Patent Publication Number: US-9887560-B2

Title: Battery management system for control of lithium power cells

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. application Ser. No. 13/458,952, filed Apr. 27, 2012, now U.S. Pat. No. 9,099,877, issued Aug. 4, 2015, and claims the benefit of priority to U.S. Provisional Application No. 61/522,192, filed Aug. 10, 2011 and U.S. Provisional Application No. 61/480,286, filed Apr. 28, 2011, and to all of the patents and applications in the chain, all of which are incorporated herein in their entireties by reference. 
     This application is related to U.S. application Ser. No. 13/459,004, entitled “Latch Mechanism For Battery Retention,” filed Apr. 27, 2012; U.S. application Ser. No. 13/458,973, now U.S. Pat. No. 8,729,951, entitled “Battery Management System MOSFET Boost System,” filed Apr. 27, 2012; U.S. application Ser. No. 13/458,988, entitled “System and Method For Automatic Detection Of Battery Insertion,” filed Apr. 27, 2012; U.S. application Ser. No. 13/458,996, entitled “System and Method For Tracking And Archiving Battery Performance Data,” filed Apr. 27, 2012; U.S. application Ser. No. 13/459,007, entitled “Viral Distribution of Data, Operating Parameters and Software Using A Battery As A Carrier,” filed Apr. 27, 2012; the entireties of which are hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention is directed to a battery pack for providing power to a device. More specifically, the invention is directed to a battery pack and battery management system for managing the charging and discharging of the battery pack, and also having a capability of receiving, transporting, and downloading data, information and software programs to devices suitably configured to accept such. 
     BACKGROUND OF THE INVENTION 
     Cardiopulmonary resuscitation (CPR) is a well-known and valuable method of first aid used to resuscitate people who have suffered from cardiac arrest. CPR requires repetitive chest compressions to squeeze the heart and the thoracic cavity to pump blood through the body. Artificial respiration, such as mouth-to-mouth breathing or a bag mask apparatus, is used to supply air to the lungs. When a first aid provider performs manual chest compression effectively, blood flow in the body is about 25% to 30% of normal blood flow. However, even experienced paramedics cannot maintain adequate chest compressions for more than a few minutes. Hightower, et al., Decay In Quality Of Chest Compressions Over Time, 26 Ann. Emerg. Med. 300 (September 1995). Thus, CPR is not often successful at sustaining or reviving the patient. Nevertheless, if chest compressions could be adequately maintained, then cardiac arrest victims could be sustained for extended periods of time. Occasional reports of extended CPR efforts (45 to 90 minutes) have been reported, with the victims eventually being saved by coronary bypass surgery. See Tovar, et al., Successful Myocardial Revascularization and Neurologic Recovery, 22 Texas Heart J. 271 (1995). 
     In efforts to provide better blood flow and increase the effectiveness of bystander resuscitation efforts, various mechanical devices have been proposed for performing CPR. In one variation of such devices, a belt is placed around the patient&#39;s chest and an automatic chest compression device tightens the belt to effect chest compressions. Our own patents, Mollenauer et al., Resuscitation device having a motor driven belt to constrict/compress the chest, U.S. Pat. No. 6,142,962 (Nov. 7, 2000); Bystrom et al., Resuscitation and alert system, U.S. Pat. No. 6,090,056 (Jul. 18, 2000); Sherman et al., Modular CPR assist device, U.S. Pat. No. 6,066,106 (May 23, 2000); and Sherman et al., Modular CPR assist device, U.S. Pat. No. 6,398,745 (Jun. 4, 2002); and our application Ser. No. 09/866,377 filed on May 25, 2001, our application Ser. No. 10/192,771, filed Jul. 10, 2002 and our application Ser. No. 12/726,262, filed Mar. 17, 2010 show chest compression devices that compress a patient&#39;s chest with a belt. Each of these patents or applications is hereby incorporated by reference in their entireties. 
     Since seconds count during an emergency, any CPR device should be easy to use and facilitate rapid deployment of the device on the patient. Our own devices are easy to deploy quickly and may significantly increase the patient&#39;s chances of survival. 
     One important aspect of such devices is the need for small, powerful yet reliable power supply to power the device. However, such a need is not limited to mechanical compression devices such as those described above. There are many medical devices which require a light weight power supply, typically a battery, which must be capable of delivering sufficient energy to the device for an extended period of time. Moreover, the power supply must be relatively light weight, so as to enhance portability of the medical device, yet it must deliver its power for an extended period of time without significant voltage or current drop off to ensure consistency of operation throughout the period the device is being used. 
     Modern batteries typically require additional electronic circuitry or a battery management system designed to manage the operation of the battery. Such a battery management system should be able to monitor the operation of the battery, including charging and discharging of the battery without significantly adding to the internal resistance of the battery. Moreover, a battery management system should also be capable of accommodating different battery chemistries as battery technology evolves. To accomplish this, the battery management system will also need to have sufficient processing power and memory capacity to record events that occur during the lifetime of the battery, and to be able to analyze and/or communicate data related to those events to a centralized or distributed server system for analysis. The battery management system will also include various safety related systems designed to protect the battery, the devices being powered by the battery, and users of the battery or powered device from various environmental or operational factors that may occur. The present invention satisfies these, and other needs. 
     SUMMARY OF THE INVENTION 
     In a most general aspect, the invention provides a high-performance battery pack capable of reliably providing high power and current to power a device for a prolonged period of time. Moreover, the battery pack includes a battery management system that monitors and controls all aspects of battery operation, including charging and discharging of the battery. The battery management system is also capable of recording events that occur during charging and discharging of the battery pack, and communicating those events for later analysis. The battery management system is further capable of being updated with enhanced or improved operating parameters, and is capable of managing various battery chemistries to provide for forward and backwards compatibility. 
     In one aspect, the battery management system of the present invention includes a circuit that monitors the charging of each cell of the battery pack to prevent voltage imbalances between cells. A bleed circuit is included that is configured to bleed off excess charge from each individual cell. 
     In another aspect, the battery management system includes a primary protection circuit that monitors the charging and discharging of each battery cell in the battery pack. If a fault is observed, the primary protection circuit provides a signal that drives the gate of one or more n-FET devices to open a main bus of the battery to interrupt current flow from the battery. 
     In still another aspect, the battery management system includes catastrophic fault protection to monitors the cells of the battery pack for over voltage or over current conditions. When a fault is sensed, a signal is provided that drives the gate of one or more n-FET devices to open the main bus of the battery to interrupt current flow from the battery. 
     In yet another aspect, the battery management system includes a gas gauge function that monitors the charging and discharging of the battery pack and then determines, based on stored information and current information a value related to an estimated amount of charge available from the battery pack. 
     In a further aspect, the battery management system of the present invention includes circuitry configured to boost the voltage provided to drive the gate of one or more n-FET devices to full on so as to allow for the maximum flow of current through the n-FET devices. 
     In a still further aspect, the battery management system of the present invention may also include circuitry designed to isolate the terminals of the battery pack unless the battery pack is inserted into a charger or a device to be powered by the battery. Such isolation results in a zero voltage across the battery terminals when the battery is not inserted into a charger or device to be powered, thus preventing inadvertent shorting of the terminals which could lead to catastrophic discharge of the battery and possible harm to equipment or to a user. 
     In yet another further aspect, the battery management system of the present invention may include one or more processors for controlling the various functions of the battery management system. In another aspect, the one or more processors may be configured to communicate with internal and/or external memory storage or devices to provide for storage of events that occur during charging, discharging and storage of the battery pack. In still another aspect, the processors may be configured to communicate not only with internal or external storage media or devices, but may also be configured to communicate over a network with other processors, storage media or devices, or even other batteries or battery chargers. The network may be either wired or wireless. 
     In another aspect, the present invention includes a battery pack for powering a device, comprising: a rechargeable cell having a positive and a negative terminal; a main bus connecting the positive terminal of the rechargeable cell with a positive connector of a battery pack connector and also connecting the negative terminal of the rechargeable cell with a negative connector of the battery pack connector; and a battery management system in electrical communication with the rechargeable cell, including a processor for monitoring an input related to the state of charge of the cell, a switch disposed in the front-side bus between the positive terminal of the cell and the positive connector of the battery pack, the switch responsive to a signal generated by the processor in response to the input related to the state of charge of the cell to control the charging or discharging of the cell. 
     In one alternative aspect, the switch is a n-FET device. Another alternative aspect further comprises a second rechargeable cell connected in series with the first rechargeable cell; and a monitoring circuit, in electrical communication with the processor, the monitoring circuit monitoring the voltage of the first and second rechargeable cells and providing a signal for each rechargeable cell to the processor, the signal representative of a level of voltage measured across a negative and positive terminal of each cell, the monitoring circuit including a first switch connecting the first rechargeable cell to a first bleeder circuit, the first switch responsive to an overvoltage signal from the processor to close the first switch to prevent overcharging of the first rechargeable cell, the monitoring circuit also including a second switch connecting the second rechargeable cell to a second bleeder circuit, the second switch responsive to an overvoltage signal from the processor to close the second switch to prevent overcharging of the second rechargeable cell. 
     Still another aspect further comprises a temperature monitoring circuit for monitoring the temperature of the battery pack and for providing a signal related to the temperature of the battery pack to the processor; and wherein the processor receives and analyses the temperature signal to determine if the temperature of the battery pack is within a pre-selected range of temperature, and if the temperature of the battery pack is not within the pre-selected range of temperatures, the processor provides a signal to the switch to disconnect the positive terminal of the cell from the positive connector of the battery pack. 
     Yet another aspect further comprises a state of charge monitoring circuit for providing an indication of the amount of charge available from the battery pack for powering a device. Still another aspect further comprises a battery pack isolation circuit in electrical communication with the processor, the battery isolation circuit configured to provide a connection signal to the processor indicating that the connector of the battery pack are connected to a battery charger or device to be powered; and wherein the processor responds to the connection signal by sending a signal to the switch to close the switch to provide a voltage across the positive and negative terminals of the connector. 
     In yet another aspect, the battery pack isolation circuit includes a mechanical switch that has a closed condition when the battery connector of the battery pack is connected to a battery charger or device to be powered; and wherein the processor responds to the closed condition of the mechanical switch to provide a voltage across the positive and negative terminals of the connector. 
     Still another aspect includes a communication bus connecting the processor to the connector and to other components of the battery management system, the communication bus configured to provide communication between the processor and the other components of the battery management system, and to other systems, processors, networks or storage media external to the battery pack. 
     Yet another aspect includes indicator means mounted on an exterior of the battery pack, the indicator means providing a visual indication of information related to the operation of the battery. In one alternative aspect, the indicator means is at least one light emitting diode. 
     In still another aspect, the present invention includes an intelligent battery pack, comprising: a housing; a connector mounted on an end of the housing and configured to engage a connector disposed in or on a device to be powered; a plurality of rechargeable battery cells, each of the cells having a positive and a negative terminal, and each of the cells connected in series by a main bus having a positive side connected to a positive terminal of the connector and a negative side connected to a negative terminal of the connector, the plurality of cells disposed within the housing; a switch disposed in the positive side of the main bus between the positive terminal of the connector and the plurality of cells; a processor for controlling the operation of the battery pack; a primary protection circuit in electrical communication with the processor, the primary protection circuit including: a plurality of monitoring circuits in electrical communication, each one of the plurality of monitoring circuits for monitoring a parameter related to the state of charge of an individual one of the plurality of cells, and a plurality of bleed circuits, each of the plurality of bleed circuits connected to an individual one of the cells to bleed the cells in response to the parameter for the cell monitored by the monitoring circuit connected to that cell to prevent an overcharge of the cell; a secondary protection circuit for protection of battery pack from short circuits; a battery management system in electrical communication with the connector, the switch and the plurality of monitoring circuits, the battery management system configured to monitor the charging and discharging of the cells and to provide a signal to the switch to open the switch if a fault is detected; a state of charge monitoring circuit, including a sensor for determining an amount of current flowing through the main bus, a state of charge processor in electrical communication with the sensor, a state of charge memory in electrical communication with the state of charge processor, the memory for storing information related to the battery cells and also to the amount of current flowing through the main bus, wherein the state of charge processor, based on the information stored in the state of charge memory and input from the sensor determines the amount of current available from the battery; and a communication bus in electrical communication with the connector, the processor, the primary protection circuit, the secondary protection circuit, the battery management system and the state of charge monitoring system for providing for communications between the connector, the processor, the primary protection circuit, the secondary protection circuit, the battery management system and the state of charge monitoring system. 
     In an alternative aspect, the switch is an n-channel field effect transistor. In still another alternative aspect, the primary protection circuit includes a primary protection processor configured to monitor the charging and discharging of the cells. In a further alternative aspect, the primary protection circuit includes a primary protection processor configured to control the charging and discharging of the cells. 
     In yet another aspect, the processor is also configured to communicate information through the communication bus to the state of charge processor, the information used by the state of charge processor to determine the amount of current available from the battery cells and to store the determined amount of current available in the state of charge memory. 
     Still another aspect includes a temperature monitoring circuit to determine the temperature of the battery back, the temperature monitoring circuit communicating the temperature of the battery pack to the primary protection circuit; and wherein the primary protection circuit is configured to send a signal to the switch to open the switch if the temperature of the battery pack is not within a pre-selected range of temperature values. 
     In yet another aspect, the intelligent battery pack further comprises a battery pack isolation circuit in electrical communication with the processor, the battery isolation circuit configured to provide a connection signal to the processor indicating that the connector of the battery pack is connected to a battery charger or device to be powered; and wherein the processor responds to the connection signal by sending a signal to the switch to close the switch to provide a voltage across the positive and negative terminals of the connector. 
     In another alternative aspect, the present invention includes a device to be powered by either the battery pack or the intelligent battery pack described above. In one alternative aspect, the device is a medical device. In another alternative aspect, the device is a mechanical compression device. In a further aspect, the battery pack or intelligent battery pack is configured to power a mechanical compression device for at least thirty minutes. In still another aspect, the battery pack or intelligent battery pack is configured to power a mechanical compression device for at least 20 minutes when the mechanical compression device is used to treat a larger than average size patient. In yet another aspect, the battery pack or intelligent battery pack 
     Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a method of performing chest compressions on a patient by using a mechanical chest compression device. 
         FIG. 2  is perspective view of the mechanical chest compression device of  FIG. 1  showing the bottom and front sides of the device 
         FIG. 3  is a perspective view of the mechanical chest compression device of  FIG. 1  showing the bottom and rear cover plates removed. 
         FIG. 4A  is perspective view of a battery pack in accordance with the present invention showing a battery latch disposed on a front side of the battery pack. 
         FIG. 4B  is a perspective view of the battery pack of  FIG. 4A  showing a connector, indicator and button disposed on a back side of the battery pack. 
         FIG. 5  is an exploded perspective view illustrating various components of one embodiment of a battery pack of the present invention. 
         FIG. 6  including  FIG. 6 - a ,  FIG. 6 - b ,  FIG. 6 - c ,  FIG. 6 - d  is a schematic diagram of an embodiment of battery management system in accordance with principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The various embodiments of the present invention are directed to providing a rechargeable battery for powering mobile equipment, particularly medical devices. The embodiments of the invention are particularly advantageous when the battery is required to provide a large amount of current over a predictable period of time. Moreover, the embodiments of the invention include a battery management system that controls all aspects of the operation of the battery, and also includes a memory in which events related to the battery that occur during the battery&#39;s lifetime are stored. Moreover, embodiments of the battery management system include the capability of accommodating batteries using different battery chemistries, and are also capable of being updated through a communication port. 
     While the various embodiments of the invention are described with reference to a mechanical compressions device, those skilled in the art will immediately appreciate that those embodiments are not to be limited to powering such a device. Indeed, such a use is merely exemplary, and a battery in accordance with the various embodiments of the present invention may be used to power any device, and in particular, medical devices, wherein the design requirements of the device are met by the capabilities of such a battery. 
     When a battery in accordance with the various embodiments of the present invention is used with a mechanical compression device, the battery must be capable of powering the mechanical compression device for long enough to treat the patient not only in the field, but also during transport of the patient from the field to a care center. Experience has shown, however, that a patient&#39;s size and weight are factors that determine the amount of current drain on the battery during treatment. Accordingly, treatment of a larger than average patient results in a larger current draw on the battery. 
     For example, studies have found that the chest depth, chest breadth and chest circumference are factors influencing the amount of current drain on a battery powering a mechanical compression device. Other studies have observed that the mean chest depth of an average adult human male is 9.4 inches, mean chest breadth is 12.2 inches and mean chest circumference is 39.7 inches. See, Young, J W, R F Chandler, C C Snow, K M Robinette, G F Zehner, M S Lofberg, Anthropometric and Mass Distribution Characteristics of the Adult Female, FAA Civil Aeromedical Institute, Oklahoma City, Okla., Report No. FAA-AM-83-16, 1983; Anthropometry and Mass Distribution for Human Analogues: Volume 1: Military Male Aviators, Report No. USAFSAM-TR-88-6, March, 1988; Haslegrave, C M, “Characterizing the anthropometric extremes of the population”, Ergonomics, 29:2, pp. 281-301, 1986; Diffrient, N, A R Tilley, J C Bardagy, Human Scale 1/2/3, The MIT Press, Cambridge, Mass., 1974; and PeopleSize Pro Software, Open Ergonomics Ltd., 34 Bakewell Road, Loughborough, Leicestershire, LE11 5QY, United Kingdom, the entireties of which are hereby incorporated by reference herein. A battery that can sustain operation of the mechanical compression device for at least thirty minutes for an average size patient, and at least twenty minutes for a larger than average size patient is advantageous. 
     Referring now to the drawings in detail, in which like reference numerals indicate like or corresponding elements among the several figures, there is shown in  FIG. 1  a chest compression belt fitted on a patient  1 . A chest compression device  2  applies compressions with the belt  3 , which has a right belt portion  3 R and a left belt portion  3 L. The chest compression device  2  includes a belt drive platform  4  and a compression belt cartridge  5  (which includes the belt). The belt drive platform includes a housing  6  upon which the patient rests, a means for tightening the belt, a processor and a user interface disposed on the housing. The belt includes pull straps  18  and  19  and wide load distribution sections  16  and  17  at the ends of the belt. The means for tightening the belt includes a motor attached to a drive spool, around which the belt spools and tightens during use. The design of the chest compression device, as shown herein, allows for a lightweight electro-mechanical chest compression device. The fully assembled chest compression device weighs only 29 pounds, and is thus hand-portable over long distances. The device itself weighs about 22.0 to 23.0 pounds, with the battery, in at least one embodiment of the present invention, weighing between 2 and 5.0 pounds, and preferably about 3 pounds. The belt cartridge weighs about 0.8 pounds and the straps to secure the patient weigh about 1.6 pounds. 
       FIG. 2  shows the posterior side  23  of the chest compression device as seen from the superior direction. In the perspective of  FIG. 2 , the average sized patient&#39;s buttocks and the back of the patient&#39;s legs would extend past the inferior bumper  40 . The device is built around a sturdy channel beam  41  that is laterally oriented with respect to the housing. The channel beam supports the device against the forces created during compressions. The channel beam also serves as the structure to which the belt cartridge is attached. 
     The channel beam  41  forms a channel extending across the lateral width of the device. During compressions, the belt is disposed in and travels along the channel. The belt is attached to a drive spool  42  that spans the channel. 
       FIG. 3  shows internal components of the chest compression device  2 . A motor  79  is operable to provide torque to the drive spool  42  through a clutch  80  and a gearbox  81 . A brake  82 , attached to the superior side of the motor, is operable to brake the motion of the drive spool. The brake hub connects directly to the rotor shaft of the motor. 
     The motor  79  and brake  82  are controlled by a processor unit  83 , motor controller  84  and power distribution controller, all of which are mounted to the inside of the anterior cover plate  60 . The processor unit includes a computer processor, a non-volatile memory device and a display. 
     The processor unit is provided with software used to control the power controller and the motor controller. Together, the processor unit, power controller and motor controller make up a control system capable of precisely controlling the operation of the motor. Thus, the timing and force of compressions are automatically and precisely controlled for patients of varying sizes. 
       FIGS. 2 and 3  also show the location of a battery compartment  121  near the head of the patient. The location and design of the battery pack and battery compartment allow for rapid exchange of batteries. A spring in the back of the compartment forces the battery pack out unless the battery pack is fully and correctly inserted in the compartment. A latch on one end of the battery pack engages a receiver in the battery compartment  121  to hold the battery pack within the batter compartment when the battery pack is inserted into the battery compartment. Recesses  120  indicate the location of the springs inside the battery compartment  121 . Plastic grills  122  at the end of the battery compartment reinforce the recesses. 
       FIGS. 4A and 4B  are perspective views of a battery pack  200  showing the front and back sides  205 ,  210  of the battery pack respectively. Front side  205  of the battery pack faces outward and is visible to the user when the battery pack is inserted into the battery compartment  121  ( FIG. 3 ). As shown in  FIG. 4A , front side  205  includes a latch  215  that engages a receiver within battery compartment  121  to hold the battery pack  200  within the battery compartment. Also shown in  FIG. 4A  are a pair of raised tabs  217  disposed on the tops side of the front end of the battery pack. These tabs cooperate with the latch to ensure that the battery is properly seated in the battery compartment by prevent the top of the battery from riding up during battery insertion, forcing the latch into proper engagement with a battery latch receiver or lip of the battery compartment. 
     The back side  210  of the battery pack, as seen in  FIG. 4B , includes a connection  220  that connects to a connector within battery compartment  121  to enable electrical communication between the controller or processor of the mechanical compression device and the battery pack  200 . This connector not only allows for the flow of current from the battery pack to power the mechanical compression device, but it also provides for the flow of data, programming commands and other information, such as battery charge status, discharge rate, time remaining until discharged, and the like between the battery pack and the processor or computer controlling the operation of the mechanical compression device. Similarly, connector  220  may be configured to be connected to a connector in a battery charger to charge the cells of the battery pack, as well as to provide for the flow of data, software programs or commands and/or other information between the charger and the battery pack. It is also contemplated that connector  220  may be used to connect the battery pack to a communication network that would allow for flow of information between the battery pack and other computers, servers, processor or devices that are also connected to the network. It will be understood that the network may be a wired network, such as, for example, an Ethernet, or it may be a wireless network. The network may be a local network, or it may be a wide area network, such as a WLAN or the Internet. 
     A status indicator  225 , which may be, for example, one or more light emitting diodes (LEDs) or similar devices, is also disposed on the back end  210  of battery pack  200  to provide a visual indication of, for example, the charge/discharge status of the battery pack, the presence of any faults that would affect the operation of the battery pack, or other information that might be useful to the user of the battery. A push button  230  is also included; button  230  may be used, for example, to initiate a reset of the battery pack. Alternatively, button  230  may be used to initiate a diagnostic test, the results of which may be indicated by status indicator  225 . In other embodiments, pressing button  230  may initiate other functions of the processor in the battery pack, including, for example, and not by way of limitation, determining the remaining capacity of the battery, display of fault codes through the use of status indicator  225  and the like. 
       FIG. 5  is an exploded perspective view of the battery pack  200 . The battery pack  200  in this exploded view has been inverted from the view of  FIGS. 4A and 4B . The battery pack has a bottom enclosure  234  and a top enclosure  232 . A battery latch assembly having a battery latch  236 , a lever base  238  and a lever latch  240  is mounted at the side of the battery pack facing outward when the battery pack is inserted into a battery compartment, and is held in place by the bottom and top enclosures. Lever latch  240  has a wing  241  that is inserted into a groove or slot  243  formed in a face of battery latch  236 , and lever base  238  is mounted on the bottom enclosure to pivotally hold lever latch  240  in the enclosure. Compression springs  254  are disposed between a bottom end of battery latch  236  and top enclosure  232 . A projection  255  is disposed a top end of battery latch  238 , and is configured to protrude through slot  251  that extends through a thickness of the bottom enclosure  234 . In this manner, battery latch  236  may be manipulated by a user to engage and disengage projection  255  from the latch receiver located in the mechanical compression device to insert the battery pack into the battery compartment and to release the battery pack for removal from battery compartment  121 . 
     Disposed at the back end  210  of the battery pack is a battery entry board  242  to which is mounted connector  220 , indicator  225  and button  230  ( FIG. 4B ). Entry board  256  is mounted to bottom enclosure  232  using one or more screws  250 . The entry board may also be fastened to top enclosure  252  using one or more screws  256 . In some embodiments, a water resistant gasket  262  may be used to resist the incursion of fluids into the interior of the battery pack. Moreover, a label  260  may be used to provide information to a user relative to the various indications that may be provided by indicator(s)  225 . 
     A battery management board  244  to which is mounted processors, memory and electrical circuitry for managing the various operations of the battery (which is described in more detail below) is mounted using screws or other fasteners  258  to battery cell assembly  246 . Battery cell assembly  246  includes one or more battery cells  248 . Battery cells  248  may be cells utilizing a variety of battery chemistries, such as, for example, nickel metal hydride, lithium hydride, lithium-ion and the like. Battery management board  244  and battery cell assembly  246  may also include a pair of splatter shields  266  mounted on left and right sides of battery cell assembly  246  to protect the terminals of the individual battery cells  248  from inadvertent contact with other parts of the assembly, thus providing a shield against short circuit of the battery cells. 
     Battery pack  200  also includes at least one vent  264 , shown disposed in the top enclosure to allow for venting of the battery pack to prevent buildup of potentially flammable or explosive gases produced by the battery cells  248  during charging or discharging of the battery pack. While shown disposed in the top enclosure, those skilled in the art will appreciate that the vents may be disposed through any wall or side of the battery pack. Vents  264  may be a simple hole extending through the wall or side of the battery pack. Alternatively, vent  264  may include a filtering means  265 , such as a screen or hydrophobic membrane to prevent the incursion of particulates or fluids or moisture into the interior of the battery pack. An additional advantage of such vents is that the vent or vents provide for equalization of pressure between the interior and exterior of the battery pack, such as may occur when the battery pack is transported to a higher or lower altitude. 
     The mechanical compression device described above requires a reliable power source to operate. It is not unusual for the device to be required to be used for thirty minutes or more to provide resuscitation to a patient in an emergency. The torque and power requirements of the motor of the mechanical compression device require up to a peak of seventy amperes of current during compression. If enough current cannot be delivered by the battery to the motor controlling the compressions, the voltage falls off and the motor may not be capable of generating enough torque to ensure complete compression of a patient&#39;s chest. 
     The inventors of the present invention have realized that having a very low total internal resistance is key to ensuring extended and reliable operation of the battery when the battery is under a constant power drain. One such battery chemistry that has been found to be useful in devices requiring high power is a battery using Lithium Ion chemistry, such as the model ANR26650M1-A or ANR26650M1-B Lithium Ion cell available from A123 Systems, Incorporated. 
       FIG. 6  is a schematic diagram illustrating one embodiment of a battery pack  300  in accordance with the present invention. Battery pack  300  includes eleven Li-Ion chemistry cells, such as the model ANR26650M1-A or ANR26650M1-B cells described above. Each cell provides 3.3 volts, and the eleven cells are connected in series to provide a total of 36.3 volts. Using such cells, one embodiment of a battery pack in accordance with principles of the present invention can be manufactured that weighs approximately three pounds. Such a battery has been observed to deliver between 1550 and 2000 watts, and preferably deliver a peak power of 1800 watts. This provides a desirable weight to power ratio. Moreover, such an embodiment has also been found to be able to deliver energy of slightly less than 100 watt/hours. While eleven battery cells are used in this exemplary embodiment, more or less cells could be used depending on the requirements of the device to be powered. 
     To provide the amount of current required to operate the motor of the compression device, the inventors have discovered that it is important to minimize the internal resistance of the battery pack. Accordingly, the Lithium Ion (Li-Ion) cells used should have low internal DC resistance, preferably below 15 milliohms, and more preferably below 12.5 milliohms per cell. 
     While Li-Ion batteries are capable of providing the voltage and current required to operate a mechanical compression device for extended periods of time, care must be taken during both the discharge phase, and recharge of the batteries to ensure that the batteries continue to function for their desired lifetimes. It is well known that Li-Ion cells should not be over charged, nor should they be over discharged. Accordingly, various embodiments of the present invention include the capability of monitoring and controlling both the discharge of the cells and the recharging cycle. Those embodiments will be discussed in more detail below. 
     As described previously, the eleven Li-Ion cells  310  are connected in series by a main power bus  320 . Bus  320  has both a positive side and a negative, or ground side, as is typical for a DC circuit. Bus  320  delivers the direct current provided by the battery cells to a load (in this example, the mechanical compression device) through an interface  330 . As shown in  FIG. 6 , interface  330  is a pin connector having seven connection pins. Alternatively, a socket could be used, or a combination of pins and sockets, with more or less than seven pins or sockets could be used. 
     The positive side of bus  320  is connected to pin  7  of interface  330 . Similarly, the negative side of bus  320  is connected to pin  6  of interface  330 . Pins  1 - 5  of interface are used to communicate various signals involved in the monitoring and control of the battery pack, as well as for communication to the device that is being powered, enabling the exchange of information and control signals between the battery pack and the powered device. Various exemplary embodiments of the invention incorporating those features will be discussed in more detail below. 
     Returning again to  FIG. 6 , the positive side of bus  320  includes a fuse  342  to protect the circuit from over current conditions. Fuse  342  may be, for example, a 30 ampere fuse. In such a case, a sustained current flow through fuse  342  of more than 30 amperes would cause the fuse to open, breaking the circuit created by bus  320  and stopping the flow of current from the battery cells. While not shown, there is also a blown fuse detector circuit that monitors the fuse, and, if the fuse is blown, provides a signal to the pack controller that the fuse is blown. The pack controller may then provide a signal indicating that the battery is not suitable for use. Such a signal, for example, may be the change in color of and LED, or activation or de-activation of some other status indicator. Alternatively, the pack control may provide a signal to the equipment powered by the battery, which may then provide an indication to a user that the battery is not ready for use. 
     The positive side of main bus  320  also includes a number of n-channel field effect transistors (n-FET)  340 ,  350  and  360 . These n-FETs provide for switching and control of the circuit. N-FETs are used because they provide a very low resistance switch in keeping with the design requirement of minimizing the total internal resistance of the battery. Another unique capability of the n-FET is that they are capable of conducting high current loads, without damage and without generating excessive amounts of heat. One example of an n-FET that has been found suitable for use in various embodiments of the present invention is the model IRLS3036, available from Digi-Key Corporation. 
     In typical designs, p-FET devices would be used as the switches and would be placed in the high side of the main bus. However, p-FETS have two or more times the on resistance of an n-FET device. Thus, to handle the same current as an n-FET device, several p-FETs wired in parallel would be needed. Moreover, use of several p-FETS may also require the use of a heat sink to dissipate heat generated while the p-FET is on. This is disadvantageous where space within the battery pack is limited. 
     Similarly, an n-FET device would typically be used in the low side of the main bus to switch the current on and off in the bus. However, use of an n-FET in this situation breaks the ground of the battery, which may cause noise in the circuit and interfere with communication between the various elements of the battery management system circuitry. Accordingly, the inventions have placed the n-FET switches in the high side of the bus, which provides efficient switching of the bus without the generation of excess heat that may occur when p-FETs are used. Placing the n-FETs in the high side of the bus also eliminates the problem of breaking the ground of circuit. 
     In some embodiments, one or more resistors, such as resistors  370  and  380 , may be inserted in the negative, or low, side of the main bus circuit. These resistors provide the ability to tap the main bus to monitor various aspects of the current flowing through the circuit. For example, in one embodiment, resistor  370  is connected across the input lines of a cell balancing and primary protection circuit, which is discussed in more detail below. A typical value for resistor  370  is, for example, 2.5 milliohms. 
     In another embodiment, resistor  380  may be connected across a state-of-charge monitor, also known as a “gas gauge.” In this embodiment, the value of resistor  380  may be, for example, 5 milliohms. 
     Each of the cells  310  is individually monitored during both charging and discharging to control the rate of charging and discharging, respectively. In one exemplary embodiment, as shown in  FIG. 6 , a separate cell tap line  390  is connected to each cell and to a cell monitoring and balancing circuit  400 . 
     Primary Protection 
     During charging, the voltage of each cell is independently monitored to prevent overcharging of the cell. In one exemplary embodiment, a monitoring system on a microchip, which may be, for example, battery pack protection and monitor integrated circuit (IC)  410  such as an OZ890 available from O 2  Micro, is used to control the charging of the various cells. In such an arrangement, cell monitoring line  390  provides a positive signal to a representative pin input of IC  410 . For example, cell  1  is monitored using input line BC 1  of IC  410 , and so forth up to cell  11 , which is monitored using input line BC 11  of IC  410 . 
     If the control circuitry of IC  410  detects an imbalance in a cell, IC  410  provides a signal on an appropriate external bleed control line CB 1 -CB 11 . As shown in  FIG. 6 , when the signal on the appropriate external bleed control line is applied to the gate of n-FET  420 , current is allowed to pass between the source and the drain of n-FET  420  and then through resistor  430 , which results in bypassing the cell and halting charging of the cell. As can be seen from  FIG. 6 , each cell has its own dedicated combination of resistor and n-FET in electrical communication with IC  410  for monitoring each cell and preventing each individual cell from over charging. 
     Cell balancing and primary protection IC  410  may also be used in some embodiments to monitor the total voltage of the battery cell pack. For example, when all of the cells have attained their maximum voltage, IC  410  can send a low signal to the gate of n-FET  350  to open the channel between the source an drain of n-FET  350  and thus open the main bus circuit  320 . This results in a cessation of charging current through the cells, and thus halts the charging process. 
     Similarly, IC  410  monitors the voltage across the cells during discharge of the battery. When the voltage across the cells drops below a threshold level, for example, 21 volts, IC  410  drives the signal on line  450  low, which in turn shuts off n-FET  360  which interrupts the main bus circuit. This prevents damage to the battery cells that may be caused by removing too much of the charge in the cells, which may result in decreased life of the cell. 
     IC  410  may also include and control a temperature measurement capability designed to monitor the temperature of the battery pack and/or individual battery cells to prevent overheating. In this embodiment, one or more thermistors  460  are used to provide temperature signals over line  470  to the IC  410 . If IC  410  determines that the temperature of the battery is either too high or too low, IC  410  may drive either or both of n-FETs  350  and  360  low, opening the main bus  320  and isolating the battery pack. It will be understood that while only a single line  470  is shown for clarity, line  470  includes an appropriate number of conductors to monitor the function of all thermistors used in the temperature monitoring circuit in communication with IC  410 . 
     IC  410  may additionally provide a visual indication of fault state by providing a fault signal on line  480  that may then be used to cause LED  490  to light up. This visual signal of a fault condition sensed by the primary protection circuit indicates that the battery pack has been rendered non-functional by IC  410  and that repair or maintenance of the battery pack may be required. 
     Secondary Protection 
     Some embodiments of the present invention may also include secondary protection against catastrophic faults or over voltage protection. Such secondary protection may be provided by a variety of circuits designed to monitor the voltage of the battery pack and/or the current flow through the main bus and to take action when certain thresholds values of current or voltage are exceeded. In one embodiment, such protection may be provided by an integrated circuit  500 , such as, for example, the OZ8800 available from O 2 Micro. Those skilled in the art will know that, depending on the number of cells used in the battery pack, more than one IC  500  may be required. For example, the OZ8800 secondary level battery protection integrated circuit can monitor three to seven individual cells. Thus, where eleven cells are used, two OZ8800s will be required. 
     IC  500  monitors the voltage of each cell over monitoring line  312 . In some embodiments, a time delay may be employed that provides for temporary over voltage conditions to exist. Where the time threshold is exceeded because the voltage did not drop back into an acceptable range, IC  500  sends a low signal over fault line  510  to n-FET  340  to turn off n-FET  340 . Each cell is monitored by a similar circuit. 
     It should be apparent from  FIG. 6  that the n-FETs described above are in a normally off state unless a positive voltage is applied to the gate of each n-FET. Thus, any fault that results in voltage decrease to a level below the threshold of the n-FET at the gate will cause the n-FET to open, thus providing additional protection to the cells and battery management circuitry. 
     Gas Gauge 
     Another embodiment of the present invention includes a “gas gauge” function that monitors the amount of useful charge remaining in the battery pack. Such gas gauge functionality can be provided using integrated circuits designed to carry out vary tasks, such as to calculate remaining battery capacity for use and standby conditions based on time of use, rate of discharge and the temperature of the battery. Such a circuit may also determine the true battery capacity in the course of a discharge cycle from near full charge to near full discharge states. 
       FIG. 6  illustrates one example of such a gas gauge circuit  600 . Monitoring of the battery pack is accomplished using an integrated circuit  610 , such as a bq2060A available from Texas Instruments, Inc. IC  610  works in conjunction with an external EEPROM  620 . EEPROM  620  stores configuration information for IC  610 , such as the chemistry used in the battery cells, the self-discharge rate of the battery, various rate compensation factors, measurement calibration, and battery design voltage and capacity. All of these settings can be changed to allow the system to be used with a variety of battery types. Moreover, IC  610  can communicate with a central processor and memory over a back-side bus circuit  630 . In this manner, IC  610  and EEPROM  620  may be configured, using control signals from the central processor, to accommodate different types of batteries that are detected and identified by other circuitry included in the system, or which are manually identified by a user. In an alternative embodiment, IC  610  may also cooperate with the pack controller to augment the reporting accuracy of the gas gauge in instances of low current draw using appropriate control commands embedded into the software that controls the operation of the pack controller and the gas gauge to implement the algorithms necessary to carry out this function. 
     In general, the gas gauge functions are carried out by IC  610  in conjunction with IC  800  to determines full charge battery capacity and the capacity remaining at any point in time by monitoring the amount of charge input or removed from the battery cells. In addition, IC  610  measures battery voltage, battery temperature and current as detected across resistor  380 . IC  610  also, in some embodiments, may estimate the self-discharge rate of the battery, and also monitors for low-voltage thresholds of the battery. As described, IC  610  measures the amount of charge and discharge of the battery by monitoring the voltage across resistor  380 , which is located between the negative terminal of the first cell  310  (of the series connected cells) the negative terminal of the battery pack. Available battery charge is determined from this measured voltage and correcting the measurement for environmental and operating conditions. 
     IC  610  may also measure the temperature of the battery pack so as to carry out the estimations and adjustments described above. In one embodiment, a thermistor  640  is mounted adjacent to a cell or cells of the battery pack in such a manner as to be able to measure the temperature of the cell or cells of the battery pack. IC  610  drives the gate of n-FET  650  high by providing an appropriate signal over line  660  to connect a bias voltage source to thermistor  640  while the temperature of the cell or cells is being measured. Once the measurement is completed, IC  610  drives the gate of n-FET  650  low, opening the n-FET and thus disconnecting thermistor  640  from the bias source. 
     IC  610  may be reset each time the battery is charged so that the reported amount of charge remaining in the battery is accurate. A battery pack supervisor circuit or pack controller  800 , to be described in more detail below, provides a signal over reset line  670  to drive the gate of n-FET  680  high. This causes the current to flow through n-FET  680 , resulting in a reset signal being provided to IC  610  to reset the battery capacity counter of IC  610 . 
     In another embodiment, IC  610  may include a seal/unseal function that prevents unauthorized access to the parameters stored in the IC  610  or EEPROM  620 . Pack controller  800  may send a signal over line  680  that drives the gate of n-FET  690  high, which closes n-FET  690  allowing for commands and data to flow between IC  610  and EEPROM  630 . Such data may include, for example, updated calibration information and the like. In an alternative embodiment, the flow of data between IC  610  and EEPROM  630  may be controlled using only software commands from the pack controller to control IC  610  and EEPROM  630  without requiring n-FET  690 . 
     Pack Controller 
     In another embodiment of the present invention, the battery management system includes a pack controller  800  which serves as an overall supervisor for the various functions carried out by the battery management system. Pack controller  800  will typically be an integrated circuit, although discrete circuitry carrying out the same functions could be used, depending on the amount of space available within the confines of the battery pack. 
     For example, pack controller  800  may be a low or ultra-low power microcontroller such as the MSP430F2418 Mixed Signal Controller available from Texas Instruments Incorporated. Such a controller may include memory, such as random access memory or flash memory to provide for rapid and efficient execution of the various functions of the battery management system. Pack controller  800  also has the capability of communicating with peripheral devices, circuitry or memory over one or more communication buses, such as backside bus  630  and front-side  810 . The communication busses typically use a communication protocol such as, for example, the I 2 C bus (a Trademark of Philips Incorporated) or the System Management Bus (SMBus). The SMBus is described in more detail below. 
     Appropriate software commands are used to program the functions of the pack controller  800 . Such software includes commands configuring the communication protocol interface, such as for example, the SMBus interface. The software would also configure the pack controller to monitor critical battery pack parameters, which are made available to it over communication lines  810 ,  820 ,  822 , back-side bus  630 , front side bus  810  and detect line  824 , as well as other communications lines not shown or which may be added in the future. 
     When appropriately programmed, pack controller  800  is also in communication with one or more memory devices, such as, for example, an event archive EEPROM  900 . Such an archive has, for example, although not limited to, 64 kilobytes of memory that can be used to store history of various events that occur during the charge and discharge cycles of the battery pack, such as, for examples, the total amount of charge, total amount of energy discharged, the temperature of the battery cells, any faults that occur, or other information related to the individual battery cells and/or the various circuits employed to manage and control the operation of the battery. 
     Pack controller  800  may also be programmed to communicate with memory and/or processors such as, for example, EEPROM  1000 . In the exemplary embodiment shown in  FIG. 6 , EEPROM  1000  may be located in a mechanical compression device that is powered by the battery pack, or it may be incorporated into the battery pack and configured to be accessed by the devices to be powered by the battery. In this example, pack controller  800  communicates with EEPROM  1000  and/or a processor in the mechanical compression device over front side bus  810 , which accesses a similar bus in the mechanical compression device through connector  330 . In this manner, a two-way communication connection may be established between the battery pack and a device powered by the battery pack to allow for exchange of information between the battery pack and the powered device. For example, updated operating parameters or commands, including updated software, may be loaded into the battery pack from the powered device when the battery pack is put into communication with the powered device. Similarly, information contained in the event archive EEPROM  900  may be transmitted to the EEPROM  1000 , or any other memory (such as a portable memory device) configured for communication over back-side bus  810  from any of the memories present in the battery pack. 
     It will be understood that this communication capability also allows the battery to communicate with other devices besides devices which will be powered by the battery. For example, typically, the battery pack will be removed from a powered device to be recharged. When the battery pack is connected to a battery charger, the battery charger may be used to retrieve information from the memory or memories of the battery pack, and/or transmit updated data, information, programming commands or software to the battery through front-side bus  810 . This communication process will typically be managed using various handshaking and communication dialog as set forth in the communication protocol used to enable the communication, such as the SMBus protocol, exchanged between a processor residing in the charger or other device and the pack controller  800  of the battery pack. In some embodiments, the battery may also be trickle charged when the battery is inserted into the device to be powered, where the device being powered is also connected to an external power supply. 
     Still other embodiments of the present invention may include a capability, managed by pack controller  800 , of recognizing when the battery pack is inserted into a battery charger or device to be powered, such as a mechanical compression device. For example, pack controller  800  may be configured using appropriate software and/or hardware commands to provide a signal to IC  410  and IC  500  to provide a high signal to the gates of n-FETs  340 ,  350  and  360  to close those switches and thus provide full battery voltage at the positive and negative pins of connector  330  only when detect circuit  1100  sends an appropriate signal to pack controller  800  over line  824  indicating that the battery pack is properly mounted into a device to be powered. 
     In one embodiment, pack controller  800  monitors a line connected to a mechanical switch or interlock that is forced closed when the battery is properly inserted into a charger or a device to be powered. In another embodiment, pack controller  800  monitors a signal line connected to one or more pins of the battery connector. When an appropriate signal is received over this signal line, pack controller  800  determines that the battery is inserted into a charger or a device to be powered, and provides a high signal to the gates of n-FETs  340 ,  350  and  360  as described above. This embodiment is particularly advantageous in that pack controller  800  may be programmed to respond only when a particular signal is received, ensuring that the battery is inserted into a specific type or make of a charger or device to be powered designed to accommodate the battery before providing the high signal to the gates of n-FETs  340 ,  350  and  360 . 
     These embodiments are advantageous in that discharge of the battery in the event of an accidental short circuit across the positive and negative terminals of connector  330  is prevented. Given the amount of energy stored in the cells of the battery pack, such a discharge could be catastrophic. Thus, in this embodiment, there is no voltage between the positive and negative terminals of connector  330  of the battery pack unless the battery pack is properly mounted in a device configured to provide the appropriate signal to the detect circuit  1100 , thus providing for safe handling, storage and transport of the battery back when the battery pack is not connected to either a charger or a device to be powered, such as the mechanical compression device described above. 
     Pack controller  800  may also be programmed to provide password access to allow for changing of settings and parameters stored in EEPROMs  900  and  620 , as well as to provide appropriate signals to drive LED  490  in the event of a fault. Additional capabilities may also be included, configured using appropriate software and/or hardware commands, to provide additional functions to the battery management system. For example, such functions may include driving a display that indicates the total amount of charge remaining in the battery and the like. A more complete description of the various capabilities that can be incorporated into pack controller  800 , particularly when pack controller  800  is an MSP430F2418 (or other members of this controller family) is contained in a document entitled “MSP430F241x, MSP430F261x Mixed Signal Microcontroller,” SLAS541F—June 2007—Revised December 2009 available from Texas Instruments Incorporated, the entirety of which is hereby incorporated herein by reference. 
     Smart Bus Communications 
     As will be apparent, the various processors and integrated circuits and logic systems incorporated into the various embodiments of the present invention are capable of functioning as a unified system due to their ability to communicate with one another over the front side bus  320  and the back side bus  630 . In some embodiments, the communications over these buses are carried out using the System Management Bus (SMBus) specification. The SMBus is a two-wire interface through which various system component chips, such as IC  410 , IC  610 , the secondary protection system  500 , event archive  900 , EEPROM  1000  and pack controller  800 , among other circuitry, can communicate with each other and with the rest of the system. Further information related to the SMBus specification is contained in “System Management Bus (SMBus) Specification Version 2.0,” SBS Implementers Forum, Aug. 3, 2000, the entirety of which is hereby incorporated herein by reference. 
     Boost Circuit 
     The inventors have observed that, in some embodiments of the invention, the voltage required to drive the n-FETs closed exceeds the voltage that is available from the battery pack. For example, using an n-FET that requires a bias voltage of 10 volts, the n-FET requires a drive voltage of the bias voltage plus the battery voltage to drive the n-FET sufficiently to allow an appropriate voltage to pass through the n-FET to charge or discharge the battery. Accordingly, a voltage boost circuit is included to boost the voltage supplied to the gates of the n-FETs to drive the n-FETs to conduct the current supplied by the battery cells. 
     Those skilled in the art will understand that the use of n-FETs in the present invention results in the need for complex circuitry, such as the boost circuit. Such complexity could be eliminated using p-FETs. Use of p-FETs however, has been found to be disadvantageous because several p-FETs may be needed to handle the same current that can be handled by a single n-FET. Additionally, the heat generated using multiple p-FETs may require the addition of one or more heat sinks to dissipate the heat, which may require more space in a compact battery than which is available. Moreover, it is well known that p-FETs have at least twice the on resistance of n-FETs, which would increase the overall internal resistance of the battery pack. 
     While several particular foams of the invention have been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention.