Abstract:
Implantable medical device adapted to provide a therapeutic output to a patient. A therapy module, operatively coupled to a battery, is adapted to provide the therapeutic output. A control circuit provides an action indicative of recharging the battery when the voltage of the battery reaches a recharge voltage wherein the recharge voltage is varied as the battery ages. Also a method of providing a therapeutic output to a patient using an implantable medical device having a battery having a voltage. An action indicative of recharging the battery is provided when the voltage of the battery reaches a recharge voltage. The recharge voltage is varied as the battery ages.

Description:
FIELD  
       [0001]     This invention is generally related to implantable medical devices and, in particular, to rechargeable implantable medical devices.  
       BACKGROUND  
       [0002]     Implantable medical devices are commonly used to treat patients suffering from a variety of medical conditions. Implantable medical devices can be used to treat any number of conditions such as pain, incontinence, movement disorders such as epilepsy and Parkinson&#39;s disease, and sleep apnea. Additional therapies appear promising to treat a variety of physiological, psychological, and emotional conditions.  
         [0003]     Such implantable medical devices include implantable drug infusion pumps and implantable electrical stimulators. Implantable drug infusion generally infuse a specific amount of a drug contained in a reservoir of the implantable drug infusion pump to the patient in accordance with a scheduled program assigned by a medical professional. Implantable electrical stimulators generally deliver an electrical stimulation to a nerve or other tissue of a patient, e.g., to control pain. In both cases, electrical power may be required to operatively drive the therapeutic componentry associated with each device, e.g., a stepper motor for a drug infusion pump and a pulse generator for an electrical stimulator. In addition, both types of implantable medical devices may also require electrical power for ancillary control and communication circuitry.  
         [0004]     An implantable battery is often used as a source of electrical power for an implantable medical device. It is also common to use a rechargeable battery in such an implantable medical device in order to achieve maximum life and small size of the implantable medical device as well as provide robust functions requiring an ample electrical power source. Such implantable medical devices depend upon periodic charging or recharging of the rechargeable battery or batteries in order to maintain continued function of the implantable medical device.  
         [0005]     Various techniques and procedures have been established to perform and achieve charging or recharging of an implanted battery of an implantable medical device typically through the use of electromagnetic coupling.  
         [0006]     A battery of a typical implantable medical device has a voltage when fully charged. As the battery of the implantable medical is depleted through use the voltage of the battery will typically decline. At a suitable point in time, the battery may then be charged or recharged, e.g., through such electromagnetic coupling, to restore the capacity of the battery.  
         [0007]     It is common to utilize the voltage of battery of an implantable medical device as a barometer as to when to charge or to recharge the battery. Typically, the voltage of the battery is monitored and when the voltage of the battery reaches a predetermined voltage, the recharge voltage, the battery is then suitably recharged. The implantable medical device, either alone or in conjunction with an external programmer or control device or monitor, typically signals the user, e.g., the patient, when or following the point in time when the recharge is reached in order that the patient may take appropriate steps to have the battery recharged.  
         [0008]     This recharge voltage is typically selected in order to provide the patient with a reasonable useful life, i.e., reasonable functional time between charges, as well as to provide a safety margin or time between the time at which the recharge voltage is reached and a time when the voltage of the battery declines to a point at which the implantable medical device is no longer functional. The time period between the point in time when the recharge voltage is reached and the point in time when the implantable device may no longer function is typically referred to as the compliance period, i.e., the period of time in which the patient should comply with recharging of the battery without risking operation of the device from a low voltage from the battery. The compliance period generally should be long enough to allow the patient a reasonable period of time to accomplish recharging. For example, if special equipment is needed for recharging, then the compliance period should be reasonably long enough to allow the patient in normal situations to obtain and utilize the special equipment needed for recharging.  
         [0009]     Generally, the voltage of a fully charged battery when new can be determined within reasonable accuracy. Also, the recharge voltage can be selected for a new battery to achieve a reasonable operational period, i.e., the time period between charges, and also to allow for a reasonable compliance period. It is recognized that a lower recharge voltage will generally provide a longer operational period but a shorter compliance period. The converse is also true. A higher recharge voltage will generally provide a shorter operational period but a longer compliance. Thus, the recharge voltage is generally set at a compromise level to ensure a reasonable operational period and a reasonable compliance period.  
         [0010]     However, as the battery of an implanted medical device ages the fully charged voltage will typically decline and, more importantly, the capacity of the battery between its “fully charged” state and its “needs recharging” state may be drastically reduced. For example, a new battery may have a 300 ampere-hour capacity while an aged battery may only have a 30 ampere-hour capacity. Thus an aged but “fully charged” battery may have a lower fully charged capacity and voltage than a “fully charged” battery that is brand new. Thus, the capacity and voltage between the “fully charged” state and the time at which the battery reaches its recharge voltage may be significantly less for an aged battery. As a result, the operational period of an aged battery may be significantly less than the operational period of a brand new battery.  
         [0011]     Also, as the battery of an implanted medical device ages the time between the time the battery recharge voltage is reached and the time the lowest operational voltage is reached may also decrease. As a result, the compliance period of an aged battery may be significantly less than the compliance period of a brand new battery.  
       SUMMARY  
       [0012]     As the battery of implanted medical device ages, the operational period or the compliance period of an implanted medical device may also vary.  
         [0013]     As the operational period becomes shorter, the patient is required to recharge the battery more frequently. Such more frequent rechargings increase the number of cycles of the battery and may actually cause the battery to age more quickly exacerbating the issue.  
         [0014]     As the compliance period becomes shorter, it is more crucial for the patient to promptly recharge the battery of the implanted medical device. In order to compensate for shorter compliance periods with aged batteries, the compliance period for new batteries may be lengthened to account for such shortening ensuring that even under the worst case conditions an adequate compliance period will be maintained. Unfortunately, maintaining a worst case compliance period may shorten an otherwise longer operational period requiring the patient to recharge the battery more frequently. Such more frequent rechargings increase the number of cycles of the battery and may actually cause the battery to age more quickly also exacerbating the issue.  
         [0015]     The recharge voltage may be varied as the battery of an implantable or implanted medical device ages in order to adjust the operational period, the compliance period, or both. For example, the recharge voltage may be decreased over time as the battery of the implantable medical device ages in order to maintain the operational period. Conversely, the recharge voltage may be increased over time as the battery of the implantable medical ages in order to maintain the compliance period. Alternatively, the recharge voltage may be otherwise varied over time in order to achieve other goals related to operational period and/or compliance period or other objectives.  
         [0016]     In an embodiment, the present invention provides an implantable medical device adapted to provide a therapeutic output to a patient. A therapy module, operatively coupled to a battery, is adapted to provide the therapeutic output. A control circuit provides an action indicative of recharging the battery when the voltage of the battery reaches a recharge voltage wherein the recharge voltage is varied as the battery ages.  
         [0017]     In an embodiment, the recharge voltage is decreased in value as the battery ages.  
         [0018]     In an embodiment, the battery has a fully charged capacity which declines as the battery ages and wherein the recharge voltage is varied in order to tend to equalize a value of a difference in voltage between a fully charged voltage and the recharge voltage as the battery ages.  
         [0019]     In an embodiment, the difference is maintained to be approximately equal.  
         [0020]     In an embodiment, the battery has a fully charged capacity which declines as the battery ages and wherein the recharge voltage is varied in order to tend to equalize an amount of charge imparted to the battery during recharge between a fully charged voltage and the recharge voltage.  
         [0021]     In an embodiment, the charge is maintained as approximately equal as the battery ages.  
         [0022]     In an embodiment, the action comprises indicating that the battery should be recharged.  
         [0023]     In an embodiment, the voltage is varied such that a time between a first recharge of the battery and a second recharge of the battery is made more uniform as the battery ages.  
         [0024]     In an embodiment, the time is approximately equal.  
         [0025]     In an embodiment, the recharge voltage is increased in value as the battery ages.  
         [0026]     In an embodiment, the battery has an off voltage below which the therapy module of the implantable medical device may cease to operate (either completely or partly), wherein the implantable medical device has a patient recharge compliance period commencing upon the action and ending when the voltage of the battery reaches the off voltage and wherein the recharge voltage is varied such that the patient recharge compliance period is maintained to be more uniform as the battery ages.  
         [0027]     In an embodiment, the battery has an off voltage below which the therapy module of the implantable medical device may cease to operate (either completely or partly), wherein the implantable medical device has a patient recharge compliance period commencing upon the action and ending when the voltage of the battery reaches the off voltage and wherein the recharge voltage is varied in order to tend to equalize an amount of charge remaining in the battery during the patient recharge compliance period.  
         [0028]     In an embodiment, the charge is maintained as approximately equal as the battery ages.  
         [0029]     In an embodiment, the recharge voltage is varied in order that the patient recharge compliance period is maintained to be approximately equal as the battery ages.  
         [0030]     In an embodiment, the recharge voltage is decreased in value as the battery ages in an early stage of a life of the battery and wherein the recharge voltage is increased in value as the battery ages in a later stage of the life of the battery.  
         [0031]     In an embodiment, the recharge voltage is increased in value as the battery ages in an early stage of a life of the battery and wherein the recharge voltage is decreased in value as the battery ages in a later stage of the life of the battery.  
         [0032]     In an embodiment, the present invention provides a method of providing a therapeutic output to a patient using an implantable medical device having a battery having a voltage. An action indicative of recharging the battery is provided when the voltage of the battery reaches a recharge voltage. The recharge voltage is varied as the battery ages.  
         [0033]     In an embodiment, the recharge voltage in the varying step is decreased in value as the battery ages.  
         [0034]     In an embodiment, the battery has a fully charged capacity which declines as the battery ages and wherein the recharge voltage is varied in order to tend to equalize a value of a difference in voltage between a fully charged voltage and the recharge voltage as the battery ages.  
         [0035]     In an embodiment, the difference is maintained to be approximately equal.  
         [0036]     In an embodiment, the battery has a fully charged capacity which declines as the battery ages and wherein the recharge voltage is varied in order to tend to equalize an amount of charge imparted to the battery during recharge between a fully charged voltage and the recharge voltage.  
         [0037]     In an embodiment, the charge is maintained as approximately equal as the battery ages.  
         [0038]     In an embodiment, the action step comprises indicating that the battery should be recharged.  
         [0039]     In an embodiment, the voltage is varied such that a time between a first recharge of the battery and a second recharge of the battery is made more uniform as the battery ages than it would be with a constant recharge voltage.  
         [0040]     In an embodiment, the time is approximately equal.  
         [0041]     In an embodiment, the recharge voltage is increased in value as the battery ages.  
         [0042]     In an embodiment, the battery has an off voltage below which the therapy module of the implantable medical device may cease to operate properly, wherein the implantable medical device has a patient recharge compliance period commencing upon the action and ending when the voltage of the battery reaches the off voltage and wherein the recharge voltage is varied such that the patient recharge compliance period is maintained to be more uniform as the battery ages.  
         [0043]     In an embodiment, the battery has an off voltage below which the therapy module of the implantable medical device may cease to operate properly, wherein the implantable medical device has a patient recharge compliance period commencing upon the action and ending when the voltage of the battery reaches the off voltage and wherein the recharge voltage is varied in order to tend to equalize an amount of charge remaining in the battery during the patient recharge compliance period.  
         [0044]     In an embodiment, the charge is maintained as approximately equal as the battery ages.  
         [0045]     In an embodiment, the recharge voltage is varied in order that the patient recharge compliance period is maintained to be approximately equal as the battery ages.  
         [0046]     In an embodiment, the recharge voltage is decreased in value as the battery ages in an early stage of a life of the battery and wherein the recharge voltage is increased in value as the battery ages in a later stage of the life of the battery.  
         [0047]     In an embodiment, the recharge voltage is increased in value as the battery ages in an early stage of a life of the battery and wherein the recharge voltage is decreased in value as the battery ages in a later stage of the life of the battery. 
     
    
     DRAWINGS  
       [0048]      FIG. 1  is a block diagram of an implantable medical device;  
         [0049]      FIG. 2  illustrates a voltage versus percent of depth of discharge of an exemplary battery for an implantable medical device illustrating particular voltages ranges;  
         [0050]      FIG. 3  illustrates a voltage versus percent of depth of discharge of an exemplary battery for an implantable medical device;  
         [0051]      FIG. 4  is a flow diagram illustrating varying the recharge voltage as the battery of an implantable medical device ages;  
         [0052]      FIG. 5  is a flow diagram illustrating decreasing the recharge voltage as the battery of an implantable medical device ages;  
         [0053]      FIG. 6  is a flow diagram illustrating increasing the recharge voltage as the battery of an implantable medical device ages;  
         [0054]      FIG. 7  is a flow diagram illustrating varying the recharge voltage of the battery of an implantable medical device in order to tend to equalize a recharge parameter;  
         [0055]      FIG. 8  is a flow diagram illustrating varying the recharge voltage of the battery of an implantable medical device in order to tend to equalize the difference between the voltage of a fully charged battery and the recharge voltage;  
         [0056]      FIG. 9  is a flow diagram illustrating varying the recharge voltage of the battery of an implantable medical device in order to tend to equalize the amount of charge imparted to the battery during recharge;  
         [0057]      FIG. 10  is a flow diagram illustrating varying the recharge voltage of the battery of an implantable medical device in order to tend to make the time between recharge more uniform;  
         [0058]      FIG. 11  is a flow diagram illustrating varying the recharge voltage of the battery of an implantable medical device in order to tend to maintain the patient compliance period;  
         [0059]      FIG. 12  is a flow diagram illustrating varying the recharge voltage of the battery of an implantable medical device in order to tend to maintain an amount of charge in the battery during the patient compliance period;  
         [0060]      FIG. 13  is a flow diagram illustrating decreasing the recharge voltage during an early stage of life of the battery of an implantable medical device and increasing the recharge voltage during a later stage of life of the battery of an implantable medical device; and  
         [0061]      FIG. 14  is a flow diagram illustrating increasing the recharge voltage during an early stage of life of the battery of an implantable medical device and decreasing the recharge voltage during a later stage of the life of the battery of an implantable medical device. 
     
    
     DETAILED DESCRIPTION  
       [0062]     The entire contents of U.S. Pat. No. 6,820,019, Kelly et al, Device and Method For Determining and Communicating the Remaining Life of a Battery in an Implantable Neurological Tissue Stimulating Device, filed Jul. 31, 1999, issued Nov. 16, 2004, is hereby incorporated by reference.  
         [0063]     The entire contents of U.S. patent application Ser. No. 09/562,221, Torgerson et al, Battery Recharge Management For an Implantable Medical Device, filed Apr. 28, 2000, is hereby incorporated by reference.  
         [0064]      FIG. 1  is a block diagram of implantable medical device  10 . Implantable medical device  10  is powered by battery  12  driving both therapy module  14  and control circuit  16 .  
         [0065]     Therapy module  14  is conventional and produces a therapeutic output  18 . Examples of therapeutic output  18  include, but are not limited to, therapeutic substance infusion and/or electrical stimulation. Battery  12  is also a rechargeable battery and is conventional. Examples of battery chemistries include, but are not limited to, lithium ion, lithium polymer, nickel cadmium and nickel metal hydride.  
         [0066]     Control circuit  16  may be conventional except as it relates to controlling operation and/or response to changing battery voltages and conditions as described below. In general, control circuit  16  may consist of a general purpose microprocessor programmed in order to accomplish the tasks identified below. In addition to those tasks identified below, control circuit  16  may perform conventional command, control and communication functions commonly associated with implantable medical devices.  
         [0067]      FIG. 2  is a graphical illustration of the voltage of battery  12  versus percent depth of discharge. While the graph of  FIG. 2  is illustrated in terms of percent of discharge, it is to be recognized and understood that the graph also generally represents an illustration of battery voltage versus time since over time the percent of discharge of battery  12  will increase.  
         [0068]     As shown in  FIG. 2 , a fully charged, i.e., zero percent (0%) discharged, battery  12  has a voltage of approximately 4.1 volts. The voltage of battery  12  gradually discharges generally linearly during operation of implantable medical device  10  until reaching approximately the ninety percent (90%) discharge point on the curve. At this point, battery  12  has a voltage of approximately 3.6 volts. As battery  12  continues to discharge from ninety percent (90%) toward one hundred percent (100%) discharge, the voltage of battery  12  drops significantly. The normal operating range of battery  12  is from a fully charged 4.1 volts to approximately 3.52 volts, at approximately the ninety percent (90%) discharged point in time, also known as the recharge voltage or therapy off voltage.  
         [0069]     The recharge interval, or the operational period  20 , is the time period between the fully charged 4.1 volts, zero percent (0%) discharged, point to the 3.52 volts, recharge voltage, approximately ninety percent (90%) discharged, point.  
         [0070]     The patient compliance period  22  is measured from the 3.52 volt recharge voltage, approximately ninety percent (90%) discharged, point to the POR voltage, or power-on-reset voltage, of 1.8 volts, approaching one hundred percent (100%) discharged. Below 1.8 volts, operation of implantable medical device  10  may not be assured. Thus, the patient should have battery  12  recharged at least by the end of the patient compliance period  22 .  
         [0071]     As battery  12  ages and its operational characteristics fade over time, the patient compliance period  22  between the recharge voltage of 3.52 volts and the POR voltage of 1.8 volts decreases. This effect is illustrated by referring to the illustration of  FIG. 3 . Curve  24  in  FIG. 3  illustrates the voltage discharge characteristics of an exemplary new battery  12 . As shown in  FIG. 2 , new battery  12  has a fully charged voltage of 4.1 volts and a recharge voltage of 3.52 volts at point  26 . As battery  12  ages, battery  12  will not quite reach a fully charged voltage of 4.1 volts but the fully charged voltage of an aged battery  12  may be somewhat less, perhaps 4.0 volts as shown by curve  28 . Curve  28  illustrates that the recharge voltage of 3.52 volts will be reached earlier at point  30  resulting in a shorter operational period. Similarly, the time period between recharge voltage and POR voltage for curve  28  representing an aged battery  12  is also shorter.  
         [0072]     Lithium ion battery chemistry has fade, i.e., aging, characteristics that can be split into two fade mechanisms. The first, calendar fade, is approximately six percent (6%) per year and is generally constant over the life of the battery. The second, cycle fade, is approximately 0.003 percent (0.003%) per year based upon normal usage and occurs each time battery  12  is fully discharged. These two fade characteristics help determine how recharge voltage should be varied to improve the operation of implantable medical device  10 .  
         [0073]     Since calendar fade can be approximately six percent (6%) per year, recharge voltage can be decreased by a specific amount over time, e.g., by decreasing the recharge voltage by about six percent (6%) every year. This decrease could be done at a single time each year or it could be split up into an interval of 0.5% per month or any other interval.  
         [0074]     In a patient recharged implantable medical device  10  approximately once per week, battery  12  would be recharged approximately fifty-two (52) times each year resulting in an additional 0.16% of cycle fade every year. For this example, recharge voltage could be reduced by approximately 6.16% per year to account for both calendar fade and cycle fade.  
         [0075]     Further, implantable medical device  10  could keep track of the number of recharge cycles encountered and adjust the recharge voltage based upon the actual number of recharge cycles. If the patient recharges battery  12  of implantable medical device daily, the recharge voltage could change more quickly to account for the additional cycle fade. Cycles could be calculated by using therapy settings and the battery capacity or the amount of time spent charging battery  12  and the battery capacity, for example.  
         [0076]     These adjustments decrease the recharge voltage over time as battery  12  ages in order to tend to maintain the operational period  20  of implantable medical device  10  over time. Maintaining operational period  20  relatively constant over time provides an implantable medical device  10  that is more consistent and easier to operate since recharging won&#39;t be required at increasingly shorter intervals.  
         [0077]     Recharge voltage may also be varied by increasing the recharge voltage over time as battery  12  ages. As battery  12  ages, the patient compliance period  22  also decreases. A decreasing patient compliance period  22  may be problematic as patients have a shorter and shorter time in which to recharge battery  12  once receiving a recharge warning. Increasing the recharge voltage over time as battery  12  ages would tend to maintain the patient compliance period  22  over time and enable the patient to have a consistent time over which to recharge battery  12 . A consistent patient compliance period  22  may provide the patient with enough time to recharge battery  12  before battery  12  is damaged or implantable medical device  10  ceases to function. This may be important even if battery  12  is zero-volt compliant and is not damaged since implantable medical device  10  may enter POR and could not communicate until battery  12  is charged to a certain level. Implantable medical device  10  may lose settings stored in volatile memory and would not be able to communicate until battery  12  is charged to a certain level.  
         [0078]     As above, recharge voltage could be done at a constant rate or could be increased as a function of the number recharge cycles to which battery  12  is subjected.  
         [0079]      FIG. 4  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then varied ( 410 ) to account for aging of battery  12  over time. Such variation of the recharge voltage could be as described above or differently in order to alter the operational period  20  and/or patient compliance period  22  of implantable medical device  10 .  
         [0080]      FIG. 5  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then decreased ( 510 ) to account for aging of battery  12  over time. Decreasing the recharge voltage over time will tend to maintain operational period  20 . In an embodiment, the recharge voltage is decreased to maintain operational period  20  of implantable medical device  10  approximately constant.  
         [0081]      FIG. 6  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then increased ( 610 ) to account for aging of battery  12  over time. Increasing the recharge voltage over time will tend to maintain patient compliance period  22 . In an embodiment, the recharge voltage is increased to maintain patient compliance period  22  of implantable medical device  10  approximately constant.  
         [0082]      FIG. 7  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then varied ( 710 ) to tend to equalize a recharge parameter. Recharge parameters that may tend to be equalized include, but are not limited to, the operational period  20  and/or the patient compliance period  22 . In an embodiment, the recharge voltage is varied to maintain a recharge parameter of implantable medical device  10  approximately constant.  
         [0083]      FIG. 8  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  shouldS be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then varied ( 810 ) to tend to equalize the difference in voltage between a fully charged voltage of battery  12  and the recharge voltage of battery  12 . In an embodiment, the recharge voltage is varied to maintain the difference in voltage between a fully charged voltage of battery  12  and the recharge voltage of battery  12 .  
         [0084]      FIG. 9  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then varied ( 910 ) to tend to equalize the amount of charge imparted to battery  12  during recharge. In an embodiment, the recharge voltage is varied to maintain the amount of charge imparted to battery  12  during recharge.  
         [0085]      FIG. 10  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then varied ( 1010 ) to tend to equalize the time between recharge cycles of implantable medical device  10 . This is similar to maintaining the operational period  20 . In an embodiment, the recharge voltage is varied to maintain the time between recharge cycles of implantable medical device  10 .  
         [0086]      FIG. 11  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then varied ( 1110 ) to tend to equalize the patient compliance period  22 . In an embodiment, the recharge voltage is varied to maintain the patient compliance period  22 .  
         [0087]      FIG. 12  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . The recharge voltage is then varied ( 1210 ) to tend to maintain the amount of charge remaining in battery  12  during the patient compliance period  22 . In an embodiment, the recharge voltage is varied to maintain the amount of charge remaining in battery  12  during the patient compliance period  22 .  
         [0088]      FIG. 13  is a flow chart illustrating an embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . If ( 1310 ) battery  12  is in an early stage of battery life, e.g., battery  12  is relatively new, then the recharge voltage is decreased ( 1312 ) in order to tend to maintain operational period  20 . However, as battery  12  ages and patient compliance period  22  becomes increasingly shorter, there may come a time during the life of battery  12  that maintaining the patient compliance period  22  may become more important than maintaining the operational period  20 . If then ( 1314 ) battery  12  is in a later stage of battery life, e.g., battery  12  is relatively used, then the recharge voltage is increased ( 1316 ) in order to tend to maintain patient compliance period  22 .  
         [0089]      FIG. 14  is a flow chart illustrating a converse embodiment of the invention. The voltage of battery  12  in monitored ( 110 ). An action indicative of battery recharge is initiated ( 112 ) when the voltage of battery  12  reaches the recharge voltage. As an example, an alarm or other indication could be issued ( 114 ) warning the user that battery  12  should be recharged. The patient would know that implantable medical device is then entering the patient compliance period  22 . If ( 1410 ) battery  12  is in an early stage of battery life, e.g., battery  12  is relatively new, then the recharge voltage is increased ( 1412 ) in order to tend to maintain patient compliance period  22 . However, as battery  12  ages and operational period  20  becomes increasingly shorter, there may come a time during the life of battery  12  that maintaining the operational period  20  may become more important than maintaining the patient compliance period  22 . If then ( 1414 ) battery  12  is in a later stage of battery life, e.g., battery  12  is relatively used, then the recharge voltage is decreased ( 1416 ) in order to tend to maintain operational period  20 .  
         [0090]     The above description may cover only some of the possible ways in which the recharge voltage may be varied in order to advantageously alter the operational characteristics of implantable medical device  10 . It is to be recognized and understood that the recharge voltage may be varied in other ways within the contemplation of the present invention in order to alter operational characteristics of implantable medical device  10  in other ways.  
         [0091]     Thus, embodiments of the invention are disclosed. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.