Abstract:
A battery pack features a shock-absorbing and sealed construction and an electronic control module that provides automatic recovery circuitry in the event of a short circuit in the load whereby the power is terminated and then restarted at a lower level so that removal of the short circuit may be detected. Full power is restored to the load when the short circuit is removed. In addition, the electronic control module of the battery pack uses the battery pack load, such as a cap lamp, to provide an indication of a low battery charge level. The electronic control module also provides a soft-start feature where the power provided to the bulb is ramped up to avoid current in-rush to the bulb during startup.

Description:
CLAIM OF PRIORITY 
     This application claims priority from U.S. Provisional Patent Application Ser. No. 60/880,330, filed Jan. 12, 2007. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to battery packs and, more particularly, to a battery pack that features a durable construction and operation directed by an electronic control module. 
     BACKGROUND OF THE INVENTION 
     Rechargeable battery packs find use in many industrial applications due to their portability, dependability and low maintenance cost. A common usage of rechargeable battery packs is to power lamps mounted on hard hats worm by miners. Such cap lamps provide illumination in underground mine shafts. Cap lamps are well known in the mining equipment industry and provide illumination while the miner&#39;s hands remain free to perform tasks. 
     The battery pack is typically secured to the user&#39;s waist and electrical wiring delivers power from the battery pack to the lamp on the helmet. Normally, at the end of each working shift, the helmet and battery pack are removed by the miner and the battery pack is placed in a recharging device so that it is ready for use during a future shift. An example of such a cap lamp and rechargeable battery pack arrangement is disclosed in U.S. Pat. No. 4,481,458 to Lane. 
     Lithium-ion (Li-ion) batteries have a higher energy-to-weight ratio then any other commercially available rechargeable batteries. This makes them very desirable as a power source for portable devices, such as cap lamps. Most Li-ion battery packs, including those used to power mining cap lamps, must have a safety protection circuit to protect them from over-voltage, under-voltage and over-discharge conditions. 
     In addition, Li-ion battery packs often feature an electronic control module in series between the batteries and the cap lamp (or other load) to control operation of the battery pack. Such electronic control modules may include circuitry or a microprocessor that functions to provide an indication of a low battery, control battery charging and other functions. A need exists, however, for a low battery indicator that is easier to detect and that provides extended cap lamp operation so that a mine may be exited. 
     Electronic control modules may also cause a Li-ion battery pack to go into protection mode in the event of a short circuit. Such short circuits may be caused by, for example, worn parts in the cap lamp assembly or wires leading thereto. When the battery pack goes into protection mode, the cap lamp (or other load) is automatically turned off. Prior art designs require the user to manually turn the lamp off and then back on to reset the electronic control module or other circuitry and allow current to resume flow to the cap lamp after the short circuit condition is removed. An electronic control module that automatically turns the lamp (or other load) back on when the short circuit condition is removed is desirable. 
     A mine provides a very harsh atmosphere for equipment, including battery packs. The mine atmosphere contains an abundance of dirt, dust, coal particles and moisture. In addition, there is always the potential of a build-up of explosive gases in a mine. As a result, it is important to effectively seal a battery pack so that harmful elements can&#39;t reach the battery or the related wiring and circuitry inside. Furthermore, battery packs used in mines may suffer mechanical abuses during use as they are banged against machinery and rock, dropped and/or jostled as they ride on the user&#39;s waist. As a result, a need exists for a battery pack that can withstand shocks and vibrations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded top front perspective view of a battery pack including an embodiment of the electronic control module of the present invention; 
         FIG. 2  is a perspective view of the battery cell bundle without a wrap or pads with an electronic control module and protection circuit assembled thereto; 
         FIG. 3  is a bottom plan view of the battery cell bundle of  FIG. 2 ; 
         FIG. 4  is a perspective view of the battery pack of  FIG. 1  after being assembled; 
         FIG. 5  is a block diagram illustrating the primary components of the electronic control module of the battery pack of  FIGS. 1-4 ; 
         FIGS. 6A-6B  are an operation flow diagram of the microprocessor of the electronic control module of  FIG. 5 . 
         FIG. 7  is a schematic of the charging section circuit of the electronic control module of  FIG. 5 ; 
         FIG. 8  is a schematic of the low battery warning/indication circuit of the electronic control module of  FIG. 5 ; 
         FIG. 9  is a schematic of the microprocessor and associated circuitry of the electronic control module of  FIG. 5 ; 
         FIG. 10  is a schematic of the battery sensing circuit of the electronic control module of  FIG. 5 ; 
         FIG. 11  is a schematic of the overload sensor circuit of the electronic control module of  FIG. 5 ; 
         FIG. 12  is a schematic of charge current sensor circuit of the electronic control module of  FIG. 5 ; 
         FIG. 13  is a schematic of the LED driver circuit of the electronic control module of  FIG. 5 . 
         FIG. 14  is a perspective view of a cap, a cap lamp and insulated power wires in an embodiment of the present invention; 
         FIG. 15  is a perspective view of a handheld lamp embodiment of the present invention; 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     While the battery pack of the invention is described below in terms of use in powering a cap lamp of the type used in the mining industry, it may find application in other industries with other battery-powered devices. Indeed, the electronic control module of the invention may be integrated into a battery-powered device itself or a load attached to the battery pack, instead of a separate battery pack. In addition, while the battery pack described below features Lithium-ion (Li-ion) battery cells, the battery pack of the invention may feature other types of battery cells. 
     An embodiment of the battery pack of the present invention is illustrated in an exploded view in  FIG. 1 . The battery pack includes a battery housing or jar  7 , that is preferably made of polycarbonate, with an open top end. A cover  8 , also preferably made of polycarbonate, removably covers the open top of the battery jar, as illustrated in  FIG. 4 . 
     As illustrated in  FIG. 1 , a battery cell bundle  9  is positioned within the battery jar  7 . The bundle features battery cells, indicated at  10  in  FIGS. 2 and 3 , wrapped with a foam vibration-reducing wrap  11 . The foam wrap is preferably composed of neoprene and ethylene propylene diene monomer (EPDM) and is preferably approximately 2″×7.5″× 1/16″ thick. In addition, a pair of pads, one of which is indicated in phantom at  12  in  FIG. 1 , are positioned on opposite sides of the bundle, between the cells and wrap. Each pad  12  is preferably constructed from the same material as the wrap and is preferably approximately 1.25″×2.5″× 1/16″ thick. 
     Enlarged views of the battery cell bundle  9  of  FIG. 1  with the wrap and pads ( 11  and  12  in  FIG. 1 ) removed are provided in  FIGS. 2 and 3 . While eight battery cells  10  are illustrated, the battery pack could include an alternative number of cells. In addition, the cells preferably are Li-ion battery cells. As an example only, the battery pack may have a maximum voltage of 4.2 Volts DC and a minimum voltage of 2.5 Volts (V) DC. The battery pack may discharge at up to 2 amp, and may charge at up to 2.5 amp (A), also as an example only. The terminals of the battery cells  10  engage contact plates  13   a  ( FIG. 1) and 13   b  ( FIG. 3 ) which, as will be explained in greater detail below, are joined to a protection circuit, illustrated at  15  in  FIGS. 2 and 3 . 
     As illustrated in  FIG. 1 , a separator plate  17  (preferably also made of polycarbonate) is positioned over the battery cell bundle  9  so that a battery compartment is formed below and is secured within the battery jar  7  by adhesive, preferably so that the edges seal against the interior walls of the battery jar  7 . As a result, an electronic control module compartment is defined within the jar or housing  7  above the separator plate. An electronic control module (ECM)  20 , which contains circuitry and a microprocessor, as described in greater detail below, is positioned on top of the separator plate  17 , and communicates with the protection circuit  15  of  FIGS. 2 and 3 , and thus the battery cell bundle  9 , via a pair of wires  19  ( FIG. 2 ) that travel through notches  21  ( FIG. 1 ) of separator plate  17 . The circuitry and microprocessor of the ECM is preferably potted in a potting compound for protection. Potting compounds for circuitry and the like are well known in the art. 
     The protection circuit  15  of  FIGS. 2 and 3  is in circuit with the wires leading from the battery pack to the ECM and provides under-voltage cutoff, over-voltage cutoff and over-current cutoff protection. The protection circuit may be a standard, off-the-shelf circuit, such as the VC3053 from Venture Inc. As illustrated in  FIGS. 2 and 3 , the protection circuit  15  preferably is housed in a box-like structure composed of thermally conductive potting compound. This protects the printed circuit board and components from stress and vibration. 
     As illustrated in  FIGS. 1 and 2 , the ECM  20  includes positive and negative posts  14   a  and  14   b  and a charging status light emitting diode (LED)  16 . As illustrated in  FIG. 1 , a pair of O-ring seals  18  are positioned over the positive and negative posts of the ECM  20  so that they are sandwiched, and thus form a seal, between the top surface of the ECM  20  and the bottom surface of the battery post holder  22 . 
     The battery jar  7  and battery post holder  22  are preferably sonically welded together to seal the battery cell bundle, ECM, and other internal components inside the battery jar where they are protected from dirt and moisture. The cover  8  is reversible and secured to the battery jar  7  with cover hold down screws  24  ( FIG. 1 ) and a gasket, which may be molded into the cover  8 , for easy service and removal as well as effective sealing. The back side of the battery jar may be provided with a clip (not shown) so that the battery pack may be mounted on the belt of a user and may also feature a plug  26  ( FIG. 1 ) that seals a corresponding hole formed in the battery jar  7  so as to serve as a pressure relief valve. 
     The cover  8  includes a cord strain relief  28  ( FIGS. 1 and 4 ), preferably constructed of a rubber material, that receives insulated wires ( 23  in  FIG. 14 ) that attach to positive and negative posts  14   a  and  14   b  to provide power to the cap lamp ( 25  in  FIG. 14 ). An example of such a cap lamp is provided in U.S. Pat. No. 4,481,458 to Lane, the contents of which are hereby incorporated by reference. Alternatively, the lamp may be directly connected to the power pack, such as a handheld lamp arrangement ( FIG. 15 ). The cover also features elongated, transparent windows  32   a  and  32   b  ( FIG. 4 ) which are illuminated by the LED  16  ( FIGS. 1 and 2 ). 
     The operational features of the ECM  20  preferably include the charging status LED ( 16  in  FIGS. 1 and 2 ), short circuit protection, a low battery warning, a soft-start feature and a 2:1 charging/discharging ratio. In addition, the ECM preferably includes a charging voltage and current converter so that the battery pack may be used with chargers originally designed for lead-acid type batteries. 
     A block diagram illustrating the primary components and circuitry of the ECM  20  of  FIGS. 1 and 2  is provided in  FIG. 5 . As illustrated in  FIG. 5 , the ECM includes a microprocessor  34 . The ECM also includes a charging section circuit  36 , a low battery warning circuit  38 , an LED driver circuit  40 , a battery sensing circuit  42 , a charge current sensor circuit  44  and an overload sensor circuit  46 , all of which communicate with the microprocessor  34 . 
     A flow chart illustrating the programming of the microprocessor  34  of  FIG. 5  is provided in  FIGS. 6A-6B . As indicated by block  47  of  FIGS. 6A-6B , when microprocessor  34  is initially powered up, that is, connected to power, a number of default settings for the ECM occur. More specifically, transistors Q 4  and Q 5  of the charging section circuit, illustrated in  FIG. 7 , are turned off. As will be explained in greater detail below, transistors Q 4  and Q 5  of the charging section circuit are responsible for controlling current flow to and from the battery pack during charging and discharging. 
     In addition, the charging status LED  16  ( FIGS. 1 ,  2  and  13 ) is turned off as a default setting of the ECM. The charging status LED  16  is controlled by the microprocessor via the LED driver circuit  40  ( FIGS. 5 and 13 ) and illuminates windows  32   a  and  32   b  of the battery pack ( FIG. 4 ) with either a red or green color to indicate charging status. More specifically, a red LED is an indication that the battery is connected to a charger and is accepting a charge current. A green LED is an indication that the battery is connected to a charger, but it is no longer accepting charge current because it is fully charged and ready for operation. The operation of the LED driver circuit will be explained in greater detail below. 
     A “LAMP_WAS_ON” bit that is internal to the microprocessor is also set to “1” as the default setting of the ECM. This bit is an indication of whether the fully charged battery pack was used after being charged. This prevents the battery pack from being charged if it is disconnected and reconnected to a charger without application of a load. Charging of the battery pack may occur only if the bit is set to “1.” 
     Next, as illustrated at  48  in  FIGS. 6A-6B , the LED_GREEN pin of the microprocessor is checked for a high or low setting. The LED_GREEN pin is illustrated at  49  in  FIG. 9  as is microprocessor  34 . The high setting of the LED_GREEN pin corresponds to the charging status LED  16  being illuminated in green, and thus corresponds to the battery pack being in a fully charged condition. If this is the case, the battery pack goes into monitoring mode, as illustrated at block  50  in  FIGS. 6A-6B , where the battery capacity is monitored. If the battery voltage falls below a threshold due to self-discharge, and the battery pack is connected to a charger, charging restarts, as will be explained below. 
     When the LED_GREEN pin  49  ( FIG. 9 ) of the microprocessor is set to high, this is communicated to the to the LED driver circuit  40  ( FIGS. 5 and 13 ) via connection  43  of  FIG. 13  so that, as noted above, the charging status LED is illuminated in green. Power is received by this portion of the LED driver circuit  40  by connection  45  ( FIG. 13 ). 
     If the LED_GREEN pin of the microprocessor is low, the charging status LED is not illuminated in green. If this is the case, as indicated at  51  in  FIGS. 6A-6B , the microprocessor checks the battery pack for an over-discharged condition. More specifically, the battery sensing circuit  42  of  FIG. 5  is illustrated in greater detail in  FIG. 10  and features a voltage divider or measurement portion, indicated in general at  53 . The voltage measurement portion  53  of  FIG. 10  communicates via connection  55  with line  56  of the charging section circuit of  FIGS. 6A-6B , and thus the positive and negative terminals of the battery cell bundle, illustrated at  15   a  and  15   b , respectively, in  FIG. 8 , and determines the battery cell voltage. The battery cell voltage is communicated by the battery sensing circuit of  FIG. 10  to the microprocessor via the connection  57  (BAT) of  FIG. 10  and corresponding input pin  59  ( FIG. 9 ) of the microprocessor. If the battery cell voltage is equal to or less than 2.5V, the battery pack is in an over-discharged condition and, as indicated at  61  in  FIGS. 6A-6B , the charging status LED and transistors Q 4 , Q 5  and Q 8  ( FIGS. 6A-6B ) are shut off. As will be explained in greater detail below, pulse transistor Q 8  is responsible for controlling current during pulse width modulation operation of the battery pack. If the battery cell voltage is greater than 2.5V, the next step of  FIGS. 6A-6B  is performed by the microprocessor. 
     As indicated at  63  in  FIGS. 6A-6B , the microprocessor next checks for a fault condition, such as a short circuit or overload condition. As described previously, the ECM must handle a short circuit or overload (the term “short circuit” being used to mean either situation herein), such as caused by worn parts in the load or wires leading thereto, by causing the battery pack to go into protection mode so that the load (a cap lamp in the present example) is turned off. Prior art designs require the user to manually turn the cap lamp off and back on to reset the associated circuit prior to allowing current flow back to the cap lamp after the short circuit condition is removed. The ECM of the present invention features circuitry that automatically turns the cap lamp (or other load) back on after the short circuit condition is removed. In other words, the user does not have to manually turn the cap lamp off and back on to reset the battery pack. 
     With reference to  FIG. 5 , the automatic recovery feature is provided by the microprocessor  34 , charging section circuit  36  and low battery warning circuit  38  of the ECM. As noted previously, schematics illustrating the details of an embodiment of the charging section and low battery warning circuits are provided in  FIGS. 7 and 8 , respectively, while a schematic illustrating the microprocessor  34  and associated circuit is provided in  FIG. 9 . 
     With reference to  FIG. 7  and as noted previously, the positive and negative terminals or posts of the battery pack are illustrated at  14   a  and  14   b , respectively. During the discharge of the battery (such as when it is powering a load/cap lamp) current from the load and post  14   b  flows through ground point  52  ( FIG. 7 ) to ground point  54  ( FIG. 8 ), through resistor R 25  and negative terminal  15   b  of the battery cell bundle ( 9  in  FIG. 1 ) into the battery cell bundle. Current from the battery cell bundle flows through battery cell bundle positive terminal  15   a , line  56  ( FIG. 8 ) and line  58  ( FIG. 7 ). As illustrated in  FIG. 7 , the current traveling through line  58  encounters transistor Q 5  and then transistor Q 4  before traveling to the positive post of the battery pack  14   a  and out to the cap lamp load. 
     In addition to the microprocessor pins already described, as illustrated in  FIG. 9 , microprocessor  34  features a number of input and output pins which are connected to the various circuits illustrated in  FIG. 5 . The input pins are illustrated on the left side of the microprocessor  34  in  FIG. 9  while the output pins are illustrated on the right side. The charging section circuit  36  of  FIG. 7  communicates with the microprocessor voltage input pin Uinp  62  ( FIG. 9 ) via connection  64  ( FIG. 7 ). In addition, with reference to  FIG. 7 , connections  66  and  68  (CHARGE ON) and  72  (LOAD OFF) of charging section circuit  36  communicate with corresponding output pins  74  and  76  of the microprocessor  34 . The low battery warning/indication circuit  38  of  FIG. 8  features connections  78  (BATT ON) and  80  (DATA 1 ) that communicate with corresponding pins  82  and  84 , respectively, of the microprocessor  34  of  FIG. 9 . 
     A coulomb counter, illustrated at  85  in  FIG. 8 , senses the discharge current flowing through resistor R 25 . The sensed current is outputted from the coulomb counter  85  through connection  86  (Is). The sensed current is monitored via overload sensor circuit  46  ( FIGS. 5 and 11 ) as the circuit receives the sensed current through connections  86  ( FIG. 8) and 88  ( FIG. 11 ). As illustrated in  FIG. 11 , an operational amplifier  92  receives the sensed current from  88  and is programmed to check for the short circuit condition (indicated by a high current flow). When such a condition is detected, a signal indicating a short circuit condition is provided to the microprocessor via connection  94  ( FIG. 11 ) and microprocessor input pin  96  ( FIG. 9 ) so that the microprocessor input pin  96  (Overload Sens) is set to high. When conditions are normal (no short circuit), the Overload Sens input pin  96  of the microprocessor is set to low. 
     When a short circuit is sensed, as indicated at  63  and  97  in  FIG. 6 , the microprocessor turns off transistor Q 5 , and thus the load (cap lamp), via pin  76  ( FIG. 9 ) and, with reference to  FIG. 7 , connection  72  and switch Q 2  so that current may flow through line  99  and thus pulse transistor Q 8 . In addition, transistors Q 4  and Q 8  are turned off by the microprocessor via output pin  74  ( FIG. 9 ) and, with reference to  FIG. 7 ), connections  66  and  68  and switches Q 1  and Q 10 . 
     Next, as illustrated at  100  in  FIGS. 6A-6B , the voltage level at the terminals of the battery pack (Uinp) is measured using connection  64  of  FIG. 7  and corresponding input pin  62  ( FIG. 9 ) of the microprocessor to determine if the short condition still exists. If so, as indicated by block  101  in  FIG. 6 , pulse width modulation using pulse transistor Q 8  ( FIG. 7 ) occurs until the load/cap lamp turns on. The pulsing of transistor Q 8  allows small amounts of current to flow, all being sensed by the comparator circuit, indicated in general at  102  in  FIG. 7 . If the short circuit is still present, the comparator  102  will detect a rapid current rise when transistor Q 8  is turned on. The microprocessor will be so signaled by the comparator through the overload sensor circuit as connection  104  ( FIG. 7 ) of the comparator communicates with connection  88  of the overload sensor circuit ( FIG. 11 ). When the short circuit is still present, the microprocessor will continue to pulse transistor Q 8  while sensing the current. 
     When the short circuit is removed, the microprocessor turns transistor Q 5  on so that full current is restored to the cap lamp. As a result, the circuitry provides a self-resetting mechanism so that when the battery is shut down due to a short circuit, the load/cap lamp is automatically re-powered when the short circuit or is removed. No additional action is required by the user. 
     While the ECM of the present invention offers an automatic recovery feature for short circuits, a battery pack or load may optionally also feature a push-button or switch that resets the system and re-powers the load after the battery is shut down due to a short circuit when the short circuit is removed. 
     The charging section circuit  36  of  FIGS. 5 and 7  of the ECM also preferably provides the battery pack with a “soft-start” feature to avoid a massive inrush of current into the cap lamp bulb at start up, and thus increase bulb life. When the cap lamp is shut off, the microprocessor shuts off transistors Q 4  and Q 5  so that when the cap lamp is switched on or connected to the battery pack terminals, current must flow through branch  99  of  FIG. 7 . The ramp-up of electrical current (soft-start) is accomplished by pulse width modulation via transistor Q 8  as controlled by the microprocessor  34 . More specifically, transistor Q 8  is controlled in this manner as current flows to the cap lamp until full current is achieved and communicated to the microprocessor. Once full current is achieved, transistors Q 4  and Q 5  are turned on by the microprocessor and transistor Q 8  is turned off. Full current then flows to the cap lamp as described above. 
     Returning to  FIGS. 6A-6B , if no short circuit condition exists, the microprocessor checks for the presence of a charging current, as indicated at  106 . More specifically, a charge current circuit sensor circuit  44  ( FIGS. 5 and 12 ) receives the current sensed in the circuit of  FIGS. 7 and 8  via connections  86  ( FIG. 8) and 108  ( FIG. 12 ). If a charge current is sensed, with the assistance of operational amplifier  110  of  FIG. 12 , input pin  112  (ICharge) of the microprocessor ( FIG. 9 ) is notified via connection  114  ( FIG. 12 ) so that ICharge&gt;0 for purposes of  106  in  FIGS. 6A-6B . The flow chart then branches to the charge mode, as illustrated by  FIGS. 6A-6B . 
     For recharging, the battery pack is placed in a charging rack having a connector that engages a corresponding charging connection on the cap lamp. Such charging racks are well-known in the art. During recharging, the charging current enters the battery pack through the positive post  14   a  ( FIG. 7 ) of the battery pack and travels the reverse of the battery pack discharge route described above so that the charging current passes through transistor Q 4  and then transistor Q 5 . The charging current exits the battery pack through negative post  14   b . The charge ratio for the battery pack preferably is 2:1. Therefore, for every twelve hours of use, it will take six hours to recharge the battery pack. 
     As illustrated at  116  in  FIGS. 6A-6B , the LAMP_WAS_ON internal bit of the microprocessor  34  is again checked to ensure that it is set to 1, so that charging is permitted. If the LAMP_WAS_ON bit is set to 0, the ECM is set to default for discharge mode whereby the charging status LED is illuminated in green, Q 4  is turned off and Q 5  is turned on, as indicated at  118  and  120  in  FIGS. 6A-6B . In addition, as indicated at  120 , the coulomb counter count is set to 16 amp hours (Ah) as an indication of full charge for the battery pack via output pin  84  ( FIG. 9 ) of the microprocessor and connection  80  of  FIG. 8 . Flow then branches back to step  51 , as illustrated in  FIGS. 6A-6B , so that the top portion of the flow chart, including the short circuit check section, is performed. 
     If LAMP_WAS_ON=1, the battery pack has been discharged an unknown amount and must go into active charge mode and the next step,  122  of  FIGS. 6A-6B  is performed. At  122 , the battery cell voltage is checked by the microprocessor (via measurement portion  53  of the circuit of  FIG. 10 , connection  57  of  FIG. 10  and microprocessor input pin  59  of  FIG. 9 ). If the battery cell voltage is less than or equal to 4.2V, the flowchart branches to current mode, as illustrated in  FIGS. 6A-6B . In current mode, as indicated at  124 , a timer ( 125  in  FIG. 9 ) is started and the charging status LED ( 16  in  FIGS. 1 ,  2  and  13 ) is illuminated in red. With regard to the latter, the microprocessor sends a signal to the LED driver circuit  40  ( FIGS. 5 and 13 ) via microprocessor output pin  126  ( FIG. 9 ) and connection  128  of  FIG. 13 . Power is received by this portion of the LED driver circuit by connection  130 . In addition, during current mode, pulse width modulation via resistor Q 8  is activated. 
     As indicated at  132  in  FIGS. 6A-6B , the charging current Ibat (or Icharge) is monitored by the microprocessor. This occurs via the charge current sensor circuit  44  of  FIGS. 5 and 12  and input pin  112  of the microprocessor ( FIG. 9 ). The microprocessor adjusts the charging current by increasing or decreasing the pulse width modulation duty cycle of transistor Q 8  ( FIG. 7 ), as indicated by  134   a  and  134   b  in  FIGS. 6A-6B . As a result, a 2.5 A mean charge current is achieved while the battery charging state is at a constant current. Flow then branches back to step  51 , as illustrated in  FIGS. 6A-6B , so that the top portion of the flow chart, including the short circuit check section, is performed. The current mode of charging occurs until the battery cell voltage is greater than 4.2V, at which time voltage mode is initiated. 
     As illustrated at  136   FIGS. 6A-6B , the pulse width modulation of transistor Q 8  continues and the charging status LED is illuminated in red during the voltage mode of charging. As indicated at  138 , the timer  125  ( FIG. 9 ), which was turned on at  124  of  FIGS. 6A-6B , is checked to determine if it is greater than the timeout value (Tmax). If so, as illustrated in  FIGS. 6A-6B , the charging status LED is illuminated in green, charging is stopped and the discharge mode is initialized as indicated at  118  and  120  in  FIGS. 6A-6B . The timer is used for safety purposes and voltage mode rarely terminates due to the timer exceeding the timeout value. 
     If the timeout value has not been exceeded at  138  in  FIGS. 6A-6B , the charging current is checked at  142  by the microprocessor to determine if it is greater than the value Imax10%. Imax10% is equal to 10% of the maximum constant current (Imax) in the current mode. This is the typical termination mechanism for charging. If the charging current is not greater than Imax10%, the charging status LED is illuminated in green, charging is stopped and the discharge mode is initialized as indicated at  118  and  120  in  FIGS. 6A-6B . 
     Returning to  106  in  FIGS. 6A-6B , if no charging current is present, the microprocessor, and thus the ECM, enters the discharge mode, as indicated at  144 . As indicated by  146  in  FIGS. 6A-6B , capacitors Q 4  and Q 5  ( FIG. 7 ) are turned on and the charging status LED is illuminated in green. Next, as indicated at  148 , the coulomb counter ( 85  in  FIG. 8 ) count is checked by the microprocessor as an indication of the charge level of the battery pack. If the count is greater than or equal to 2 Ah, normal discharge mode continues and processing loops back to step  51  as illustrated in  FIGS. 6A-6B . As a result, a short circuit and general monitoring mode is performed continuously, whether the battery pack is in charge or discharge mode. 
     If the coulomb counter count is less than 2 Ah, the battery pack goes into low power mode where a low battery charge warning is provided. More specifically, as indicated at  152  in  FIGS. 6A-6B , the microprocessor turns transistors Q 4  and Q 5  ( FIG. 7 ) off and operates Q 8  in pulse width modulation mode so that the discharge of the battery pack occurs at low power. This causes the cap lamp load to dim. The dimmed light provides extended time for a miner to depart from the mine and obtain a fully charged battery pack. In addition, as indicated at  154  and  156  in  FIGS. 6A-6B , every two minutes the microprocessor turns on transistors Q 4  and Q 5  for one second so that the cap lamp flashes with full power, which acts as a warning of a low battery charge condition. As indicated at  158 , operation of Q 8  in pulse width modulation mode resumes after the flash so that the cap lamp is again dim. 
     The microprocessor  34  of  FIGS. 5 and 9  requires a constant voltage to run. This is provided by the voltage regulator  162  of the circuit of  FIG. 10 . More specifically, as noted previously, the circuit of  FIG. 10  receives voltage from the battery cell bundle (VDD) via connection  55 . This is converted by the voltage regulator  162  to voltage (VCC) that is provided to the microprocessor, and other components of the ECM such as the coulomb counter  85  of  FIG. 8  and the operational amplifiers  92  and  110  of  FIGS. 11 and 12 , respectively, via connection  164  ( FIG. 10 ). 
     As noted previously, the battery pack is provided with a protection circuit illustrated at  15  in  FIGS. 2 and 3  that provides under-voltage cutoff, over-voltage cutoff and over-current cutoff protection. The protection circuit therefore acts as a backup to the ECM circuitry and microprocessor programming discussed with respect to  FIG. 6 . As examples only, an over-voltage condition may occur if the protection circuit detects a voltage of 4.35V or greater, while an under-voltage condition may occur if the protection circuit detects a voltage of 2.5V or less. An over-current condition may exist if the current exceeds 4.5 A. If any of these conditions exist, the protection circuit is tripped like a circuit breaker. As a result, the protection circuit must be reset before the battery pack may be used again. 
     The protection circuit is reset using the capacitor bank circuit indicated in general at  172  in  FIG. 8 . Transistor QB 6  ( FIG. 8 ) permits energy to flow into the capacitor bank circuit  172 , but does not permit it to escape until so directed by the microprocessor. As a result, energy is stored in the capacitor bank circuit  172 . When the protection circuit ( 15  of  FIGS. 2 and 3 ) is tripped, input pin  59  ( FIG. 9 ) of the microprocessor goes to zero and the microprocessor signals the capacitor bank circuit  172  to release the stored energy via connection  78  ( FIG. 8 ) and microprocessor output pin  82  ( FIG. 9 ). This release of energy causes the battery protection circuit to reset. 
     The voltages, currents and times of  FIGS. 6A-6B  are presented as examples only and are in no way to limit the scope of the invention. 
     While the preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the appended claims.