Patent Publication Number: US-6664763-B2

Title: System for managing power to an implanted device based on operating time, current drain and battery capacity

Description:
This a division of application Ser. No. 09/649,918 filed on Aug. 30, 2000 now U.S. Pat. No. 6,278,258, which is a division of application Ser. No. 09/299,858, filed Apr. 26, 1999, now U.S. Pat. No. 6,166,518. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and circuitry for safely regulating the charge and discharge cycles of implantable grade, rechargeable power sources, utilizing inductively coupled radio frequency energy. Patient safety and power source longevity are vastly improved by the method and circuitry of the system of the present invention. Such safety and longevity are obtained by the steps of: (1) measuring and recording, each charge/discharge cycle, to obtain the corrected capacity of the power source in order to calculate and display, upon interrogation, the remaining operating time of the implanted device, (2) providing within the implanted medical device circuitry for disconnecting the power source upon reaching a pre-selected low voltage in order to prevent deep-discharging the power source below safe limits, (3) providing circuitry for using variable constant current charge rates, (4) providing circuitry for switching to constant voltage to top-off the power source at the completion of the charge cycle, in order to prevent overcharging beyond safe limits, (5) providing within the implanted medical device circuitry for disconnecting the charging circuit from the power source upon the power source reaching a preselected high voltage level, (6) providing circuitry for full-time RF powered operation, in case of failure of the internal power source or for operation of the implanted medical device requiring extremely high power consumption (rather than being powered from the internal power source of the implanted device), (7) providing circuitry for transmitting to a remote receiver, via a telephone link, critical data that can be used by the physician and/or the device manufacturer to assess the performance and condition of the rechargeable power source and the Implantable Medical Device, and (8) providing circuitry for transmitting to the implantable medical device, via a telephone link, new operation parameter value(s). 
     2. Description of the Prior Art 
     A number of new, state-of-the-art, implantable medical devices are powered by a rechargeable electrical power source, such as a small volume, large value capacitor (known as a Super-capacitor), or a rechargeable electrochemical cell. These power sources need to be periodically recharged by an external Radio Frequency (RF) Transmitter via inductive coupling in a manner known in the art. 
     Each type of power source has a different charge and discharge methodology which must be faithfully followed to prevent permanent damage to the power source. In the prior art, the charge/discharge methodology has been factory preset via a specific hardware circuitry, suitable only for the specific power source used to power the implantable device. Furthermore, the prior art circuitry is incapable of properly regulating the charge/discharge cycles of new implantable-grade powersources, such as a Lithium-Ion cell battery. 
     Heretofore various battery power source charging systems have been proposed. Examples of some of these previously proposed systems are disclosed in the following U.S. patents: 
     
       
         
           
               
               
             
               
                   
               
               
                  U.S. Pat. No. 
                 Patentee 
               
               
                   
               
             
            
               
                  5,411,537 
                 Munshi et al. 
               
               
                 5,631,537 
                 Armstrong 
               
               
                 5,670,862 
                 Lewyn 
               
               
                 5,675,235 
                 Nagai 
               
               
                 5,764,030 
                 Gaza 
               
               
                 5,811,959 
                 Kejha 
               
               
                 5,818,199 
                 Beard 
               
               
                 5,880,576 
                 Nagai 
               
               
                   
               
            
           
         
       
     
     SUMMARY OF THE INVENTION 
     The present invention provides the method, software and hardware to (a) support the correct charge/discharge regimen for different types of power sources, (b) the capability of selecting, via software, the correct regimen of current and voltage limits, and (c) the capability of non-invasively up-grading the regimen, by down-loading, via a direct telemetry link or telephone link, new software revisions incorporating new improvements. 
     Some new state-of-the-art implantable medical devices are powered by a rechargeable Super-capacitor. One limitation of a capacitive power source is the small amount of charge that it can hold relative to an electrochemical rechargeable cell. In the case of a Super-capacitor powered Implantable Medical Device, when the device requires very high power consumption, its power source must be recharged very frequently. This makes the Super-capacitor impractical as a power source for use in high power consumption implantable medical devices. One obvious solution is to replace the Super-capacitor with an electrochemical cell. However, most implantable-grade, rechargeable electrochemical cells exhibit other critical limitations when used in a hermetically sealed implantable unit. These limitations must be surmounted during the design phase of the charge/discharge regulating circuit for the implanted power source. 
     One of the power sources most suitable for use in hermetically sealed, rechargeable implantable medical devices, is the Lithium-Ion cell. It offers many advantages, such as relatively high energy density (high capacity), no out-gassing during charge and discharge, high current delivery capabilities and relatively high output voltage. However, it also has some disadvantages, such as some loss of capacity with each recharge cycle (called “fade”), and the cell may be permanently damaged if allowed to be deeply discharged or overcharged. The continual loss of capacity (fade), requires the capability of measuring and up-linking (a) the corrected capacity value in mA-hrs, and (b) the power consumption of the Implanted Medical Device, in order to accurately calculate and display the operating time for the Implanted Medical Device. Having the capability of displaying the accurate operating time is extremely helpful to elderly patients for scheduling the next recharge session. 
     The power management system of the present invention provide a method and circuitry for measuring, on a real-time basis, the current power consumption and elapsed time since the last full charge. This data is used by a microcontroller to calculate (a) the actual capacity (corrected for fade) of the power source, and (b) the “operating time” for the Implantable Medical Device. This operating time can be up-linked by the Implantable Medical Device to the RF Transmitter/Charger where it can be displayed to the patient. Thus, the patent is provided, at any time, with an accurate prediction of the operating time as the cell&#39;s capacity slowly fades. 
     If desired, the work performed by the microcontroller in the power management system/module can be performed by a microcontroller of the Implantable Medical Device. In either event, the following functions are performed: 
     1. Detecting whether or not an RF sensor line has switched high or low. 
     2. Controlling the charging rate. 
     3. Non-invasively changing the charge high voltage limit. 
     4. Switching to a constant voltage mode to top off the charge on the power source. 
     5. Non-invasively changing the low voltage limit when the power source is disconnected during discharge. 
     6. Disconnecting the power source when it reaches the low voltage limit. 
     7. Reconnecting the power source upon sensing the transmission of RF energy. 
     8. Disconnecting the power source upon sensing a high temperature. 
     9. Reconnecting the power source when the temperature drops to a normal level. 
     10. Measuring the power consumption of the circuitry for the Implantable Medical Device. 
     11. Measuring the elapsed time since the last full charge. 
     12. Tracking the actual capacity of the power source. 
     13. Calculating the operating time left for the Implantable Medical Device. 
     It is an aspect or objective of the present invention to provide: (1) a method and circuitry for measuring the current drain of the Implantable Medical Device, (2) a method and circuitry for measuring the elapsed time since the last full charge, (3) a method for calculating the actual capacity of the power source (corrected for fade) based on the variable of current drain and the variable of elapsed time, (4) a method for calculating the operating time based on the variable of current drain and the variable of the actual capacity of the power source, (5) a method and circuitry for measuring the voltage of the power source, (6) a method and circuitry to signal the Implantable Medical Device when the power source voltage has reached a certain low value which requires disconnection from the power source, (7) a method and circuitry for disconnecting, during discharging, the power source from the Implanted Medical Device upon the power source reaching a certain low voltage in order to prevent deep discharging of the power source and subsequent damage, (8) circuitry for precisely limiting the charging voltage to the power source in order to prevent overcharging beyond safe limits, (9) a method and circuitry for disconnecting, during charging, the power source from the charging circuit upon the power source reaching a certain high voltage in order to prevent overcharging of the power source and subsequent damage, (10) circuitry for sensing when the electromagnetic waves being transmitted by the RF Transmitter/Charger induce a voltage level above a certain value at the RF Receiver of the Implanted Power Management System, (11) circuitry for reconnecting the power supply inputs of the Implanted Medical Device to the power source upon sensing this induced high voltage level, (12) a method and circuitry for monitoring the temperature of the power source during charging and discharging, (13) circuitry for disconnecting the charging circuitry from the power source if the temperature of the power source raises above a certain level during charging, (14) circuitry for reconnecting the charging circuitry to the power source when the temperature of the power source drops below a certain low value during charging, (15) circuitry for disconnecting the Implanted Medical Device from the power source if the temperature of the power source raises above a certain level during discharging, (16) circuitry for reconnecting the Implantable Medical Device to the power source when the temperature of the power source drops below a certain low value during discharging, (17) a method and circuitry for transmitting to a remote device, via a telephone link, data that can be used by the physician and/or the device manufacturer to assess the performance and condition of the rechargeable power source and the Implantable Medical Device, and (18) a method and circuitry for transmitting via a telephone link to, and setting in, the Implantable Medical Device, new operational parameter value(s). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is block plan view of one embodiment of the power management system of the present invention and shows a charge monitor, an RF transmitter, an Implanted Medical Device (Neural Stimulator) with exterior RF pick up coil and a telephone link. 
     FIG. 2 is block plan view of another embodiment of the power management system of the present invention, similar to the view shown in FIG. 1, and shows a charge monitor, an RF transmitter, an Implanted Medical Device (Neural Stimulator) without an exterior RF pick up coil and a telephone link. 
     FIG. 3 is a block plan view of a physician programmer and a telephone link for communicating with the power management system shown in FIG. 1 or in FIG.  2 . 
     FIG. 4 is a block electrical schematic circuit diagram of the power management module located inside the Implanted Medical Device. 
     FIG. 5 is a block schematic circuit diagram for a voltage regulator having an output whose voltage value is adjusted by a bus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     FIG. 1 illustrates the power management system  1  of the present invention. The system  1  utilizes an implanted RF receiving antenna  2  located outside of an Implantable Medical Device  4 . This RF receiving antenna  2  is used for capturing RF electrical energy  5  being transmitted by an RF Transmitting Antenna  6  located outside the human body tissue  7 . The Implanted Medical Device  4  is typically enclosed in a hermetic titanium housing  4 A in order to prevent intrusion of the body fluids which would permanently damage its sensitive electronic circuitry  8 . This titanium housing  4 A significantly attenuate and reduces the RF energy that can be coupled through the titanium enclosure  4 A. Therefore, in FIG. 1, the RF receiving antenna  2  is placed outside of the Implanted Medical Device  4  but inside the human body, using insulated wires in a cable  9  to bring the coupled RF energy to the Implanted Medical Device  4  in order to recharge its power source  10 . 
     FIG. 2 shows an embodiment of the Power Management System  1  without the external Rf antenna  2  and, instead, shows an RF receiving antenna  3  which is located inside of the Implantable Medical Device  4  for capturing the RF electrical energy  5  being transmitted by the RF transmitting antenna  6  located outside the human body. In this embodiment, a more compact Implantable Medical Device  4  is provided by placing the RF receiving antenna  3  inside the hermetic titanium enclosure  4 A of the Implanted Medical Device  4 . This compactness is achieved at the expense of reducing the amount of RF energy that can be coupled into the Implanted Medical Device  4 . This requires transmitting substantially higher levels of RF energy, significantly decreasing the longevity of the battery powering an RF Transmitter Unit  13 . 
     FIG. 4 is a block schematic circuit diagram of the circuitry  8  for a Power Management System Module  11  located inside the Implanted Medical Device  4 . The function of the Power Management Module  11  is to supervise the charging and discharging operations of the rechargeable powersource  10  powering the Implantable medical Device  4 , in order to improve the safety, efficacy and reliability of the rechargeable power source  10 . This Power Management Module  11  incorporates distinctive circuitry and methods for operating same to: (a) sense when the RF energy coupled into the RF Receiver exceeds the minimum level for safe operation of the Implantable Medical Device, (b) adjust the rate of charge to the power source  10 , (c) precisely regulate the voltage used to charge the power source, (d) non-invasively adjust the level of this charge voltage, (e) allow unidirectional current flow from the voltage regulator to the power source, (f) provide a low impedance path from the power source to the VDD connection supplying the operating power to a Power Management Module Controller  100  when the RF signal is not sensed, (g) sense the temperature of the power source, (h) disconnect the V-supply to the Implantable Medical Device  4  upon sensing a battery temperature which exceeds a safe value during discharging, (i) reconnect the V+ supply to the Implantable Medical Device upon the battery temperature dropping to a safe value during discharging, (j) disconnect a charging circuit  60 A from the power source  10  upon sensing a battery temperature exceeding a safe value during charging, (k) reconnect the charging circuit  60 A to the power source upon the battery temperature dropping to a safe value during charging, (I) disconnecting the charging circuit  60 A from the power source upon sensing a “full” voltage level at the power source  10 , (m) non-invasively adjust the value of this “full” voltage, and (n) reconnect the charging circuit  60 A to the power source when the RF energy coupled into the RF Receiver exceeds the minimum level for safe operation of the Implantable Medical Device  4 . 
     Referring again to FIG. 1, there is illustrated therein the main components of the Power Management System  1  used for maximum RF efficiency, where the RF receiving antenna  2  is placed outside the hermetic titanium enclosure  4 A of the Implanted Medical Device  4 . The Power Management System  1  is used to safely manage the charging and discharging of the power source  10  powering the Implantable Medical Device  4 . 
     On the right half of FIG. 1, the main components of the Power Management System comprise: (a) Charge Monitor  20  which is used to display the “remaining operating time” and “corrected capacity” of the power source  10  powering the Implantable Medical Device  4 , (b) an RF Transmitter Unit  13  used to generate the RF signal to be transmitted by antenna  6 , (c) plug  18  which is used to connect the RF Transmitter Unit  13  to antenna  6 , (d) RF Receiving Antenna  2  which is used to pick-up the RF energy  5  transmitted by antenna  6 , (e) cable  9  which are used to bring inside the Implantable Medical Device  4  the induced RF energy, (f) Power Management Module  11  which is used to safely manage the charge/discharge cycles of the power source  10  powering the Implantable Medical Device  4  and to collect performance data, and (g) rechargeable power source  10  used to power the Implantable Medical Device. 
     RF Transmitter Unit  13  can be used as a stand-alone device when the Implantable Medical Device  4  must be powered full-time via RF coupled energy. When used for full-time RF power, a switch  14  must be switched to the “RF” position. When the Implantable Medical Device  4  is to be powered by its own rechargeable power source  10 , RF Transmitter Unit  13  is used to generate the RF energy used to recharge the power source  10 . In this case, the switch  14  of RF Transmitter Unit  13  must be switched to the “self” position and a plug  17  of RF Transmitter Unit  13  must be plugged into a jack  29  of the Charge Monitor  20 . An A/C Transformer  45  can be used to power the Charge Monitor  20 , or alternatively the Charge Monitor  20  can be powered by its own internal battery. 
     Referring to the Charge Monitor  20 , a liquid crystal display  21  is used to display critical data, such as the “number of charge/discharge cycles” of the power source  10 . Push button  22  is used to scroll the display  21  to the next data, such as the “corrected capacity” of the power source  10 . The push button  22  “oper. time” is used to display the remaining operating time of the Implantable Medical Device  4  based on current power consumption and the present capacity of the power source  10 . Push button  24  is used by the patient to return the Implantable Medical Device  4  to safe “default” parameter value(s) when newly programmed values via the Phone Link do not work correctly for the patient. Push button  25  is used to abort a charge cycle to the power source  10 . Push button  26  is used to initiate a charge cycle for the power source  10 . Push button  27  is used to power-up or power-down the Charge Monitor  20 . 
     On the left side of FIG. 1, the remaining system components comprise: Phone Link coupler  33  which is used to convert digitally coded signals into audible distinctive “tones”. These converted “tones” are sent to a standard telephone  44  via jack  30 , plug  42  and cable  43 . Note that data communications between Phone Link coupler  33 , telephone  44  and the public telephone system is made via a cable  37 , plug  38  and jack  39  of telephone wall plug  40 . Also, note that data communications between Phone Link  33  and Charge Monitor  20  is made via a cable  32 , plug  31  and jack  28 . 
     Referring now to FIG. 2, there is illustrated therein the main components of the Power Management System  1  used for a maximum volumetric efficiency, where the RF Receiving Antenna  3  is placed inside the hermetic titanium enclosure  4 A of the Implanted Medical Device  4 , rather than outside. Other than this simple difference, the Power Management System shown in FIG. 2 is identical to that of FIG.  1 . 
     Referring now to FIG. 3, the other side of the telephone link circuit is completed by using a “Physician Programmer”  50  which is connected, via another Phone Link coupler  51 , to another telephone  52  having a connection established, via telephone  44  of FIG. 1, to the Charge Monitor  20  of FIG.  1 . 
     By pressing push button  53 , the physician or the manufacturer of the Implantable Medical Device  4  can retrieve data representing the condition of the rechargeable power source  10  and of the Implantable Medical Device  4 . 
     By pressing push button  54 , the physician can program new operating parameters values into the Implantable Medical Device  4 . It should be obvious that the circuitry within the Phone Link  51  can be incorporated into the Physician Programmer  50  to accomplish the same goal. 
     Referring now to FIG. 4, there is illustrated therein a block schematic circuit diagram of the circuitry  8  for the Power Management Module  11  used to safely manage the charge and discharge cycles of the power source  10  powering the circuitry  8  of the Implantable Medical Device Circuit  4 . 
     The following is a detailed narrative of the operation of each circuit component shown in FIG.  4 . 
     Coupling RF Energy into the Implantable Medical Device 
     On the top-left side of FIG. 4, there is shown a RF Receiver  55  comprising the RF Receiving Antenna  2  used to pick-up the transmitted RF energy  5 , capacitor  56  used for tuning the antenna  2  to the specific RF frequency to be transmitted, back-to-back Zener diodes  57  which are used to limit the maximum voltage that can develop across the antenna  2  in order to protect the charging circuit  60 A comprising a Voltage Regulator  61  from over-voltage, a bridge  58  used for rectifying the RF energy into a DC voltage, and a capacitor  59  used for smoothing the output  60  of the bridge  58  to a steady DC voltage. 
     Operation of the RF Sensor 
     On the top-middle of FIG. 4, there is shown an RF Sensor  67  which is used to sense when the voltage at line  60  has risen above a preset voltage indicating that the level of RF energy  5  is sufficiently high to provide the current required to charge power source  10 . When the voltage at line  60  reaches the reverse breakdown voltage of a Zener diode  68  connected as shown, sufficient voltage will develop at resistor  69  to turn on transistor  70 , causing line  72  to switch low. 
     A microcontroller  100  detects this signal change and responds by switching line  85  high which turns on transistor  87  and connects the power source  10  to the common ground. 
     Controlling the Charge Rate Using a Closed-loop Method 
     The Charge Rate Control  73  is used, under the supervision of microcontroller  100 , to regulate the constant current value used to charge the power source  10 . Microcontroller  100  applies a square wave at line  74  which is directed to the cathode of diode  75 . During each negative half-cycle, diode  75  becomes forward biased and some charge is injected into capacitor  77  through resistor  76 . However, during each positive half-cycle a smaller charge bleeds off from capacitor  77  through larger resistor  78  since diode  75  is reverse biased. The result is that a specific residual voltage develops at capacitor  77  due to the square wave at line  74 . The specific voltage value depends on the frequency and duty cycle of the square wave and the resistance ratio between resistors  76  and  78 . This residual voltage at line  82  drives transistor  79  in a constant current mode. 
     As transistor  79  sources current into the power source  10 , a voltage will develop across resistor  81 . This voltage is amplified by amplifier  83  and sampled by channel  4  of the A/D converter in micro-controller  100 . Therefore, a closed-loop charging method is created where the charge rate is precisely regulated within a wide range by microcontroller  100 . The charge rate is regulated by varying the frequency and/or duty cycle at line  74  until the desired current is measured by the A/D in the microcontroller  100 . This closed-loop method permits adjusting the charging rate to the specific value recommended by each manufacturer of the power source  10 , thus providing a universal charging method suitable for different types of power sources  10 . This closed-loop method, also permits an initial fast charge rate in order to quickly reach the minimum operating voltage of the power source  10  of the Implantable Medical Device  4  to enable therapy, and then switch to a lower rate which is more benign to the life of the power source  10 . 
     Also, since typically the Implantable Medical Device  4  incorporates a telemetry circuit to communicate with an external device, such as the Physician Programmer  50  of FIG. 3, the charge rate can be non-invasively changed after implant by down-loading new values via the Physician Programmer  50 . 
     Switching to a Constant Voltage Mode to Top-off the Cell 
     Once the power source  10  has reached a voltage close to its maximum rated voltage, charging is switched from constant current to constant voltage to preclude exceeding the maxim voltage recommended by the manufacturer. As an example, for a Lithium-Ion cell, the maximum value is typically 4.1 volts. For this example, microcontroller  100  will set the voltage regulator  61  to output 4.1 volts. Once the power source  10  has reached approximately 3.9 volts while charging at constant current, microcontroller  100  will fix line  74  high and line  64  low. This will turn off transistor  79  (constant current) and turn on transistor  65  (constant voltage), limiting the power source  10  to 4.1 volts when fully charged. 
     Disconnecting the Power Source to Avoid Deep Discharging 
     Microcontroller  100  incorporates a digital to analog converter having at least four channels: A/D 1 , A/D 2 , A/D 3  and A/D 4 . A/D 1  is used to monitor the voltage at the powersource  10 . During discharging of the power source  10 , when the voltage at line  95  reaches a preset low value, microcontroller  100  will initiate the following power-down protocol: 
     1. Microcontroller  100  will signal the circuitry  8  of the Implantable Medical Device  4  to perform the necessary housekeeping chores to prepare for a power shut-down. 
     2. The microcontroller  100  will “float” the line  85  if no RF energy is being sensed by RF Sensor  67  (line  72  is high). This will turn off transistor  87 , effectively disconnecting the power source  10  from the common ground. This is done to preclude damaging the power source  10  if allowed to be deeply discharged. Such will be the case for a Lithium-Ion cell. Note that the power is disconnected from the Power Management Module  11  and the circuitry  8  of the Implantable Medical Device  4 , thus removing all loads from the power source  10 . 
     Reconnecting the Power Source Upon Sensing the Transmission of RF Energy 
     As explained previously, RF Sensor  67  is used to sense when the level of RF energy  5  is sufficiently high to provide the current required to charge the power source  10 . When adequate proximity and alignment is achieved between the charging antenna  6  and receiving antenna  2  (or  3 ) of FIG. 1 (or FIG.  2 ), line  72  will switch low, and in response, microcontroller  100  will switch line  85  high, reconnecting power source  10  to the common ground, and getting it ready for charging. 
     Disconnecting the Power Source Upon Sensing a High Temperature at the Power Source During Discharge 
     On the bottom-right of FIG. 4, there is shown Temperature Sensor  98  whose output line  99  is connected to an A/D Converter channel A/D 3 . When the temperature of power source  10  is nearing an unsafe value which is a software loaded variable, microcontroller  100  will “float” line  104 , switching off transistor  103 . This effectively disconnects power source  10  from the circuitry  8  Implantable Medical Device  4 . Note that the power source  10  will continue to power the microcontroller  100  (through the line  80 , transistors  65  and the VDD supply) in order for the microcontroller  100  to sense when the temperature drops to a safe level by monitoring line  99 . 
     Reconnecting the Power Source when the Temperature Drops to a Safe Level 
     When the temperature of the power source  10  drops to a safe level, microcontroller  100  will switch line  104  high which will turn on transistor  103 , effectively reconnecting the power source  10  to the circuitry  8  of the Implantable Medical Device  4 . 
     Measuring the Power Consumption of the Implantable Medical Device 
     On the center right of FIG. 4, there is shown the Current Measurement circuit  88  which comprises transistor  94  and its control line  93 , voltage-dropping resistors  90  and  91 , averaging capacitor  92 , amplifier  89 , and output line  96 . As current is sourced into to the circuitry  8  of the Implantable Medical Device  4 , a voltage drop will develop across the resistance path formed by resistors  90  and  91 . This voltage drop is amplified by Amplifier  89  and directed to the A/D Converter channel A/D 2  in microcontroller  100 . Since Amplifier  89  has a gain of 100, if resistors  90  and  91  are assigned values of 1 and 9 Ohms, respectively, the voltage at line  96  will be 1 volt for a current drain of 10 mA (transistor  94  is switched on, shunting resistor  91 ). For lower current drains, microcontroller  100  will turn off transistor  94  to increase the voltage-dropping resistance to 10 Ohms. Therefore, high and low current drain scales are achieved. The circuitry  8  of the Implantable Medical Device  4  will communicate to microcontroller  100  the scale to be used depending on the parameter values presently being used by the Implantable Medical Device  4 . 
     Measuring the Elapsed Time Since the Full Charge 
     On the top center of FIG. 4 there is shown microcontroller  100  which is also used to count the elapsed time since the last full charge. When the RF Transmitting Antenna  6  of FIG. 1 is removed from the RF Receiving antenna  2  (or  3  in FIG.  2 ), RF Sensor  67  will sense this event causing line  72  to switch high. Microcontroller  100  will sense the rise of line  72  and will start counting the elapsed time in days, hours and minutes, using a typical software timing loop known in the art. 
     Tracking the Capacity of the Power Source as the Charge/Discharge Cycles are Used Up 
     The measured elapsed time from full charge to a full discharge in conjunction with the measured current drain, is used by microcontroller  100  to calculate the actual capacity of power source  10 . Therefore, the capacity value is corrected for the fading effects caused by each charge/discharge cycle. Note: A full discharge refers to a power source discharged only to the lowest voltage recommended by the manufacturer of power source  10 . In the case of a Lithium-Ion cell this low voltage is typically 3.0 volts. 
     Calculating the Operating Time of the Implantable Medical Device 
     The operating time from any point in the discharge curve of power source  10  to a full discharge, can be calculated by microcontroller  100  which measures (a) the average mA being consumed by the Implantable Medical Device  8 , (b) the elapsed time since the last charge, and (b) the actual capacity of power source  10 . The remaining operating time is calculated by: (1) multiplying the mA being consumed by the elapsed time in hours to arrive at the “consumed” capacity, (2) subtracting this “consumed” capacity from the “actual” (total) capacity to arrive at the “remaining” capacity, and (3) dividing the mA being consumed into the “remaining” capacity value of mA/hrs to arrive at the hours of operating time and dividing the answer by 24 to convert hours to days. Note that the power consumption of the Power Management Module  11  is insignificant (less than 3 uA) and therefore can be ignored in the calculation. 
     Referring now to FIG. 5, there is illustrated therein a block schematic circuit diagram of the Voltage Regulator  61  with output  62  whose voltage value is adjusted by a bus  66 . Microcontroller  100  controls the state of bus lines  106 ,  107 ,  108  and  109 . These lines in turn control the state of transistors  110 ,  111 ,  112  and  113 . These transistors are used to select the total value of resistance in the voltage sense loop for the voltage regulator  61 . By adjusting the ratio of resistor  114  to the combined resistance of resistors  106 ,  107 ,  108  and  109 , the output voltage of the voltage regulator  61  can be adjusted anywhere between 2.5 to 5.5 volts. This range covers the voltage required by most implantable grade, rechargeable power sources, including Lithium-Ion cell, Vanadium Oxide cell and a Super-capacitor. 
     From the foregoing description, it will be apparent that the method and system for power management of the present invention have a number of advantages, some of which have described above and others which are inherent in the invention. Also, it will be understood that modifications can be made to the method and system for power management of the present invention without departing from the teachings of the present invention. Accordingly, the scope of the invention is only to be limited as necessitated by the accompanying claims.