Abstract:
A method of controlling charging of a battery ( 16 ) of a totally implantable auditory prosthesis and a control system ( 50 ) therefor. The method comprises determining a first charge related characteristic of the battery ( 16 ) such as the predetermined minimum amount of charge ( 58 ), and a second charge related battery characteristic such as the preset charge level or charge rate of the battery ( 16 ). The method further comprises detecting when a charge cycle of the battery ( 16 ) commences and monitoring where the charge level of the battery ( 16 ) is in relation to the first charge related battery characteristic when the charge cycle commences and adjusting the second charge related battery characteristic depending on the relationship between the charge level and the first charge related battery characteristic at the commencement of the charge cycle. The control system and method has the potential to increase the operational life of the battery ( 16 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. application Ser. No. 10/962,459 filed Oct. 13, 2004, which claims priority from Australian Provisional Patent Application No. 2003905571 filed on Oct. 13, 2003, the contents of which is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to an implanted auditory prosthesis. The invention relates particularly, but not necessarily exclusively, to a method of, and a control system for, charging of a battery of a totally implantable auditory prosthesis such as a cochlear implant or an implanted hearing aid. 
         [0004]    2. Related Art 
         [0005]    Hearing loss, which may be due to many different causes, is generally of two types, conductive and sensorineural. In some cases, a person may have hearing loss of both types. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss is often helped by use of conventional hearing aids which amplify sound so that acoustic information reaches the cochlea and the hair cells. 
         [0006]    In many people who are profoundly deaf, however, the reason for their deafness is sensorineural hearing loss. This type of hearing loss is due to the absence of, or destruction of, the hair cells in the cochlea, which convert acoustic signals into nerve impulses. These people are thus unable to derive suitable benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because there is damage to, or absence of, the mechanism for nerve impulses to be generated from sound in the normal manner. 
         [0007]    It is for this purpose that cochlear implant systems have been developed. Such systems bypass the hair cells in the cochlea and directly deliver electrical stimulation to the auditory nerve fibres, thereby allowing the brain to perceive a hearing sensation resembling the natural hearing sensation normally delivered to the auditory nerve. 
         [0008]    Typically, cochlear implant systems consist essentially of two components, an external component, commonly referred to as a processor unit and an internal, implanted component, commonly referred to as a stimulator/receiver unit, the latter receiving signals from the processor unit to provide the sound sensation to a user. 
         [0009]    The external component includes a microphone for detecting sounds, such as speech and environmental sounds, a speech processor that converts speech into a coded signal, a power source, for example a battery, and an external transmitter antenna coil. 
         [0010]    The coded signal output by the sound processor is transmitted transcutaneously to the implanted stimulator/receiver unit situated within a recess of the temporal bone of the user. This transcutaneous transmission occurs via the external transmitter antenna coil which is positioned to communicate with an implanted receiver antenna coil of the stimulator/receiver unit. Therefore, the communication serves two essential purposes; firstly to transmit, transcutaneously, the coded signal and, secondly, to provide power to the implanted stimulator/receiver unit. The transcutaneous link is, normally, in the form of an RF link, but other links have been proposed and implemented with varying degrees of success. 
         [0011]    The implanted stimulator/receiver unit includes, in addition to the receiver antenna coil that receives the coded signal and possibly power from the external processor component, a stimulator that processes the coded signal and outputs a stimulation signal to an intracochlear electrode assembly which applies the electrical stimulation via the basilar membrane to the auditory nerve producing a hearing sensation corresponding to the originally detected sound. 
         [0012]    Recently, the Applicant has developed a totally implantable cochlear implant where all the components, including the microphone, are implanted subcutaneously. This results in a more versatile system providing the recipient with greater freedom and ability to use the implant in what would previously have been regarded as adverse environments, eg. wet environments. The Applicant&#39;s implant is described in greater detail in PCT/AU01/00769 which is incorporated herein by reference. The implant is powered by an implantable rechargeable battery which receives charging signals, when required, transcutaneously via an external charging device and an implanted receiver antenna coil. Because the battery is part of an implanted system, there is a need to make the battery life as long as possible to reduce the frequency of explantation and/or re-implantation of the implant for the purposes of battery replacement. 
         [0013]    Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application. 
       SUMMARY 
       [0014]    Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. 
         [0015]    According to a first aspect of the invention, there is provided a method of controlling charging of a battery of a totally implantable auditory prosthesis, the method comprising: 
         [0016]    determining a first charge related battery characteristic; 
         [0017]    determining a second charge related battery characteristic; 
         [0018]    detecting when a charge cycle of the battery commences; and 
         [0019]    monitoring where the charge level of the battery is in relation to the first charge related battery characteristic when the charge cycle commences and adjusting the second charge related battery characteristic depending on the relationship between the charge level and the first charge related battery characteristic at the commencement of the charge cycle. 
         [0020]    For ease of explanation, the first charge related battery characteristic is sometimes referred to in this specification as a “safety margin” of the battery. This safety margin may be a predetermined minimum amount of charge contained in the battery. 
         [0021]    In one embodiment of the invention, the second charge related battery characteristic may be a preset charge level of the battery. The preset charge level may be determined such that it is lower than a maximum possible state-of-charge of the battery. The battery may be charged to this preset charge level and, in so doing, due to the fact that the battery is not consistently being charged to its maximum battery charge, the life of the battery may be extended, because side reactions are thereby minimised. 
         [0022]    When a charge cycle commences, the method may include monitoring the charge level of the battery relative to the safety margin. If the charge level at which charging commences is higher than the safety margin, the method may include lowering the preset charge level. This may occur each time that, when a charge cycle commences, the charge level of the battery is greater than the safety margin. If, however, upon commencement of a charge cycle the charge level of the battery is lower than the safety margin, the method may include, initially, increasing the preset charge level. 
         [0023]    Typically, the method may include setting the safety margin to be a voltage corresponding to a charge required for use of the implant for a predetermined period of time. The period of time may, for example, be a complete day, i.e. a 24 hour period. 
         [0024]    In a second embodiment of the invention, the second charge related battery characteristic may be a charge rate of the battery. As in the case of the first embodiment of the invention, the first charge related battery characteristic, in this embodiment, may be the safety margin. 
         [0025]    In this second embodiment of the invention, the method may include adjusting the rate of charge of the battery, rather than the degree of charge, depending on the charge level of the battery relative to the safety margin. Thus, if the charge level is lower than the safety margin, the method may include increasing the charge rate. Conversely, if the charge level is above the safety margin, the method may include decreasing the charge rate. 
         [0026]    In respect of both embodiments, the method may include controlling the amount of charge normally stored in the battery. The amount of charge normally stored in the battery may be controlled using feedback control. For example, on a daily basis, the preset charge level (in the case of the first embodiment of the invention) or the charge rate (in the case of the second embodiment of the invention) may be adjusted. The size of the adjustment may be related to the safety margin. Typically, an adjustment step size may be approximately 1%-10% of the safety margin, more particularly, about 3%-7% of the safety margin and, optimally, about 5% of the safety margin. 
         [0027]    Still further, the method may include adjusting the safety margin to take into account ageing of the battery. The safety margin may be automatically adjusted by characterising the battery as it ages. The method may therefore include creating a look-up table in an electronic memory of the implant that provides the safety margin point for each preset charge level or charge rate, as the case may be, so that, when the preset charge level or charge rate is updated, so is the safety margin. The table may be constructed so that the safety margin is equal to a fixed period of time. 
         [0028]    The time represented by the safety margin may also need to be adjusted depending on the recipient&#39;s needs. The recipient or a third party may indicate the time that they require the safety margin to be set at and, by knowing the characteristics of the battery and the current drawn by the specific recipient&#39;s data map, a voltage corresponding to the safety margin desired by the recipient may be included in the electronic memory of the implant. The safety margin may be downloaded along with the recipient&#39;s map into the memory of the implant. 
         [0029]    According to a second aspect of the invention, there is provided a control system for controlling charging of a battery of a totally implantable auditory prosthesis, the control system comprising: 
         [0030]    a controller for controlling charging of the battery, the controller being a programmable controller having data relating to a first charge related battery characteristic and a second charge related battery characteristic and for adjusting the second charge related battery characteristic dependent upon where a charge level of the battery is relative to the first charge related battery characteristic when a charging cycle commences; and 
         [0031]    a switching arrangement for switching a charging source into charging communication with the battery when charging of the battery is required. 
         [0032]    As in the case of the first aspect of the invention, the second charge related battery characteristic may, in accordance with one embodiment of the invention, be a preset charge level of the battery and may, in accordance with a second embodiment of the invention, be a charge rate of the battery. The first charge related battery characteristic may be the safety margin, as defined, of the battery. 
         [0033]    In the case of the first embodiment of this aspect of the invention, the switching arrangement may be a switch controlled by the controller. The charging source may include a radio frequency (RF) link which provides charge to the battery when required and/or power to the implant. The switch may be connected between the RF link and the battery and may be closed under the action of the controller when the battery of the implant requires charging. 
         [0034]    The control system may include an analog-to-digital converter for converting a battery voltage to a number to be fed to the controller. 
         [0035]    The controller may store numbers corresponding to voltages that, in turn, correspond to the preset charge level, a minimum battery charge and the safety margin. The relationship between charge level and voltage is not linear but is stable so that it can be measured for the battery. A simple look-up table may be stored in a memory of the controller to be used for voltage-to-charge conversion. 
         [0036]    The controller may detect when charging of the battery has commenced, i.e. when the switch has closed, so that the controller can determine if the preset charge level needs to be adjusted. This may be done by monitoring the RF link power periodically, for example, once per minute. 
         [0037]    In the case of the second embodiment of the invention, the switching arrangement may comprise a switching circuit. The switching circuit may comprise a voltage-to-current converter feeding to a current amplifier and mirror. The converter may include an operational amplifier, an output of which is connected to a switch means in the form of a field effect transistor (FET). The operational amplifier may have a first, “current control” input which causes the output of the amplifier to turn the FET on. The FET may be connected between a pair of resistors. When the FET is turned on, current flows through a first resistor connected to a source of the FET. All the current also flows through the second resistor connected to a drain of the FET. Current may flow through the first resistor until the voltages at inputs of the current amplifier are equal. 
         [0038]    Current flow through the second resistor may induce a voltage at an input of the current amplifier. A second switch means may be connected to an output of the current amplifier with the battery to be charged being connected to the second switch means. The second switch means may also be a FET. 
         [0039]    The components of the switching arrangement may be selected so that only a small voltage differential exists between the voltage from the RF link and the battery voltage. 
         [0040]    The charge current may be adjusted by changing the “current control” voltage. This may be done using an analog voltage or may be controlled by a digital number if a digital-to-analog converter is to be used. 
         [0041]    In the case of the second embodiment of the invention, the switching arrangement may replace the switch of the first embodiment of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    The invention is now described by way of example with reference to the accompanying drawings in which:— 
           [0043]      FIG. 1  shows a schematic representation of a totally implantable cochlear implant; 
           [0044]      FIGS. 2 and 3  show graphs of charge versus time for a method of charging a battery of the implant, in accordance with a first embodiment of the invention; 
           [0045]      FIG. 4  shows a part of a graph of charge versus time for a method of charging a battery of the implant, in accordance with a second embodiment of the invention; 
           [0046]      FIG. 5  shows a block diagram of a control system, in accordance with a first embodiment of the invention for charging a battery of the implant of  FIG. 1 ; and 
           [0047]      FIG. 6  shows a part of the control system, in accordance with a second embodiment of the invention, for charging the battery of the implant of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    In  FIG. 1  of the drawings, reference numeral  10  generally designates a totally implantable cochlear implant incorporating an example of the invention. The implant  10  is a self-contained unit implanted into a skull  12  of a recipient in an excavated region of a temporal bone of the recipient&#39;s skull  12 . 
         [0049]    As described in the Applicant&#39;s above referenced International Patent Application, the implant  10  is totally implanted within the recipient&#39;s skull and has no external or externally accessible components. 
         [0050]    The implant  10  has a hermetically sealed housing  14 . The housing  14  houses a battery  16  ( FIGS. 5 and 6 ). The housing  14  also contains electronic components of the implant  10 . The electronic components include a speech processor and an electronic memory in which recipient-specific parameters are stored. In the depicted embodiment, the battery  16  is a lithium-ion battery which is rechargeable a number of times. It will be appreciated that other rechargeable battery types could be utilised. 
         [0051]    A microphone  22  is mounted on the housing  14  for receiving external sound signals for transmission to the electronic components of the implant  10  for processing. 
         [0052]    The implant  10  includes a receiver antenna coil  24  having a central magnet  26 . The receiver antenna coil  24  is mounted externally of the housing  14  and is connected to the electronic components and the battery  16  of the implant  10  via external leads and a feedthrough (neither of which is shown). 
         [0053]    Stimulation signals generated by the speech processor of the implant  10  are fed to an electrode array  32  via a lead  34 . 
         [0054]    In practice, the implant  10  is implanted in the recipient&#39;s skull  12  in a recess formed in the temporal bone adjacent an outer ear  36  of the recipient. The electrode array  32  is mounted in the cochlea  38  of the recipient. When stimulation signals are received by the electrode array  32  these signals are transmitted to the basilar membrane  40  to stimulate the recipient&#39;s auditory nerve  42 . 
         [0055]    The housing  14  of the implant  10  is of a biocompatible material. More particularly, the housing  14  is a hermetically sealed titanium housing. Prior to implantation, the housing  14  is coated with a layer of silicone or parylene as a further protective measure for the implant  10 . 
         [0056]    As indicated above, the battery of the implant  10  is a lithium-ion battery. Lithium-ion batteries have the advantage that they are of extremely long life and can be recharged a number of times. However, such batteries can only be charged a finite number of times before they “wear out”. As a lithium-ion battery wears out, its charge storage capacity decreases. In practice, after approximately 1000 charge/discharge cycles, a lithium-ion battery may only be able to store 50% of the charge that it could in comparison to when the battery was new. 
         [0057]    The tendency of a battery to wear out is greater if the battery is always fully charged each time it undergoes a charging cycle. Further, the wear out rate increases if high currents are used for charging a battery or discharging it. 
         [0058]    Totally implantable cochlear implants are intended to be used with at least an external power pack for charging the battery. Often, the implant is used together with an external speech processor where the situation demands it, or it may not be necessary for an external speech processor, in which case it can be omitted. Typically, an external speech processor is used where the recipient finds himself or herself in adverse or difficult acoustic environments or at work but the implant may be used on its own without the external speech processor where cosmetic requirements and/or convenience dictate. 
         [0059]    The external speech processor incorporates a power pack so that, when it is being used and when required, the external speech processor can be used for charging the battery  16  of the implant  10 . The charging of the battery  16  takes place under the control of a control system  50  ( FIG. 5 ), also in accordance with the invention and which will be described in greater detail below. 
         [0060]    Optimisation of the charging of the battery  16  ensures that the number of times, during the life of the implant  10 , that the implant  10  needs to be explanted and/or re-implanted for battery replacement is reduced. 
         [0061]    In accordance with a first embodiment of the invention, the charging of the battery is controlled so that the battery is charged to a level below a maximum charge level  52  (see  FIGS. 2 and 3  of the drawings). 
         [0062]    In addition, a first battery charge related characteristic, referred to as a “safety margin” is selected. The safety margin, designated at  58  in  FIGS. 2 to 4  of the drawings, is a charge level greater than a minimum battery charge level  60 . The minimum battery charge level is a level below which the battery  60  will not function. 
         [0063]    A second battery charge related characteristic is also selected. In the case of the first embodiment of the invention, the second battery charge related characteristic is a preset charge level  62 . 
         [0064]    In the drawings, it is assumed that the person uses the external speech processor (not shown) for approximately 12 hours per day and runs the implant  10  on its internal battery  16  for the remaining 12 hours of the day. When the external speech processor is in use, the battery  16  is able to be charged. In  FIG. 2  of the drawings, two curves  64  and  66  are shown. In the case of the first curve  64 , during the day, the battery  16  is charged by the external speech processor until it reaches the preset charge level  62 . While the implant  10  is running on its internal battery  16 , during the night, the battery  16  discharges to a level  68 . It is to be noted that the level  68  is a charge level exceeding the safety margin  58 . Accordingly, the control system  50  reduces the preset charge level as shown at  70  so that, when the external speech processor is applied the following day, the battery  16  is charged to a lower level than the preceding day. When the battery charge reaches the preset charge level  62 , the control system  50  ceases charging of the battery  16  as shown by plateau  72  of the curve  64 . Because the internal battery  16  of the implant  10  is not being used, the battery charge remains at the plateau  72  until the external speech processor is disconnected whereafter the implant  10  again runs on the battery  16  causing it to discharge. 
         [0065]    The battery  16  discharges during the night to a level as shown at  74  in  FIG. 2  of the drawings. Once again, this level is greater than the safety margin  58  so that the following day, when the external speech processor is reapplied to start charging the battery  16 , the control system  58  resets the preset charge level to a yet lower level as shown at  76 . When the battery reaches charge level  76 , the control system  50  causes charging of the battery  16  to stop. 
         [0066]    Curve  66  shows the ideal situation where the battery always discharges to the safety margin  58 . As a result, there is no need to adjust the preset charge level  62 . 
         [0067]      FIG. 3  shows the situation where, during running of the implant on the internal battery  16  during the first night of a period, the battery discharges to a level below the safety margin, as shown at  78  in  FIG. 3  of the drawings. Because this has occurred, the following day, when the external speech processor is applied, the control system  50  adjusts the preset charge level  62  upwardly as shown at  80  in  FIG. 3  of the drawings. In this case, only one adjustment is needed as, in the following night, when the implant is running on its battery  16 , the battery  16  discharges to the safety margin  58  and no adjustment of the preset charge level  62  is thereafter required. 
         [0068]    Referring now to  FIG. 4  of the drawings, a further curve  82  of a second embodiment of the invention for controlling charging of a battery of the implant  10  is illustrated. With reference to  FIGS. 2 and 3  of the drawings, like reference numerals refer to like parts, unless otherwise specified. 
         [0069]    In this embodiment of the invention, rather than adjusting the degree of charge of the battery  16 , the rate of charge of the battery  16  is adjusted. Regardless of the embodiment of the invention, the charging of the battery takes place such that the battery is not charged to its maximum level  52 . 
         [0070]    During the day, when the external speech processor is applied, the battery  16 , should it require charging, is charged at a predetermined rate as shown at  84 . The rate at which the battery  16  is charged is dependent on where the charge level of the battery  16  was at the time of commencement of charging of the battery  16  at the start of a charging cycle. If the charge level of the battery  16  was below the safety margin, the charge rate of the battery  16  is increased until a desired charge level, as shown at  86 , is reached. 
         [0071]    If the charge level of the battery  16  is above the safety margin, the charge rate of the battery  16  is decreased. 
         [0072]    Referring now to  FIG. 5  of the drawings, the control system  50  is described in greater detail. The control system  50  forms part of the electronic components contained within the housing  14  of the implant  10 . 
         [0073]    As indicated above, the battery  16  is charged via a radio frequency (RF) link  88  from the external speech processor. The control system  50  includes a controller  90 . The controller  90  is microprocessor based and includes an electronic memory. The electronic memory stores the safety margin  58  and the preset charge level  62  therein. The battery voltage is monitored and is converted to a number by an analog-to-digital converter  92 . The number representing the battery voltage is fed to the controller  90  from the converter  92 . The system  50  works on a feedback basis and includes a feedback loop  94  controlling a normally open switch  96 . The switch  96  is controlled by the controller  90  so that, when the battery  16  is to be charged, the switch  96  is closed and when the battery  16  is not to be charged, the switch  96  is open. 
         [0074]    Power transmitted through the link  88  is also fed to the controller  90  so that the controller  90  detects when this “RF power” is present. When RF power is present via the link  88 , the controller  90  controls a toggle switch  98 , via a control line  100  to switch a wiper of the switch  98  to contacts  98 . 1  to connect a line  102  to the implant  10  via a line  104 . Power is therefore supplied from the link  88  to the implant  10  on the line  104 . When the controller  90  detects that the link  88  is absent, the controller  90  switches the wiper of the switch  98  to contacts  98 . 2  so that the implant  10  is powered from the on-board battery  16 . 
         [0075]    The preset charge level  62 , the safety margin  58  and the minimum battery charge level  60  are all stored as numbers in the controller  90 . The relationship between charge and battery voltage is not linear but is stable so that it can be measured for the battery  16  and a simple look-up table is used for voltage-to-charge conversions. 
         [0076]    The controller  90  detects when charging of the battery  16  has started so it can determine if the preset charge level  62  needs to be adjusted. The controller  90  monitors the link power periodically, for example, every one minute. 
         [0077]    In the case of the second embodiment of the invention, the switch  96  is replaced by a switching arrangement  106  ( FIG. 6  of the drawings). The switching arrangement  106  includes a voltage-to-current converter  108  connected to a current amplifier and mirror  110 . 
         [0078]    The converter  108  includes an operational amplifier  112  having a pair of inputs  112 . 1  and  112 . 2 . The input  112 . 1  is a current control input. A voltage at the current control input  112 . 1  causes the output of the operational amplifier  112  to turn on a switch in the form of a field effect transistor (FET)  114 . When the FET  114  turns on, current flows through a first resistor  116 , connected to a source of the FET  114  until the voltages at the inputs  112 . 1  and  112 . 2  of the amplifier are equal. All the current flowing through the first resistor  116  also flows through a second resistor  118  connected to a drain of the FET  114 . This induces a voltage at input  120 . 1  of an operational amplifier  120  of the amplifier  110 . When this occurs the operational amplifier  120  turns on a second switch in the form of an FET  122  until the voltage across a third resistor  124  equals that across the second resistor  118 . This then generates a charge into the battery  16 . 
         [0079]    The circuit  106  provides a stable charge current. If suitably low values of resistors  118  and  124  are selected, this allows the battery  16  to be charged with only a small voltage differential between the voltage from the link  88  and the battery voltage  16 . The charge current, i.e. the rate at which the battery  16  is charged, can be adjusted by adjusting the voltage applied at input  112 . 1  of the operational amplifier  112 . This may be done by using an analog voltage or may be controlled by a digital number if a digital-to-analog converter is used. 
         [0080]    As described above, the circuit  106  replaces the switch  96  in the system  50  in the case of the second embodiment of the invention, i.e. when the battery charge rate is adjusted rather than the degree of charge of the battery. 
         [0081]    In the case of the second embodiment of the invention, it is not necessary to monitor the maximum voltage to which the battery charges except to ensure that it is not overcharged. However, it is still necessary to measure the minimum voltage so that the amount of charge delivered is controlled. 
         [0082]    As described above, the control system  50  uses feedback to control the amount of charge normally stored by the battery. On a daily basis, the preset charge level  62 , or the charge rate, as the case may be, are adjusted. The degree of adjustment needs to be determined A large adjustment would correspond to a high loop gain. If the adjustment were too large, then the power charge cycle would become erratic due to the minor differences in day-to-day use. Conversely, if the adjustment were too small, the system  50  could not keep track of changing battery characteristics. As battery characteristics change slowly, a slow change is all that is required. 
         [0083]    A small adjustment ensures that the charge levels are based on the average use over an extended period of time rather than a single day, which may prove to be anomalous in use. Typically, a step size adjustment of approximately 5% of the safety margin  58  is regarded as satisfactory. 
         [0084]    As the battery  16  ages, its safety margin  58  gets smaller. In other words, the time taken by the system to discharge the battery from the safety margin  58  position to a point where the battery is fully discharged will decrease. In an extreme case, it is possible that the time could be so short that if the recipient had a particularly long time of powering the device from the external battery, the implanted battery  16  may run out of power and the implant  10  could stop working. As a result, the safety margin  58  itself is adjustable. Data relating to the safety margin  58  are included in the recipient&#39;s map parameters for their implant. 
         [0085]    The safety margin  58  could also be automatically adjusted. This is effected by characterising the battery  16  as it ages. A look up table is created that provides the safety margin  58  for each preset charge level  62  or charge rate, as the case may be, so that, when the preset charge level or charge rate is updated, so is the safety margin  58 . 
         [0086]    The time represented by the safety margin  58  might also need to be adjusted depending on the recipient&#39;s needs. The recipient or a third party can indicate the time in hours that they want the safety margin to be. By knowing the battery characteristics, i.e. the relationship between the charge held by the battery  16  and the battery voltage, and the current drawn by the specific recipient&#39;s map, the voltage corresponding to the safety margin  58  as desired by the recipient is included in an electronic memory of the implant  10 . This value is loaded along with the recipient&#39;s map using conventional implant programming techniques. 
         [0087]    In another manner of adjusting the safety margin, the voltage corresponding to the safety margin  58  is measured by asking the recipient to use the implant until it stops working because the battery  16  is fully discharged. A clock within the implant  10  causes the battery voltage to be measured periodically, say every hour, and is stored. From this data the voltage corresponding to the desired safety margin required by the recipient are determined 
         [0088]    It will be appreciated that, rather than automatically controlling the charging of the battery, the control of the charge of the battery could be manually effected. In such a case, the battery voltage is monitored and is telemetered out of the implant  10  to an external controller such as a hand-held computer or similar device. The external controller includes a manual display that the recipient can use to judge the amount of charge in the battery  16 . The external controller also includes commands that the recipient or a third party uses to select either the charge rate or the maximum amount of charge in the battery  16 . The system works in the same way as the automatic control system except that the recipient is required to adjust the charge parameters. 
         [0089]    It is a particular advantage of the invention that, because the battery  16  is not charged to its maximum charge level and is not charged at a maximum charging rate, the life of the battery  16  is considerably extended in comparison with batteries charged using conventional charging techniques. The advantage of this is that the number of times that the implant  10  needs to be explanted for battery replacement is considerably reduced. Thus, trauma to the recipient in having to undergo surgery is reduced. It will be appreciated that, particularly with a recipient of relatively advanced age, this has major benefits. As the method of charging the battery is implemented largely by software, the complexity of the implant is not significantly increased. As a result, the reliability of the implant should not be adversely affected. 
         [0090]    It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.