Patent Publication Number: US-2007122661-A1

Title: Methods and apparatus for a hybrid power source

Description:
TECHNICAL FIELD  
      The present invention generally relates to power sources and, more particularly, to improved power supplies incorporating fuel cell technology.  
     BACKGROUND  
      During the normal course of operation, a mobile device—for example, a mobile terminal, a personal data assistant (PDA), or the like—will deplete its main power source. As such devices typically include important information such as user data, configuration values and state information stored in some form of memory, it is desirable to allow the main power source to be swapped out without disrupting storage of this information.  
      To maintain memory, conventional mobile devices generally incorporate some form of dedicated power supply, for example, a battery or ultra-capacitor (also referred to as a “supercap”). These types of power sources are often used in conjunction with support circuitry configured to charge the backup power source and regulate its output. This support circuitry takes up additional board space and can add significant expense to the unit.  
      In addition, such known power sources generally operate at a low power level. That is, the battery in such systems is designed merely to maintain certain information stored in the device&#39;s various memory components; it is not designed to supply enough power to allow the device to be used in a normal operation mode. Rather, the device is typically powered down or placed in stand-by mode in order to remove the main power supply. This leads to inconvenience and loss of productivity.  
      Accordingly, there is a need for systems and methods that overcome these and other limitations of the prior art.  
     BRIEF SUMMARY  
      A hybrid power supply in accordance the present invention generally includes a fuel cell plant with a reservoir (e.g., a direct methanol fuel cell (DMFC) with a methanol-filled reservoir) configured to produce a DC voltage via electrochemical conversion of a fuel. A rechargeable battery (e.g., a lithium-ion battery) is electrically coupled to the fuel cell plant. The fuel cell plant keeps the rechargeable battery substantially charged while the rechargeable battery accommodates load variations resulting from operation of the device. In this way, the hybrid power supply maintains operation of the device even when the reservoir is removed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures.  
       FIG. 1  is a schematic overview of a device with a hybrid power supply in accordance with one embodiment of the present invention;  
       FIG. 2  is a schematic overview of the device of  FIG. 1  with fuel reservoir removed; and  
       FIG. 3  is a schematic overview of a typical direct methanol fuel cell. 
    
    
     DETAILED DESCRIPTION  
      The following detailed description is merely illustrative in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.  
      The detailed description may also include functional and/or logical block components and various processing steps. It should be appreciated that such block components may be realized by any number of hardware, software, and/or firmware components configured to perform the specified functions.  
      Referring to  FIG. 1 , a device  100  incorporating a hybrid power supply in accordance with one embodiment of the present invention generally includes a fuel cell plant (or simply “plant”)  120  communicating with and receiving fuel from a fuel reservoir (or “reservoir”)  130 . A rechargeable battery (or “battery”)  110  is electrically coupled to fuel cell plant  120 . Rechargeable battery  110 , fuel reservoir  130 , and fuel cell plant  120  are collectively referred to herein as the “power supply” and/or the “hybrid power supply.” 
      The available energy of a typical battery diminishes over a relatively short time (approximately 8 hours), while the available energy of a fuel cell lasts much longer (greater than 20 hours). On the other hand, fuel cells have difficulty in handling load fluctuations—a difficulty that batteries do not share. A hybrid power source in accordance with the present invention therefore combines these two technologies such that fuel cell plant  120  produces a DC voltage that charges battery  110  and, at the same time, battery  110  accommodates variations in load current provided to device  100 . The hybrid power supply of the present invention thus allows reservoir  130  to be removed from device  102  without significantly sacrificing operational capability.  
      In one embodiment, fuel reservoir  130  may be removed from housing  102  to simplify changing of the reservoir when, for example, the fuel in reservoir  130  has been depleted. Such an embodiment is shown schematically in  FIG. 2 , which illustrates device  100  with reservoir  130  removed from housing  102 . Attachment of reservoir  130  to housing  102  may be accomplished in accordance with any convenient method. In one embodiment, a key/lock system or other security arrangement is employed to prevent accidental or unauthorized removal of reservoir  130 .  
      Having thus given an overview of a hybrid power supply in accordance with the present invention, a detailed description of the various components will now be provided.  
      Fuel cell plant  120  includes any component capable of producing electrical energy via electrochemical conversion of a fuel, which is typically a liquid. In this regard, many types of fuel cells may be used in conjunction with the present invention. In one embodiment, fuel cell plant  120  is a direct methanol fuel cell (DMFC).  
      A DMFC is a proton-exchange type fuel cell that uses a polymer membrane as an electrolyte and relies upon the oxidation of methanol on a catalyst layer to form carbon dioxide. Referring to  FIG. 3 , a DMFC  120  generally includes an anode electrode  304 , a cathode electrode  302 , and respective terminals  308  and  310 . Cathode  302  and anode  304  are separated by a membrane  306 , e.g., a polymer electrolyte membrane (PEM)  306 . During operation, methanol and water are supplied to anode  304 , producing carbon-dioxide, while oxygen is supplied to cathode  302 , producing water and resulting in the transport of protons (H+) across membrane  306 . Specifically, the half reactions within DMFC  120  are: 
 
Anode: CH 3 OH+H 2 O→CO 2 +6H + +6e − 
 
Cathode: 1.50 2 +6H + +6e − →3H 2 O
 
 and the net reaction is: 
 
CH 3 OH+1.50 2 →CO 2 +2H 2 O
 
      Thus, DMFC  120  uses methanol as a fuel, producing electrical energy, carbon dioxide, and water. In this regard, while the illustrated embodiment is discussed in the context of a DMFC, the present invention contemplates the use of other fuel cell types, including, for example, alkaline fuel cells, molten-carbonate fuel cells, phosphoric-acid fuel cells, direct borohydride fuel cells, solid-oxide fuel cells, zinc fuel cells, and the like.  
      Terminals  308  and  310  of fuel cell  120  are connected to a load external to the cell. In accordance with one embodiment of the present invention, terminals  308  and  310  are coupled to a rechargeable battery (e.g., battery  110  in  FIG. 1 ) as well as the internal electrical load associated with device  100 . That is, when reservoir  130  is removed from device  100 , battery  110  takes over and provides the required DC power in conjunction with fuel cell  120 . The positive and negative terminals of battery  110  are preferably coupled, directly or indirectly, to the anode and cathode of fuel cell  120 .  
      When battery  110  is being charged, a voltage is applied across its terminals to reverse the chemical reaction that would typically take place during normal operation of the battery (i.e., when the battery is acting as a standard voltaic cell.). Battery  110  is preferably electrically coupled to fuel cell plant  120  (and other optional control electronics, not shown) such that fuel cell plant  120  keeps battery  110  substantially charged. In one embodiment of the present invention, the output of fuel cell  120  feeds a battery charger circuit of the type known in the art, which would then feed into battery  110 .  
      Battery  110  is any suitable type of rechargeable battery now known or later developed. In one embodiment, for example, battery  110  is a rechargeable lithium-ion battery. Other battery-types may also be used, however, including various nickel-cadmium batteries, nickel-metal-hydride batteries, lithium-polymer batteries, and the like. Furthermore, battery  110  may include two or more batteries configured in parallel or series, depending upon the power requirements of the application.  
      Battery  110  is selected in accordance with known criterion depending upon, for example, required power, required voltage, anticipated recharge cycles, etc. For example, device  100  will typically have known operational power requirements for normal loads, peak loads, and loads necessary to maintain some minimum level of storage (i.e., to maintain settings and data resident in the device). In this regard, battery  110  and fuel plant  120  are preferably selected such that they are, in combination, capable of supplying power substantially equal to the operational power requirements of the device. That is, it is preferable for the device to be fully-operational even when the reservoir is removed.  
      In one embodiment, battery  110  has a nominal capacity of approximately 400 to 500 mA*hr and a supply voltage of from about 3.0 to 5.0 volts. Such a battery is of particular utility in mobile devices of the type having an input, an LCD screen, and other such components that must be carried around to locations where an external power source is not available.  
      While at least one example embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the example embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope of the invention as set forth in the appended claims and the legal equivalents thereof.