Abstract:
A battery pack topology wherein the battery pack has multiple battery sub-stacks electrically connected in parallel such that the capacity of each battery sub-stack may be utilized but one is reduced unequally as to the others. As a result, one battery sub-stack will reach a point of failure before the other, which causes a drastic, observable change in the output voltage of the battery pack, but provides sufficient reserve capacity to permit a user of a device, such as an AED, having the battery pack to be notified in a timely fashion of the need to replace the battery pack.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to automated external defibrillators, and, more specifically, to a battery pack for powering the device. 
       BACKGROUND OF THE INVENTION 
       [0002]    External defibrillators are emergency medical devices designed to supply a controlled electric shock (i.e., therapy) to a person&#39;s (e.g., victim&#39;s) heart during cardiac arrest. This electric shock is delivered via pads that are electrically connected with the external defibrillator and in contact with the person&#39;s body. 
         [0003]    To provide a timelier rescue attempt for a person experiencing cardiac arrest, some external defibrillators have been made portable, by utilizing battery power (or other self-contained power supplies). In addition, many portable external defibrillators have programming to make medical decisions making possible operation by non-medical personnel. 
         [0004]    These portable external defibrillators, commonly known as automated external defibrillators (AEDs), including automatic and semi-automatic types, have gained acceptance by those outside the medical profession and have been deployed in myriad locations outside of traditional medical settings. Due to the life saving benefits of AEDs, more and more non-medical users are purchasing and deploying AEDs in their respective environments. This allows for a rescue attempt without the delay associated with bringing the person to a medical facility, or bringing a medical facility to the person (e.g., a life support ambulance). 
         [0005]    Individuals as well as businesses are purchasing and deploying AEDs. As time is of the essence during any rescue attempt, multiple AEDs may be purchased by any particular individual or user to allow placement at multiple locations. In the case of an individual, this could be on several floors of a home, and in the case of a business, this could be for placement throughout a facility (e.g., factory, office building, or large retail center). Thus, regardless of where the victim is within the home/facility, access to an AED would only be seconds, or minutes, away. 
         [0006]    AEDs rely on batteries to provide power. More precisely, AEDs rely on battery packs that have battery stacks, which contain multiple batteries (i.e., cells). To assure that the battery pack is capable of meeting the power demands of the AED, the capacity of the battery pack is continually assessed. 
         [0007]    Generally, assessment of the present capacity of the battery pack occurs during routine AED self testing (e.g., schedule, autonomous testing conducted by the unit). If an assessment determines that the battery pack lacks sufficient capacity to perform to a predetermined level, the user is alerted to the need to replace the battery pack. 
         [0008]    When to alert a user as to the need to replace the battery pack can be extremely problematic. If a user is alerted too early, battery pack capacity is wasted, as the user replaces a battery pack that could perform. If a user is alerted to late, the AED could be out of service before the timely replacement of the battery pack can occur. 
         [0009]    Determining when to alert a user to replace a battery pack is complex. Typically, battery pack capacity is assessed by determining the voltage output delivered under specific load conditions, which places a known load on the battery such that the battery&#39;s internal resistance causes a decrease in voltage output. If the voltage output falls below a given pre-determined threshold voltage, the battery pack is considered to lack the necessary capacity. In other words, voltage output is a surrogate for remaining battery pack capacity, thus remaining battery pack life. 
         [0010]    Historically, batteries, and the battery packs that use them, had a discharge curve that exhibited a gradual voltage output decline under load. Thus, a threshold voltage output under a known load of a battery pack could be identified that equated to battery pack end of life. 
         [0011]    As battery technology has advanced, the discharge curve has flattened out, thus the gradual output voltage decline has been eliminated. More precisely, newer technology batteries, such as Lithium Battery CR-2/3A, exhibit relatively stable voltage output under a known load until near end of life when there is a precipitous drop. 
         [0012]    Presently, to provide a timely warning to an AED user of the need to replace a battery pack using newer technology batteries, the threshold voltage under a known load is being continually increased. However, as the threshold voltage under a known load is increased, due to the ever flatter discharge curves, it is becoming ever closer to the normal operating output voltage. As those skilled in the art of assessing remaining battery capacity will appreciate, as the threshold voltage output under load approaches the normal operating output voltage under load, it becomes increasing difficult, due to the ever smaller delta between the two and minor fluctuations in the output voltage due to manufacturing and operational tolerances, to discern when the threshold voltage output has been reached. As a result, to meet the need of assuring proper operation and a timely notification of users as to the need to replace the battery, users are being instructed to replace battery packs earlier than might otherwise be required. As a result, capacity in battery packs employing newer technology batteries is being wasted. 
         [0013]    What is needed in the art is a better method of assessing battery pack end of life so additional battery capacity can be utilized to lower user costs. More specifically, autonomous self-tests being conducted on the AED should be able to determine the remaining capacity of the battery pack. Then, the battery pack should remain fully functional for some reasonable period of time thereafter to permit the timely notification of a user as to the need to replace the battery pack and allow a reasonable time to allow replacement before the battery pack is depleted. 
         [0014]    Furthermore, other desirable features and characteristics of the present invention will become apparent for the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
       SUMMARY OF THE INVENTION 
       [0015]    The invention is a battery pack topology wherein the battery pack has multiple battery sub-stacks electrically connected in parallel such that the capacity of each battery sub-stack may be utilized but one is reduced unequally as to the others. As a result, one battery sub-stack will reach a point of failure before the other, which causes a drastic, observable change in the output voltage of the battery pack, but provides sufficient reserve capacity to permit a user of a device, such as an AED, having the battery pack to be notified in a timely fashion of the need to replace the battery pack. 
         [0016]    In an exemplary embodiment, the battery pack includes two battery stacks configured in parallel. As a result, each battery stack is a battery sub-stack within the battery pack. The inequality in capacity utilization between the battery sub-stacks results from a difference in voltage drop relative to each branch of the parallel circuitry. In an illustrative example, this voltage drop difference is created by employing a different number of diodes on each branch. As those skilled in the art will appreciate, other electronic devices could be used to create different voltage drops, but diodes work well as the voltage drop, which is generally constant, as it is generally independent of current being drawn, except at very low current draws, from the associated battery sub-stack. 
         [0017]    Other features, attainments, and advantages will become apparent to those skilled in the art upon a reading of the description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a top view of an illustrative AED on which the present invention may be used. 
           [0019]      FIG. 2  is a perspective side view of the AED depicted in  FIG. 1 . 
           [0020]      FIG. 3  is a functional block diagram of the components of the AED depicted in  FIGS. 1 and 2 . 
           [0021]      FIG. 4  is a block diagram of the battery stack found in the in battery pack. 
           [0022]      FIG. 5  is a chart showing battery pack voltage over time. The chart depicts the results of two separate tests. One line is for one test and the other is for a second test. The overlapping of the lines indicates the repeatability of the outcome. 
           [0023]      FIG. 6  is a chart showing current sharing between the battery sub-stacks in the battery pack where the current draw is 1 milliamp. 
           [0024]      FIG. 7  is a chart showing current sharing between the battery sub-stacks in the battery pack where the current draw is 100 milliamps. 
           [0025]      FIG. 8  is a chart showing current sharing between the battery sub-stacks in the battery pack where the current draw is 1.5 amps. 
           [0026]      FIG. 9  is a schematic drawing of an alternate load allocator. 
           [0027]      FIG. 10  is a schematic drawing of another alternate load allocator. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0028]    Turning now to the drawings,  FIG. 1  illustrates a plan view of an AED unit  100 . As seen in this  FIG. 1 , the AED unit  100  has a video display  102 , a speaker  104 , an audio output jack  105 , and a user interface  106 . The AED unit  100  further includes an ON/OFF switch  108 , a shock switch  110 , a pad connector  112 , and an active status indicator  114  (ASI) (e.g., a light source which blinks green indicating the unit is OFF but ready to operate normally, solid green indicating the unit is ON and operating normally, solid red indicating the unit is ON but having a problem, and blinking red indicating the unit is OFF but having a problem. If the ASI is not blinking, the unit is out of service). The pad connector  112  connects pads  116  to the AED unit  100 . 
         [0029]    Referring to  FIG. 2 , the AED unit  100  further includes a card port  118  for providing an electronic interface for a card  120  for data collection, a standardized interface socket  122 , e.g., and universal serial bus (more commonly known as a USB port) for connecting such items as a keyboard and/or mouse  124  or a mass storage device  125  (see  FIG. 3 ), and a network interface  130  for connecting, for example a computer  132  (see  FIG. 3 ). Further, the AED unit  100  has a pad slot  133  for securing pads  116 . 
         [0030]    The AED unit  100  includes a battery pack  126  that provides the main power. As illustrated, the battery pack  126  slides into a battery slot  128 , but it could an internal battery pack. Where the battery  126  is removably secured in the battery slot  128 , a faulty battery can generally be replaced by a user. 
         [0031]      FIG. 3  is a functional block diagram of an exemplary AED unit  100 . Circuitry and programming of AED units is well known in the art. 
         [0032]    The AED unit  100  typically have many operating modes, with some being sub-modes of primary modes. There are two primary modes—OFF and ON. The OFF mode has several sub-modes including SELF-TEST and AUXILIARY. The OFF-SELF-TEST sub-mode is the default mode. More specifically, the AED unit  100  must always be in an operational mode. Thus, when the AED unit  100  is referred to as being in the OFF mode, it is in one of the sub-modes. When the AED unit  100  is in the OFF SELF-TEST sub-mode, a user considers the AED unit  100  to be OFF. 
         [0033]    In the OFF SELF-TEST sub-mode, the circuitry  200  of the AED unit  100  utilizes minimal power to maintain basic functions of the AED such as running a clock  210  (which is shown as having a backup battery) and autonomously (i.e., without human intervention) initiating self-tests, so that scheduled self-diagnostic maintenance checks in response to the passage of time are performed. The results of the self-test in this illustrative AED  100  are displayed by an active status indicator  114 , over which the AED programming has autonomous control. 
         [0034]    For a rescue attempt, the AED unit  100  is put into the ON mode from the OFF SELF-TEST sub-mode by operation of the ON/OFF switch  108 . After the rescue attempt, the AED unit  100  may be put back into the OFF SELF-TEST sub-mode by operation of the ON/OFF switch  108 , or the programming may automatically put the AED into the OFF SELF-TEST sub-mode. 
         [0035]    Continuing with  FIG. 4 ,  FIG. 4  is a block diagram of the topology of a battery stack (generally referred to by reference no.  400 ) inside the battery pack  126  (See  FIG. 2 ). Each battery sub-stack  402 ,  404  is composed of some number of battery cells  406 . Each battery sub-stack  402 ,  404  has an initial capacity sufficient to meet the energy needs of the AED  100  for some period of time beyond a single use. 
         [0036]    For a typical AED application, a suitable battery cell  406  is a 3 v battery, such as a Duracell Lithium CR-2/3A, and a battery sub-stack  402 ,  404  is four batteries electrically connected in series giving the battery sub-stack an initial output voltage of 12 v. These batteries have an initial capacity of about 1.5 Ah. In this exemplary embodiment, each battery sub-stack  402 ,  404  is generally identical (to the degree permitted by manufacturing tolerances) as they employ the same type and number of battery cells  406 , but this is not required. 
         [0037]    The two battery sub-stacks  402 ,  404  are connected via a load allocator  407  that places the battery sub-stacks in parallel. Therefore, one battery sub-stack is on branch A, and the other on branch B. 
         [0038]    The illustrated load allocator  407  includes three identical diodes  408 ,  410 ,  412  wherein two  408 ,  412  are in series and in parallel with one  410 . The diode configuration of the load allocator  407  (two on branch A and one on branch B) creates an unequal voltage drop across the branches A, B of the battery pack  126 . Since the branch voltage drops are unequal, the current drawn over time, or the capacity used, from each individual battery sub-stack  402 ,  404  will be different. As a result of the load imbalance, battery sub-stack  404  (the battery sub-stack on the one diode branch) will be depleted prior to battery sub-stack  402 . 
         [0039]    In addition, a diode on each branch of the parallel circuit prevents one battery sub-stack  402 ,  404  from charging the other battery sub-stack in the event they should have different voltage potentials. As those skilled in the art will appreciate, the identified suitable batteries are not rechargeable; therefore, these batteries should not be subjected to a charging current. 
         [0040]    As shown in  FIG. 5 , the voltage output from the battery pack  126 , comprising Duracell Lithium CR-2/3A batteries and using identical Schottky diodes, provides a clear indication of when the battery pack should be replaced. 
         [0041]    More precisely,  FIG. 5  shows changes in the output voltage of a battery pack  126  undergoing accelerated life testing. The accelerated life test simulates an AED “battery test event” (e.g., a draw at approximately 2 amps for 2 seconds) at fixed intervals. 
         [0042]    As shown in  FIG. 5 , the battery pack  126  has an initial steady voltage output of approximately 10.25 volts under load. After a few simulated intervals, the voltage output drops to approximately 9.75 volts under load, which is generally maintained until approximately simulated interval  380 . After approximately simulated interval  380 , a significant drop, or discontinuity, in voltage output under load is observed. After the discontinuity at simulated interval  380 , the voltage output dropped to roughly 8.6 volts under load. 
         [0043]    As used herein, a voltage discontinuity means a precipitous voltage output drop of the battery pack from one operational voltage to another under a known load. An operational voltage means a voltage in combination with a remaining capacity that is capable of operating the device for at least one cycle. 
         [0044]    The voltage output discontinuity results from the failure of the ability of one battery sub-stack to provide any current. In other words, prior to the failure of one battery sub-stack, both battery sub-stacks contributed current and the resulting output voltage was 9.75 volts under load. After the voltage discontinuity, which resulted from an end of life event wherein one battery sub-stack (e.g., a failure of at least one battery cell  406  in the battery sub-stack  404 ), the current draw on the remaining battery sub-stack resulted in a voltage output under load of 8.6 volts. The testing was continued until a simulated interval  420 , where at that point the battery pack  126  was unable to provide an operational voltage. 
         [0045]    This accelerated life test indicates the battery pack  126  had sufficient operational voltage to operate for a simulated 420 intervals and give a noticeable event at approximately simulated interval  380 . This noticeable event, of output voltage discontinuity, can be used to alert a user of a need to replace the battery, which is discussed below. 
         [0046]    The different capacity being drawn from each battery sub-stack, or load sharing between the battery sub-stacks, under different load conditions is shown in  FIGS. 6-8 . Each battery sub-stack has a total capacity, or amp-hrs. When the AED is in an operational mode, while each battery sub-stack is operational (i.e., prior to the output voltage discontinuity), the amp-hrs needed to power the operational mode are provided by both battery sub-stacks. 
         [0047]    These graphs were created using an iterative test procedure using a battery pack  126  having the same construction as that used in the simulated life testing discussed above. Starting with new batteries, a 50 ohm resistor was placed across the terminals of the battery pack  126  for 40 minutes. The 50 ohm resistor was removed and the voltage output determined. Using the known voltage output, a resistor giving a load consistent with a current draw of 1 mA was connected across the battery pack  126  terminals, and the current from each battery sub-stack obtained. Then, a resistor giving a load consistent with 100 mA was connected across the battery terminals, and the current from each battery sub-stack obtained. Finally, the procedure was conducted with a resistor giving a load consistent with a 1.5 A draw. This iterative procedure was repeated some number of times. The average voltage output from the battery pack  126  over the test was 11V. 
         [0048]    As shown in  FIG. 6 , when a very low current is drawn, current is drawn predominantly from one battery sub-stack. It should also be observed that there is a change over between sub-battery stacks. Initially, the battery sub-stack  404  is providing the bulk of the current and then there is a change over to battery sub-stack  402 . This results due to the ever increasing voltage drop present in the failing battery sub-stack. 
         [0049]      FIGS. 7 and 8  show that as current drawn from the battery pack  126  increases, current sharing between the battery sub-stacks becomes less disproportionate. At the highest of current draws there is only a minor difference between the proportions of the current load being satisfied by either battery sub-stack. Thus, where the current draws are low (i.e., low load), one battery sub-stack provides the capacity. But when, the current draws are high (i.e., high load), the current draw is allocated more equally. 
         [0050]    As those skilled in AED design will appreciate, many AEDs are intended to meet a once in a life time need, but have many operational modes whether in storage or in use that use battery pack capacity at varying rates. For example, during storage, an AED continually performs scheduled self tests. These self tests vary in scope and duration. For example, a daily self test uses very little battery capacity, while weekly, monthly and quarterly self tests use ever increasing amounts. Generally, the increased amount of battery capacity used in various self tests results from degree the testing involves the shock circuit. In tests that are more frequent, the shock system may be not charged or only partially charged where in the less frequent tests it could fully, or almost be fully, charged. 
         [0051]    For example, when stored and OFF with no self-testing occurring (e.g., the AED is merely reporting operational status using an active indicator), the load and associated current draw is in single digit milliamps, but relatively continuously. When OFF and conducting a daily self-test, the load is marginally higher having a current draw in the hundreds of milliamps (e.g., 100-200) for some short duration. However, when OFF and performing weekly, monthly, or quarterly self-tests, the load can be significant with the current draw (either battery limited or device limited) approaching several amps (e.g., 2 amps) for some number of seconds, becoming longer for the less frequent tests (e.g., 2 seconds weekly, 10 seconds quarterly). In the event of the AED is used in a rescue, the load and associated current draw is generally equivalent in amount and duration to that in longest self-test. 
         [0052]    Referring to  FIGS. 6-8  and assuming an AED is maintained for a random emergency, the AED will be predominately OFF with no-self-testing occurring, thus it will operate predominately using a single battery sub-stack. Even when OFF and conducting daily self-testing, one battery stack will be predominately used. However, during extremely high current draw events, such as during non-daily self-testing and rescues, both sub-stacks will more equally participate in the operation of the AED. 
         [0053]    The above usage pattern of the battery pack  126  makes diodes preferred for the load allocator  407 , as diodes have generally constant voltage drops over a wide current range. This diode characteristic maximizes battery pack  126  life by keeping the voltage drop associated with the load allocator  407  as small as possible under all potential AED uses, even during high current events. Schottky diodes, which are illustrated, are available with forward-voltage drops between approximately 0.15-0.45 volts. Other more conventional diodes, such as silicone diodes, could be used, but the available forward-voltage drops are between approximately 0.7-1.7 volts. Precise diode selection is a matter of design choice considering such factors as maximum current flow and maximum reverse voltage. 
         [0054]    As discussed above, the significant drop, or discontinuity, in output voltage indicates a failure in battery sub-stack  404 . As those skilled in the art will appreciate, the diodes used in the battery stack affect when the significant drop in output voltage of the battery pack  126  will occur. More specifically, the objective is to create a different voltage drop between the branches of the circuit containing the battery sub-stacks. The closer the created voltage drops are, the longer the time until the significant drop will occur, assuming two equal battery sub-stacks. As a result, less residual capacity will remain in the battery pack  126 , or in the still functioning battery sub-stack. On the other hand, the greater the disparity in the voltage drops, the shorter the time until the significant drop and the greater the residual capacity in the battery pack  126  or the still functioning battery sub-stack. 
         [0055]    It, therefore, should be appreciated that there is a tradeoff between the amount of residual capacity and the timing of the occurrence of the significant voltage drop. As the significant voltage drop is used to signal the need to replace the battery pack, this will establish the duration of the notice period before AED failure, and the time in which the battery must be replaced to avoid an out-of-service condition. 
         [0056]    As addressed above, the voltage discontinuity can be used as a triggering event for the AED to notify a user of the need to replace the battery pack  126 . For example, during a self-test, the self-test could determine the output voltage of the battery pack under a known load condition, such as a “battery test event.” Then based on a pre-determine threshold voltage, determine whether to alert the user to the need to replace the battery pack. The threshold voltage would be set between the output voltage before the discontinuity and the output voltage after the discontinuity. 
         [0057]    In the alternative, self-tests that run frequently on the AED, such as periodically, would determine a change in output voltage of the battery pack  126  by comparing the ultimate output voltage with a previous output voltage. For example, a self-test is run in which an output voltage of the battery pack  126  is determine and then this ultimate output voltage is compared to the penultimate output voltage. The delta between the two, would be compared to a predetermine voltage delta and if equal to or greater than the predetermined voltage delta, the programming would trigger some type of user alert, such as through the ASI. It would also be possible for programming to compare some number of prior output voltages, such as five prior output voltages be they the last five or say five of the last 10. For those skilled in the art of programming AEDS, the programming required is straight forward based on the description of the requirements provided. 
         [0058]      FIG. 9  is another embodiment of the load allocator  407 , referred to by reference no.  900  with common components having the same reference numbers. In this embodiment, one of the series diodes is replaced with a resistor  906 . 
         [0059]      FIG. 10  is another embodiment of the load allocator  407 , referred to by reference no.  1000  with common components having the same reference numbers. In this embodiment, one of the series diodes is replaced with a MOSFET  1002 . The MOSFET is configured as a diode, and provides a low voltage drop. A suitable MOSFET is a LINEAR TECH LTC4358. It should be appreciated, that the diode  408  could be integrated into the MOSFET. 
         [0060]    In addition, other diode configurations could be used. More specifically, a single Schottky diode could be used on one branch and a single silicone diode on the other. As a result, each branch could only have one diode instead of one branch having two. As applied to the embodiment depicted in  FIG. 4 , the diode  408  and diode  412  would be combined into one diode, where the one diode would have a voltage drop greater than that of diode  410 . 
         [0061]    Alternative embodiments of the invention will become apparent to one of ordinary skill in the art to which the present invention pertains without departing from its spirit and scope. Thus, although this invention has been described in exemplary form with a certain degree of particularity, it should be understood that the present disclosure has been made only by way of example and that numerous changes in the details of the construction and the combination and arrangement of parts or steps may be resorted to without departing from the spirit or scope of the invention. Accordingly, the scope of the present invention is defined by the appended claims rather than the foregoing description.