  in fuel cell system  400  and charging circuit  428  in portable electronic device  420 . Interface  410  includes: (1) a bi-directional power link  412  that transfers electrical power between fuel cell system  400  and portable electronic device  420 , and (2) a communication link  414  that provides bi-directional communication between charging circuit  428  in portable electronic device  420  and controller  406  in fuel cell system  400 . More specifically, the coupling through communication link  414  allows controller  406  to control charging circuit  428  which, in turn, regulates the charging process on battery  426 . On the other hand, the coupling through power link  412  allows a regulated electrical power to be transferred between fuel cell stack  402  and battery  426 , wherein the power regulation is performed by controller  406  and charging circuit  428 . 
         [0068]    In one embodiment, communication link  414  is substantially the same as communication link  114  in  FIG. 1B , and hence can perform all the functions associated with communication link  114 . Note that power link  412  can be a separate link from power link  112  in  FIG. 1B , such that the former is a bi-directional power link for transferring power between fuel cell stack  402  and battery  426 , while the latter is a unidirectional power link specifically for transferring power from fuel cell stack  402  to one or more components in portable electronic device  420 . In another embodiment, however, power link  412  can perform all the functions associated with power link  112 . 
         [0069]    In some embodiments, controller  406  provides power to a portable electronic device  420  through a MagSafe™ connector (not shown) which is coupled to the end of interface  410 . Note that the connector on interface  410  is not limited to a MagSafe™ connector, and in general can include any type of connector that provides power and bi-directional communication, such as a USB connector or a 30-pin iPod™ connector. In some embodiments, controller  406  can also perform DC-to-DC conversion operations before transferring the power through interface  410  to portable electronic device  420 . 
         [0070]    Note that interface  410  may provide physical links for both power and communication. However, in an alternative embodiment, interface  410  may provide a single physical connection for the power link and a wireless bi-directional data link. In other embodiments, the power link may also be wireless. 
         [0071]    Note that, unlike fuel cell system  100  described above, fuel cell system  400  does not include a rechargeable battery, such as rechargeable battery  108  in fuel cell system  100  (or only includes a small rechargeable battery which is used to power controller  406 ). In one embodiment, fuel cell system  400  can direct excess power generated by fuel cell stack  402  over power link  412  to be stored externally on rechargeable battery  426  in portable electronic device  420 . 
         [0072]    In one embodiment, fuel cell stack  402 , controller  406 , interface  410  (and its bi-directional links), charging circuit  428 , and battery  426  form a closed control loop to servo an operational parameter of fuel cell stack  402 . In this embodiment, controller  406  may be configured to monitor the operational parameter of the fuel cell stack, and then adjust and maintain the operational parameter around a set-point. More specifically, based on the value of the operational parameter, controller  406  can command charging circuit  428 , through communication link  414 , to charge battery  426  at a variable rate. Note that, when charging circuit  428  charges battery  426  at a faster/slower rate (i.e., with a larger/smaller charging current), charging circuit  428  draws more/less power from fuel cell stack through power link  412 . This effectively servos the operational parameter of fuel cell stack  402 . 
         [0073]    In one embodiment, this operational parameter is hydrogen pressure inside fuel cell stack  402 . As described above, hydrogen is provided to fuel cell stack  402  to generate electrical current. For stable operation of fuel cell stack  402 , controller  406  should be configured to maintain hydrogen pressure provided to fuel cell stack  402  at a predetermined set-point. As described above, a hydrogen pressure can be measured as excess hydrogen exits fuel cell stack  402  through an H 2  outlet and feeds through a pressure sensor on controller  406 . However, the embodiment of  FIG. 2A  maintains stack outlet pressure at a set-point by charging/discharging an internal battery  108  on fuel cell system  100 . Rather other charging/discharging an internal battery (which may not exist on fuel cell system  400 ) to maintain the hydrogen pressure, one embodiment uses the closed control loop of fuel cell stack  402 , controller  406 , interface  410 , charging circuit  428 , and battery  426 , to maintain the stack outlet pressure. 
         [0074]    More specifically, during normal operation of fuel cell system  400  (assuming hydrogen pressure is maintained around the set-point), charging circuit  428  continuously charges battery  426  with a charging current  430  drawn from fuel cell stack  402 . In one embodiment, charging circuit  428  uses an integrated DC-to-DC converter to convert charging current  430  into a battery voltage which is suitable for charging battery  426 . 
         [0075]    Meanwhile, controller  406  continuously monitors the hydrogen pressure and determines if the hydrogen pressure is above or below the set-point. If the monitored hydrogen pressure goes above the set-point, controller  406  transmits a control signal to charging circuit  428 , through communication link  414 , to cause battery  426  to charge at a faster rate. While charging battery  426  at the faster rate, charging circuit  428  draws more power (i.e., causing charging current  430  to increase) from fuel cell stack  402 , which in turn causes the hydrogen pressure to decrease. In this manner, controller  406  continuously regulates the charging current until the hydrogen pressure returns to the set-point. 
         [0076]    Conversely, when the monitored hydrogen pressure moves below the set-point, controller  406  sends a control signal to charging circuit  428  which causes battery  426  to charge at a slower rate. While charging battery  426  at the slower rate, charging circuit  428  draws less power (i.e., causing charging current  430  to decrease) from fuel cell stack  402 , which in turn causes the hydrogen pressure to increase. In this manner, controller  406  continuously regulates the charging current until the hydrogen pressure returns to the set-point. When the hydrogen pressure is substantially equal to the set-point, controller  406  can maintain a constant charging current (a constant charging current  430 ) so that charging circuit  428  draws a substantially constant current from fuel cell stack  402  to maintain the hydrogen pressure. 
         [0077]    Note that, because the capacity of a rechargeable battery of an external electronic device (e.g., a laptop) can be sufficiently large, regulating a fuel cell operating parameter by directly charging an external battery with the fuel cell allows the control process to be highly reliable. 
         [0078]    In one embodiment, battery  426  in portable electronic device  420  can also be used to provide power to fuel cell system  400  through interface  410 . For example, assume battery  426 , which is at least partially charged, is coupled to fuel cell system  400  through interface  410  while fuel cell system  400  is off. During a boot-up process for fuel cell system  400 , controller  406  causes fuel cartridge  404  to pump hydrogen gas into fuel cell stack  402  to build up the hydrogen pressure. However, before a threshold level hydrogen pressure is reached, no current will be generated by fuel cell stack  402 . As the hydrogen pressure continues to rise past the threshold level, controller  406  starts to draw a small current from fuel cell stack  402 . As the hydrogen pressure rises toward the set-point value, the current generated by fuel cell stack  402  also increases. 
         [0079]    Note that, during this period of building up hydrogen pressure from zero toward the set-point value, fuel cell system  400  may be externally powered by battery  426 . More specifically, as battery  426  discharges, a discharging current  432  can be transferred from battery  426  to controller  406  and other components in fuel cell system  400  over bi-directional power link  412  to provide necessary power to the boot-up process. In one embodiment, the discharging process of battery  426  may be activated by controller  406 . For example, controller  406  can send a control signal through communication link  414  to charging circuit  428 , which subsequently establishes discharging current  432  from battery  426 . Charging circuit  428  can also regulate the magnitude of discharging current  432  and route discharging current  432  from battery  426  over power link  412  to power fuel cell system  400 . After the hydrogen pressure in fuel cell stack  402  reaches the set-point value, controller  406  may send another control signal to charging circuit  428  to command charging circuit  428  to terminate the discharging of battery  426 . Note that this transition point essentially reverses the current flow in power link  412 . At this transition point, the above-described closed loop control process can be engaged, and battery  426  begins to receive charging current  430  from fuel cell stack  402  flowing to portable electronic device  420  to servo the operational parameter around the set-point. 
         [0080]    Note that, while the above discussion assumes all control functions are generated by controller  406  in fuel cell system  400 , other embodiments can generate these control functions from a controller located in portable electronic device  420 . However, this embodiment requires that values of the fuel cell stack operational parameter be transmitted over interface  410  to portable electronic device  420 . 
       Controlling a Fuel Cell System Using an External Rechargeable Battery 
       [0081]      FIG. 5  presents a flow chart illustrating how fuel cell system  400  can use external rechargeable battery  426  on portable electronic device  420  to maintain hydrogen pressure around a set-point in accordance with the disclosed embodiments. The left-hand column of  FIG. 5  lists actions performed by fuel cell system  400 , and the right-hand column lists actions performed by portable electronic device  420 . During operation, fuel cell system  400  starts by increasing the hydrogen pressure in fuel cell stack  402  from a low pressure to substantially close to the set-point value (step  502 ). Step  502  is described in more detail in conjunction with  FIG. 6 . 
         [0082]    Next, controller  406  in fuel cell system  400  sends a charging command and a charging current through interface  410  to charging circuit  428  in portable electronic device  420 , wherein interface  410  comprises: a power link that transfers electrical power between the fuel cell system and the portable electronic device, and a bi-directional communication link that provides bi-directional communication between the portable electronic device and the fuel cell system (step  504 ). Next, charging circuit  428  receives the charging command and the charging current and, in response, starts charging rechargeable battery  426  using the charging current (step  506 ). 
         [0083]    Meanwhile, controller  406  in fuel cell system  400  monitors the hydrogen pressure (step  508 ) and determines if the hydrogen pressure is greater than, less than, or substantially equal to the set-point value (step  510 ). 
         [0084]    If the hydrogen pressure is greater than the set-point value, controller  406  sends a control signal to charging circuit  428  to command charging circuit  428  to charge battery  426  at a faster rate (step  512 ). Next, portable electronic device  420  receives the control signal and, in response, charges battery  426  at the faster rate (step  514 ). In doing so, charging circuit  428  draws more power (i.e., causing charging current  430  to increase) from fuel cell stack  402 , which in turn causes the hydrogen pressure to decrease. After sending the control signal, and typically after a predetermined delay, controller  406  returns to step  508  to continue the monitoring and adjusting operations. 
         [0085]    If the hydrogen pressure is less than the set-point value, controller  406  sends a control signal to charging circuit  428  to command charging circuit  428  to charge battery  426  at a slower rate (step  516 ). Next, portable electronic device  420  receives the control signal and, in response, charges battery  426  at the slower rate (step  518 ). In doing so, charging circuit  428  draws less power (i.e., causing charging current  430  to decrease) from fuel cell stack  402 , which in turn causes the hydrogen pressure to increase. After sending the control signal, and typically after a predetermined delay, controller  406  returns to step  508  to continue the monitoring and adjusting operations. 
         [0086]    If, however, the hydrogen pressure is substantially equal to the set-point value, controller  406  returns to step  508  to continue the monitoring and adjusting operations. Note that, because the hydrogen pressure is at the desired value, there is no change to the charging current or to the amount of power transferred from the fuel cell system to the portable electronic device. 
         [0087]    Although the above closed loop control process is described in terms of hydrogen pressure, the general technique can be used with other operational parameters associated with fuel cell system  400 , and hence is not limited to hydrogen pressure. 
         [0088]      FIG. 6  presents a flow chart illustrating a process of increasing the hydrogen pressure in fuel cell stack  402  from a low pressure to a pressure substantially close to the set-point value in accordance with the disclosed embodiments. 
         [0089]    The left-hand column of  FIG. 6  lists actions performed by fuel cell system  400  and the right-hand column lists actions performed by portable electronic device  420 . During operation, controller  406  commands fuel cartridge  404  to begin pumping hydrogen gas into fuel cell stack  402  to build up the hydrogen pressure (step  602 ). Controller  406  then sends a control signal through communication link  414  to charging circuit  428  to command discharging battery  426  (step  604 ). Next, charging circuit  428  receives the discharging command and, in response, causes battery  426  to discharge and produce discharging current  432  (step  606 ). Charging circuit  428  then routes discharging current  432  over power link  412  to fuel cell system  400 , wherein the power can be used by one or more components in fuel cell system  400  (step  608 ). 
         [0090]    Meanwhile, controller  406  in fuel cell system  400  monitors the hydrogen pressure (step  610 ) and determines if the hydrogen pressure has reached the set-point value (step  612 ). If not, controller  406  continues to monitor the hydrogen pressure (step  610 ), and allowing the hydrogen pressure to rise and the battery  426  to continue discharging current to power fuel cell system  400 . However, if controller  406  detects that the hydrogen pressure is substantially equal to the set-point value, controller  406  sends a control signal to charging circuit  428  to command charging circuit  428  to terminate the discharging of battery  426  (step  614 ). 
         [0091]    Next, charging circuit  428  receives the stop discharging command and, in response, terminates discharging battery  426  (step  616 ). At this point, a significant amount of current is being produced by fuel cell stack  402 . Hence, controller  406  can continue to step  504  in  FIG. 5  and transition from receiving power from battery  426  to transferring power to battery  426  using the power generated by fuel cell stack  402 . 
         [0092]    The foregoing descriptions of embodiments have been presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the present description to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present description. The scope of the present description is defined by the appended claims.