Patent Publication Number: US-2012038328-A1

Title: Systems and Methods for Charging Super Capacitors

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 60/943,135, entitled “Systems and Methods for Charging Super Capacitors,” filed Jun. 11, 2007, which is incorporated herein by reference as if set forth herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to charging super capacitors, and more particularly to charging super capacitor banks irrespective of temperature effects on the voltage ratings of the capacitors. 
     BACKGROUND 
     Super capacitors are typically used as a power reservoir to supply temporary reserve power when available input power is unable to handle a spike in power demand or when the input power supply is no longer available due to a power failure. Super capacitors provide reserve power, for example, for a telemetry device when enabling a temporary boost in transmission power, or when enabling a “last gasp” transmission opportunity in the event of power failure. 
     The maximum charging voltage for super capacitors is a function of the operating temperature. As the operating temperature is increased, the maximum voltage rating of the super capacitor is reduced. Since exceeding the maximum charging voltage reduces the life of the super capacitor, temperature compensating circuits are often used to limit the charging voltage at higher operating temperatures. 
     SUMMARY 
     The present invention provides systems and methods for charging a super capacitor bank. In one embodiment, a method for charging a super capacitor bank comprises determining a desired charge voltage for the super capacitor bank, providing a charging current to the super capacitor bank, limiting the charging current according to a corresponding worst case temperature within the super capacitor bank operating temperature range, limiting the charge voltage according to the worst case temperature within the super capacitor bank operating temperature range, and turning off the charging current upon a determination that the super capacitor bank has been charged to the desired charge voltage. 
     Another method for charging a super capacitor bank comprises determining a charging current limit for the super capacitor bank, wherein the charging current limit is the maximum charging current for the super capacitor bank at worst case temperature within the super capacitor bank operating temperature range, determining a charging voltage limit for the super capacitor bank, wherein the charging voltage limit is the maximum voltage for the super capacitor bank at worst case temperature within the super capacitor bank operating temperature range, providing a charging current to the super capacitor bank, wherein the charging current is less than the charging current limit, maintaining the charging voltage below the charging voltage limit, and turning off the charging current upon determining that the super capacitor bank has charged to a specified charge voltage. 
     In another embodiment, a method for charging a super capacitor bank comprises providing a regulated voltage, where the regulated voltage is the maximum voltage for the super capacitor bank at worst case temperature within the super capacitor bank operating temperature range, providing a charging current to the super capacitor bank, and limiting the charging current below a specified maximum charging current specified for the super capacitor bank according to worst case temperature within the super capacitor bank operating temperature range. 
     In another embodiment, a method for charging a super capacitor bank comprises determining a charging current limit for the super capacitor bank where the charging current limit is the maximum charging current for the super capacitor bank at worst case temperature within the super capacitor bank operating temperature range, providing a regulated voltage where the regulated voltage is the maximum voltage for the super capacitor bank at worst case temperature within the super capacitor bank operating temperature range, and providing a charging current to the super capacitor bank where the charging current is less than the charging current limit. 
     In yet another embodiment, a system for charging a super capacitor bank comprises a super capacitor bank for storing energy and to further provide a specified power demand to a circuit, a current charger to provide a charging current to the super capacitor bank, where the charging current is limited to a maximum charging current corresponding to a worst case temperature within the super capacitor bank operating temperature range, a voltage sense circuit to detect the voltage of the super capacitor bank, and a control configured to disconnect the current charger from the super capacitor bank upon detecting a specified voltage at the super capacitor bank. 
     Other systems, methods, features and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description and be within the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a block diagram illustrating a system for charging a super capacitor bank irrespective of temperature effects on the voltage ratings of the capacitors. 
         FIG. 2  is a block diagram illustrating an alternative system for charging a super capacitor bank using a regulated voltage. 
         FIG. 3  is a flowchart illustrating a method for charging a super capacitor bank according to the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference is now made in detail to the description of the embodiments of systems and methods for charging super capacitor banks as illustrated in the accompanying drawings. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are intended to convey the scope of the invention to those skilled in the art. Furthermore, all “examples” given herein are intended to be non-limiting. 
       FIG. 1  is a block diagram illustrating an embodiment of a system  100  for charging a super capacitor bank  140  irrespective of temperature effects on the voltage ratings of the capacitors. A charger  110  provides current from an unregulated voltage  120  for charging the super capacitor bank  140 . The charging current and voltage are maintained at a level that is safe for the range of operating conditions based on the super capacitor characteristics as provided by the super capacitor manufacturer. A voltage sense circuit  150  is provided for monitoring or detecting the voltage level of the super capacitor bank. When the super capacitor bank  140  is charged to the desired voltage, the voltage sense circuit  150  provides an input that signals the on/off control  130  to disconnect the charging current from the charger  110 . 
     Additionally, when the super capacitor bank  140  voltage drops below the desired voltage level, the voltage sense circuit  150  provides an input that signals the on/of control  130  to connect the charging current from the charger  110 . The super capacitor bank  140  begins charging again. 
     One skilled in the art will readily note that the charger  110  need not be limited to a constant current charge, so long as the charging current does not exceed the maximum recommended charging current for the super capacitors, and the charge time does not exceed the maximum allowable charge time for the application. 
     Super capacitors are typically used as a power reservoir to supply temporary reserve power for circumstances when available input power is unable to handle a spike in power demand, or other relatively sudden increases in power demand, or when the input power supply is no longer available due to a power failure. In one non-limiting example, super capacitors provide reserve power for a telemetry device when enabling a temporary boost in transmission power. In another example, super capacitors provide reserve power when enabling a “last gasp” transmission opportunity in the event of power failure. Such “last gasp transmissions” allow for notification to a server or other system devices that the application or device may not be available, for example. 
     Super capacitors are charged to a temperature-dependent maximum voltage. Exceeding the maximum charge voltage typically reduces the life of a super capacitor. Super capacitors are typically sized to supply adequate energy to, for example, a communication module, under worst case conditions. Worst case conditions typically occur near the high end of the operating temperature range (typically 85 degrees Celsius). 
     Charging circuits typically protect components from overheating by varying the charging voltage with temperature. The charging voltage is allowed to track, but not to exceed, the maximum working voltage at any given temperature. Necessarily then, a circuit for adjusting the charging voltage according to temperature increases cost as well as printed circuit board (PCB) space requirements. It should be noted that embodiments of the present invention simplify the circuitry for charging super capacitor banks according to worst case voltage requirements while also extending the life of the super capacitors. Even so, functionality is not sacrificed. 
     The energy storage capacity of the super capacitor bank  140  is given by: 
     
       
         
           
             E 
             = 
             
               
                 1 
                 2 
               
                
               
                 CV 
                 2 
               
             
           
         
       
     
     E is the stored energy (joules), C is the capacitance (farads) of the super capacitor bank  140 , and V is the voltage (volts) across the super capacitor bank  140 . An increase in temperature reduces the maximum voltage rating of the super capacitor. Thus, the energy storage capacity of the super capacitor is lower at high temperatures. The energy capacity and voltage rating of the super capacitors are configured such that even with the reduced voltage at high temperatures, e.g., 85 degrees Celsius, adequate energy is stored and available for the particular application(s). The extra energy that is available at lower temperatures is not used or needed. The super capacitor bank  140  is charged for the worst case voltage irrespective of temperature, and thus, temperature compensated charging mechanisms are not necessary. 
     In one exemplary embodiment, a product is designed for a circuit to operate in a temperature range from −40 degrees C. to +85 degrees C., where the super capacitors have a maximum voltage rating of 2 V at +85 degrees C. and a maximum voltage rating of 2.5 V at −40 degrees C. The super capacitors are charged to 2 V throughout the temperature range, even though they have the capacity for charging to 2.5 V at −40 degrees C. The functionality of the circuit is not reduced because even with a temperature compensated charging circuit, the load calculations would be based on a worst case voltage of 2 V. Such load calculations would not utilize the extra charge that would be available at higher voltages. 
     In an exemplary embodiment, a worst case voltage rating for the super capacitors of 2 V at 85 degrees Celsius, would necessitate the charging voltage being maintained at or below 2 V irrespective of the actual temperature. Similarly, a maximum safe charging current of 1 A under worst case conditions would necessitate maintaining the current at or below 1 A irrespective of the actual temperature conditions. 
     Additional benefits provided by the system  100  of  FIG. 1  include elimination of temperature compensated charging circuitry and the associated costs. Further, the circuit is typically smaller and therefore occupies less PCB space. The reduction of voltage stress on the super capacitors results in increased life, since the super capacitors are typically required to operate at maximum charge voltage conditions only at 85 degrees C. At temperatures below 85 degrees C., the super capacitors operate below their maximum charge voltage conditions. It should also be noted that reduced super capacitor voltage results in less heat dissipation in downstream linear voltage regulators. 
       FIG. 2  is a block diagram illustrating an alternate embodiment of a system  200  for charging a super capacitor bank  140  using a regulated voltage  210 . Again, the super capacitor bank  140  is charged irrespective of temperature effects on the voltage ratings of the capacitors. The regulated voltage  210  provides the correct voltage as an input to the charger  110 . A voltage regulator (providing regulated voltage  210 ) implements the control functionality, thus eliminating the need for an on/off control. The regulated voltage  210  corresponds to the maximum voltage rating of the super capacitors at the highest operating temperature. The charger  110  suffices for charging the super capacitor bank  140 , so long as a regulated voltage  210  is supplied. 
     In one exemplary embodiment, the regulated voltage  210  provides the correct charging voltage, for the super capacitor bank  140 , as the input voltage to the charger  110 . The voltage regulator effectively implements the voltage control function, thus eliminating the need for an external voltage sensing circuit and an on/off controller. 
     The embodiment of  FIG. 2  is used, for example, in intelligent devices that have a regulated switching power supply feeding the charging circuit for the super capacitors. 
       FIG. 3  is a flowchart illustrating a method  300  for charging a super capacitor bank  140 . In step  310 , a charging current limit is set for the super capacitor bank  140 . The charging current limit is the maximum charging current for the super capacitor bank  140  under worst case temperature conditions within the operating temperature range. In step  320 , a charging voltage limit is set for the super capacitor bank  140 . The charging voltage limit is the maximum voltage for the super capacitor bank  140  under worst case temperature conditions within the operating temperature range. The charging current is turned on and, limited by the charging current limit, is provided to the super capacitor bank  140  at step  330 . The charging voltage is maintained below the charging voltage limit. In step  340 , if the super capacitor bank  140  has not yet been charged to the desired voltage, the charging continues. Once the super capacitor bank has been charged to the desired voltage, the charging current is turned off at step  360 . 
     Additionally, monitoring of the super capacitor bank  140  voltage continues after the super capacitors have achieved the desired voltage. After the super capacitor bank  140  has been charged to the desired voltage during step  340 , a test is made to determine whether the charging current is still turned on at step  350 . If the charging current is on at this point, then it is turned off at step  360  as above. If the charging current is off, then the method loops back to check the super capacitor bank  140  voltage at step  340 . So long as the voltage remains at the desired level, this loop continues. If the super capacitor bank  140  voltage drops below the desired level due to usage or other leakage, then the functionality of block  330  is activated to turn on the charging current and charge the super capacitor bank  140 . In addition to charging the super capacitor bank  140 , the voltage of the super capacitor bank  140  is also monitored for loss or leakage so that the super capacitor bank  140  is charged when the voltage drops below a desired level. 
     The foregoing description of the exemplary embodiments of the invention has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching. 
     The embodiments were chosen and described in order to explain the principles of the invention and their practical application so as to enable others skilled in the art to utilize the invention and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those skilled in the art to which the present invention pertains without departing from its spirit and scope. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.