Abstract:
A splitting adapter for a battery charger having a power cord for connection to an electrical source, the charger having a single charging receptacle for receiving a connector extension of a battery pack of the type used with cordless power tools, said adapter including a housing having a housing connector extension with a sufficiently similar structural and electrical configuration as the connector extension of the battery pack so that the housing can be electrically connected to the battery charger when said housing connector extension is inserted into the charging receptacle. The splitting adapter also includes at least two charging receptacles, each of which is configured to receive a battery pack connector extension, where the charging receptacles are spaced from one another a sufficient distance that the battery packs will not interfere with one another when inserted into the adapter.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to battery chargers, and more specifically relates to an adapter apparatus for battery chargers used with cordless power tools. 
     There has been continued innovation in the field of battery chargers that are used with cordless power tools. Examples of such battery chargers are those produced under the SKIL and BOSCH brands by the S-B Power Tool Corporation of Chicago, Ill., which are used with various cordless power tools also produced under the same brands. A typical battery pack ordinarily has a generally rectangular housing and a male stem connector extending from a surface thereof. The battery chargers ordinarily used for charging these battery packs are correspondingly sized and configured to contain a female slot for receiving the stem connector of the battery pack. The battery packs are further equipped with terminals for establishing electrical contact with mating terminals that are configured on an internal circumference of the female slot of the battery charger. 
     In addition to the power tool itself, a power tool kit will often include two battery packs and a single battery charger, which is capable of charging only a single battery pack at a time. Heavy users will frequently purchase additional battery packs. Having multiple battery packs of a given type enables the user to continuously use the cordless power tool by swapping out depleted battery packs for fully charged battery packs. However, users typically have more battery packs than they have chargers, which prevents optimal charging of multiple battery packs at one time. This results an in inconvenience to the user, who must instead serially charge the battery packs. Moreover, the most common type of power tool battery pack uses Nickel-Cadmium (NiCd) batteries, which lose their charge over time when the battery packs are not inserted into a charger. Thus, users of conventional battery chargers are unable to maintain the charge in the battery packs during periods when the batteries are not in use. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a particularly efficient adapter apparatus for use with battery chargers for cordless power tools for allowing the user to charge multiple battery packs using a single charger. The present adapter apparatus couples to a battery charger and comprises a generally rectangular housing having a top portion and a bottom portion, with at least two female charging receptacles on the top portion, each charging receptacle being configured to receive a male stem connector of the battery pack. The bottom portion of the adapter also includes a male stem connector that is sized and configured to be received by a correspondingly sized and configured female charging receptacle in the battery charger. The battery charger is coupled to an AC outlet via a power cord extending from a rear surface of the battery charger. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is top perspective view of the adapter apparatus of the instant invention shown with a battery charger and battery packs. 
     FIG. 2 is a top perspective view of the adapter apparatus illustrated in FIG. 1 in use with a battery charger and battery packs. 
     FIG. 3 is a top view of the battery charger illustrated in FIG.  1 . 
     FIG. 4 is a top view of the adapter apparatus illustrated in FIG.  1 . 
     FIG. 5 is a bottom view of the adapter apparatus illustrated in FIG.  1 . 
     FIG. 6 is a side view of the adapter apparatus illustrated in FIG.  1 . 
     FIG. 7 is a bottom view of the battery pack illustrated in FIG.  1 . 
     FIG. 8 is a side view of the battery pack illustrated in FIG.  1 . 
     FIG. 9 is a circuit diagram for the internal electronic circuitry of the adapter apparatus illustrated in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to FIGS. 1-8, the preferred embodiment of the present adapter apparatus is indicated generally at  10  and includes a generally rectangular adapter housing  12  having a top surface portion  14  and a bottom surface portion  16 , where at least two female charging receptacles  18  are disposed on the top portion. Disposed on the bottom surface portion  16  is a male stem connector  20 . The adapter apparatus  10  is coupled to a battery charger apparatus  22  having a charger housing  24 , a charger top portion  26 , and a charger bottom portion (not shown), wherein the charger top portion includes a single female slot  30  at a top surface thereof. In the absence of the adapter apparatus  10  of the instant invention, a battery pack  31  is ordinarily coupled directly to the battery charger apparatus  22  used in conjunction with the instant invention. Thus, the adapter apparatus  10  of the instant invention acts as a mechanical and electrical intermediary between the charger apparatus  22  and the battery pack  31 , and accordingly includes features sized and configured to mimic the insertion of a battery pack within the charger apparatus and the receiving of a battery pack within the adapter apparatus. 
     To couple the adapter apparatus  10  to the battery charger apparatus  22 , and referring to FIG. 5, the stem connector  20  of the adapter apparatus matingly engages the single female slot  30  on the charger top portion, which is sized and configured to receive the stem connector of the adapter apparatus. To this end, the stem connector  20  of the adapter apparatus  10  is configured to matingly engage the female slot  30  of the charger apparatus  22 . The stem connector  20  includes a first end portion  32  and a second end portion  34  that are configured to be unitary with one another, wherein the first end portion is generally arcuate at its circumferential edge, and the second end portion is generally rectangular at its circumferential edge. In the preferred embodiment of the instant invention, stem connector  20  is disposed in a corner portion of the bottom surface portion  16  of adapter apparatus  10 , with the arcuate first end portion  32  nearest an edge of the bottom surface portion and the generally rectangular second end portion  34  nearest a center of the bottom surface portion. Preferably, the stem connector  20  is configured and arranged so that a longitudinal axis of the stem connector generally bisects the plane of the bottom surface portion  16  in a diagonal direction. 
     The arcuate first end portion  32  of the stem connector  20  includes a pair of generally rectangular female cavities  36  that are adjacent one another on either side of a first wall  38  separating them. At least one of the pair of female cavities  36  includes a slot  40  at a bottom surface thereof. Disposed above the pair of female cavities  36  is a top channel  42 , which is a three sided depression having an open front edge, and includes a front slot  44  extending downward from a bottom surface of the top channel. The top channel  42  is separated from the female cavities  36  by a second wall  46 . Disposed below the pair of generally rectangular female cavities  36  is a pair of second female channels  48  that are separated from the female cavities by a third wall  50  and separated from each other by a fourth wall  52 . The second female channels  48  are open at an edge opposite the common fourth wall  52 , and each second female channel has a side slot  54  extending downward from a bottom surface of the second female channels. 
     Accordingly, the single female slot  30  of the battery charger (see FIG. 3) is disposed in a corner portion of the top portion  26  of the battery charger apparatus  22 . The female slot  30  is generally obround in shape, being generally rectangular with top and bottom arcuate end portions  56 ,  58 . The top arcuate end portion  56  is sized and configured to have a radius of curvature corresponding to a radius of curvature of the first portion  32  of the stem connector  20 . A bottom surface of the female slot  30  is divided into a first surface portion  60  and a second surface portion  62 , wherein the second surface portion is elevationally higher than the first surface portion with respect to the top portion  26  of the battery charger apparatus  22 . The second surface portion  62  therefore is a shelf-like surface within the female slot  30 , and has an arcuate end coextensive with the bottom arcuate end portion  58  of the female slot  30  and a generally linear front edge  64 . The linear front edge  64  is sized and configured to correspond to the second portion  34  of the stem connector  20 . Thus, the female slot  30  is sized and configured to nestingly engage with stem connector  20  wherein the top arcuate end portion  56  abuts the correspondingly sized and configured first portion  32  of the stem connector  20  and the linear front edge  64  of the of the second surface portion  62  of the female slot  30  abuts the second portion  34  of the stem connector  20 . Moreover, the female slot  30  is preferably disposed on the battery charger apparatus  22  in an orientation similar to the orientation of the stem connector  20  on the adapter apparatus  10 . Specifically, the top arcuate end portion  56  is most proximate an edge of the battery charger apparatus  22  whereas the bottom arcuate end portion  58  is most distal, and a longitudinal axis extending therebetween generally bisects the battery charger apparatus in a diagonal direction. 
     In the preferred embodiment, the surfaces of both the female slot  30  and the stem connector  20  are configured to matingly engage one another. Extending from the first surface portion  60  of the bottom surface of the female slot  30  are a plurality of male projections, including a pair of rear slotted projections  66 , a pair of generally middle projections  68 , and a front slotted projection  70 . The rear slotted projections  66  are generally rectangular and oppose one another along an inner circumference of the female slot  30  and are unitary with the circumference but separated from one another at sides most proximate one another by a portion of the first surface portion  60 . Each rear slotted projection  66  includes a slot portion  72  running therethrough in a direction generally parallel to the longitudinal axis of the female slot  30 , and each slot portion includes two side walls and a bottom wall, but are preferably open at the front and rear sides. The pair of middle projections  68  are also generally rectangular and are disposed above the rear slotted projections  66 , separated by a portion of the first surface portion  60  of the bottom surface. Like the rear slotted projections  66 , the middle projections  68  are separated from one another at sides most proximate to one another by a portion of the first surface portion  60 . The sides of the middle projections  68  that are most distal to one another include generally rectangular cavities  74  having four side walls and a bottom wall. The front slotted projection  70  is preferably disposed above the middle projections  68  having a front wall that abuts an inner circumferential surface of the female slot  30  and a rear wall that is separated from the front wall by a slot portion  76  extending the width of the front slotted projection  70  in a direction generally perpendicular to the direction of the slot portion  62  of the rear slotted projection  66 . 
     Therefore, when the stem connector  20  is inserted into the female slot  30 , the top channel  42  of the stem connector  20  nestingly receives the front slotted projection  70 . Each of the female cavities  36  nestingly receives a corresponding one of the middle projections  68 . Similarly, each of the second female channels  48  nestingly receives a corresponding one of the rear slotted projections  66 . Additionally, an internal circumferential surface of the female slot  30  of the charger apparatus  22  preferably includes at least two guide ribs  78 , wherein a single guide rib is vertically disposed at each of the top and bottom arcuate end portions  56 ,  58 , and are configured to oppose one another along the internal circumference of the female slot. A plurality of side wall guide ribs  80 , preferably three, are also disposed on the internal circumferential surface of the female slot  30  along a side wall portion. Opposite the side wall portion containing the side wall guide ribs  80  is a guide channel  82  defined on each side by generally planar raised portions  84 . The stem connector  20  of the adapter apparatus  10  includes corresponding projections and recesses that engage these configurations on the internal circumferential surface of the female slot. In this manner, the stem connector  20  is mechanically coupled to the female slot  30 . 
     Electrical coupling between the stem connector  20  and the female slot  30  is also established via a plurality of terminal connectors disposed within the various projections of each. More specifically, the stem connector  20  generally includes a terminal contact (not shown) disposed within the at least one slot  40  of the female cavities  36 , the front slot  44  within the top channel  42 , and the side slots  54  of each of the pair of second female channels  48 . The terminal contacts (not shown) of the preferred embodiment are made of steel, but it is contemplated that the terminal contacts may be made from any electrically conductive metal typically used in batteries. The terminal contacts engage a respective slot or cavity disposed within the female slot  30 . More specifically, the slot portion  72  disposed within the rear slotted projections  66  matingly engage the terminal contacts within the second female channels  48 , the cavities  74  disposed within the middle projections  68  matingly engage the terminal contacts of the female cavities  36 , and the slot portion  76  of the front slotted projection  70  matingly engages the terminal contact within the top channel  42 . 
     Generally, the adapter apparatus  10  includes physical configurations that mimic the insertion of a battery, thus adapting the battery charger to receive multiple battery packs within the charging receptacles  18  on the adapter apparatus. Numerous conventional commercial battery packs are contemplated for use with the instant invention. In the preferred embodiment of the instant adapter apparatus  10 , an 18-volt power tool battery manufactured by S-B Power Tool of Chicago, Ill. is used. Typically, a battery pack  31  of the type used with the instant invention a main power pack  94  and a stem connector  96  for establishing and maintaining electrical coupling with adapter apparatus  10 . 
     For purposes of description and illustration, the 18-volt battery having a main power pack  94  and a stem connector  96  will be shown and described. The power pack  94  includes a generally cylindrical housing and includes a first side  98  and a second side  100 , where the stem connector  96  extends from an upper section of the second side  100  of the power pack  94  in a direction generally perpendicular to a plane of the second side  100 . The first side  98  is disposed opposite of the second side  100 . Since the battery pack  31  is typically either coupled with either the tool with which it is used or with a battery charger apparatus  22 , the stem connector  96  of the battery pack  31  includes surface configurations at a distal end thereof that matingly correspond to the projections extending from a bottom surface of the female slot  30  on the battery charger apparatus. Accordingly, since the instant adapter apparatus  10  matingly engages the same battery pack  31  that is ordinarily coupled directly to the battery charger apparatus  22 , the charging receptacles  18  disposed on top surface portion  14  of adapter apparatus  10  are sized and configured to mimic the female slot  30  of the charger apparatus. 
     In the preferred embodiment of the instant invention, and referring to FIG. 4, two charging receptacles  18  are disposed on the top surface portion  14  of the adapter apparatus  10  and are separated from one another by a portion of the top surface portion  14  sufficient to allow the second side  100  of the power pack  94  of one battery pack  31  to abut the surface of the top surface portion without contacting the power pack of a second battery pack. Like the female slot  30  on the charger apparatus  22 , each of the charging receptacles is obround in shape, being generally rectangular but having arcuate top and bottom end portions  102 ,  104 . The charging receptacles  18  are oriented so that the longitudinal axis of each is generally parallel to one another, with the arcuate top end portions  102  and arcuate bottom end portions  104  of each oriented in the same direction. The rectangular side portions are similarly generally parallel to one another. A bottom surface of each charging receptacle  18  is divided into a first surface portion  106  and a second surface portion  108  wherein the second surface portion is elevationally higher than the first surface portion with respect to the top surface portion  14  of the adapter apparatus  10 . The second surface portion  108  therefore is a shelf-like surface within the charging receptacle  18 , and has an arcuate end coextensive with the bottom arcuate end portion  104  of the charging receptacle and a generally linear front edge  110 . 
     Extending from the first surface portion  106  of the bottom surface of the charging receptacle  18  are a plurality of male projections corresponding to those male projections extending from the first surface portion  60  of the female slot  30 , including a pair of rear slotted projections  112 , a pair of generally middle projections  114 , and a front slotted projection  116 . Like those disposed within the female slot  30  of the battery charger apparatus  22 , the rear slotted projections  112  of the charging receptacle  18  are generally rectangular and oppose one another along an inner circumference of the charging receptacle  18  and are unitary with the circumference but separated from one another at sides most proximate one another by a portion of the first surface portion  106 . Each rear slotted projection  112  includes a slot portion  118  running therethrough in a direction generally parallel to the longitudinal axis of the charging receptacle  18 , and each slot portion includes two side walls and a bottom wall, but are preferably open at the front and rear sides. The pair of middle projections  114  are also generally rectangular and are disposed above the rear slotted projections  112 , separated by a portion of the first surface portion  106  of the bottom surface. Like the rear slotted projections  112 , the middle projections  114  are separated from one another at sides most proximate to one another by a portion of the first surface portion  106 . The sides of the middle projections  114  that are most distal to one another include generally rectangular cavities  120  having four side walls and a bottom wall. The front slotted projection  116  is preferably disposed above the middle projections  114  having a front wall that abuts an inner circumferential surface of the charging receptacle  18  and a rear wall that is separated from the front wall by a slot portion  122  extending the width of the front slotted projection  116  in a direction generally perpendicular to the direction of the slot portion  118  of the rear slotted projection  112 . 
     Just as the stem connector  20  of the adapter apparatus includes configurations that matingly engage corresponding configurations disposed within the female slot  30  of the battery charger apparatus  22 , the stem connector  96  of the battery pack  31  includes configurations at a distal end thereof for mechanically and electrically coupling to corresponding configurations disposed within the charging receptacles  18 . The configurations on the distal end of the stem connector  96  of the battery pack  31  are therefore similar to those disposed on the end of the stem connector  20  extending from the adapter apparatus. 
     The distal end of the stem connector  96  of the battery pack  31  includes a first portion  124  and a second portion  126  that are configured to be unitary with one another, wherein the first portion is generally arcuate at its circumferential edge, and the second portion is generally rectangular at its circumferential edge. The arcuate first portion  124  of the stem connector  96  includes a pair of generally rectangular female cavities  128  that oppose one another on either side of a first wall  130  separating them. At least one of the pair of female cavities  128  includes a slot (not shown) at a bottom surface thereof. Disposed above the pair of female cavities  128  is a top channel  134 , which is a three sided depression having an open front edge, and includes a front slot (not shown) extending downward from a bottom surface of the top channel. The top channel  134  is separated from the female cavities  128  by a second wall  138 . Disposed below the pair of generally rectangular female cavities  128  is a pair of second female channels  140  that are separated from the female cavities by a third wall  142  and separated from each other by a fourth wall  144 . The second female channels  140  are open at an edge opposite the common fourth wall  144 , and each second female channel has a side slot (not shown) extending downward from a bottom surface of the second female channels. 
     The engagement of the stem connector  96  of the battery pack  31  with the charging receptacles  18  of the adapter apparatus  10  is accomplished in the same manner as the engagement of the stem connector  20  of the adapter apparatus with the female slot  30  of the battery charger apparatus  22 . When the stem connector  96  is slidably inserted into one of the charging receptacles  18 , the top channel  134  of the stem connector nestingly receives the front slotted projection  116 . Each of the rectangular female cavities  128  nestingly receives a corresponding one of the middle projections  114 . Similarly, each of the second female channels  140  nestingly receives a corresponding one of the rear slotted projections  112 . Additionally, an internal circumferential surface of each of the charging receptacles  18  preferably includes at least two guide ribs  148 , wherein a single guide rib is vertically disposed at each of the top and bottom arcuate end portions  102 ,  104 , and are configured to oppose one another along the internal circumference of the charging receptacle. A plurality of side wall guide ribs  150 , preferably three, are also disposed on the internal circumferential surface of the charging receptacle  18  along a side wall portion. Opposite the side wall portion containing the side wall guide ribs  150  is a guide channel  152  defined on each side by generally planar raised portions  154 . The stem connector  96  of the battery pack  31  includes corresponding projections and recesses that engage these configurations on the internal circumferential surface of the charging receptacle. In this manner, the stem connector  96  of each battery pack  31  is mechanically coupled to a respective one of the charging receptacles  18  disposed on the adapter apparatus. 
     As with the stem connector  20  and the female slot  30 , electrical coupling between the stem connector  96  of the battery pack  31  and one of the charging receptacles is also established via a plurality of terminal connectors disposed within the various projections of each. More specifically, the stem connector  96  includes terminal contacts  162  disposed within a slot (not shown) disposed within at least one of the female cavities  128 , a front slot (not shown) within the top channel  134 , and side slots (not shown) of each of the pair of second female channels  140 . The terminal contacts  162  of the preferred embodiment are made of nickel-plated steel, but it is contemplated that the terminal contacts may be made from any electrically conductive metal typically used in batteries. The terminal contacts  162  engage a respective slot or cavity disposed within one of the charging receptacles  18 . More specifically, the slot portion  118  disposed within the rear slotted projections  112  matingly engage the terminal contacts  162  within the second female channels  140 , the cavities  120  disposed within the middle projections  114  matingly engage the terminal contacts  162  of the female cavities  128 , and the slot portion  122  of the front slotted projection  116  matingly engages the terminal contact  162  within the top channel  134 . 
     Thus, as shown in FIGS. 1 and 2, when the adapter apparatus  10  of the instant apparatus is in use, the stem connector  20  of the adapter apparatus is mechanically and electrically coupled to the female slot  30  of the charger apparatus  22 . The charger apparatus is electrically coupled to an AC power outlet or other suitable power source via a power cord  164  extending from the charger apparatus  22 . Either prior to or following mating of the adapter apparatus to the charger apparatus  22 , one or more battery packs  31  are each coupled to a respective charging receptacle  18  disposed on the adapter apparatus. In this manner, the adapter apparatus  10  is electrically and mechanically coupled to both the charger apparatus  22  and one or more battery packs  31 . 
     Turning now to FIG. 9, charging of the battery packs coupled to the adapter apparatus  10  of the instant invention is promoted by internal electronic circuitry within the adapter apparatus. While the instant invention contemplates use of circuitry capable of charging numerous charging multiple batteries, a circuit diagram for charging two battery packs  31  is shown and will be described. Very generally, the internal electronic circuitry, designated generally at  166 , senses the relative charge of each of the two battery packs  31  disposed within the charging receptacles  18  of the adapter apparatus  10 , and is capable of sending a charging current to one or the other battery pack. The internal electronic circuitry is capable of sending various charge intensities, and may specifically send either a “fast charge” or a “trickle charge.” Also preferably included within the internal electronic circuitry  166  is circuitry for visual feedback to indicate which battery pack is being charged and the relative state of charge of that battery pack. A preferred example of this visual feedback is a light emitting diode (LED) display  168  (best shown in FIG.  1 ). Additionally, the preferred embodiment of the instant invention includes internal electronic circuitry  166  allowing the adapter to pass a “trickle charge” for maintaining a full charge on at least one of the battery packs  31 , thereby maintaining a full charge on both battery packs for an indefinite period of time. 
     The internal electronic circuitry  166  will operate with all existing one-hour chargers and battery packs currently manufactured by Bosch or Skil. One-hour chargers have three connections: battery plus, battery minus, and negative temperature coefficient (NTC) thermistor. Thus, the internal electronic circuitry of the instant invention includes connections for first battery plus  170 , first battery minus  172 , first battery NTC  174 , second battery plus  176 , second battery minus  178 , and second battery NTC  180 . The internal electronic circuitry  166  senses the difference between fast and trickle charging and switches over from the first battery pack to the second battery pack. 
     Transformer chargers typically lack an open-circuit voltage, while the switching type one-hour chargers used with the preferred embodiment of the instant invention have a predetermined open-circuit voltage, for example 42V. Thus, circuit energy originates from the battery pack  31  itself. 
     The internal electronic circuitry  166  used with the preferred embodiment uses a double pole double throw (DPDT) relay  181 . Normally closed contacts charge one of the batteries in a known manner. When a start button  182  is activated by a user, the DPDT relay  181  is switched to the second battery and into the “fast charge” mode. Preferably, no charge is sent to the first battery pack until the second battery pack is completely charged, and then it is automatically sent back to the first battery. Following the complete “fast charge” of both batteries, the first battery continues to receive a trickle charge for an indefinite period of time. The DPDT relay  181  switches the battery plus and NTC contacts, and the battery minus is placed at common ground. 
     Preferably, a current sensitive resistor  184  senses the charging current. A 1-ampere charge rate will yield about a 10-millivolt (mV) signal; a 2.5 ampere charge rate will create a 25-mV signal, while a 10-amp charge rate will have a 100-mV signal. The second battery NTC  180  signal is preferably 10 millivolts above ground. 
     To accommodate a predetermined battery pack voltage range, preferably a range from 12 to 24 volts, a voltage regulator integrated circuit  186  is used in the internal electronic circuitry  166  to regulate the circuit electronics at a constant 12 volts. However, if a lower battery voltage is desired, the voltage regulator  186  may be reduced, for example, to 9 volts, but the relay coil energizing current would consequently increase. 
     Capacitors  188 ,  190  are included in circuit with the regulator  186  for input and output filtering. 
     A low pass filter comprising a resistor  192  and a capacitor  194  is connected between the current sensing resistor  184  and an operational amplifier  196 . The low pass filter is configured to attenuate any signal above 1 kHz, and the filtered signal is fed to the operational amplifier  196  to amplify the signal. Resistors  198 ,  200  define the amount of the amplification as is known in the art. For example, a 10-mV signal will preferably be amplified to approximately 1 volt, and a 25-mV signal will be amplified to about 2.5 volts. 
     However, while the transformer signal is discontinuous and will exceed 1 volt after amplification, the switching power supply has a steady, continuous signal when amplified. A capacitor  202  is a filter capacitor, and is connected to the output of the operational amplifier  196  and produces an output signal that is continuous with a triangular ripple. However, this signal is periodically interrupted by the charger microprocessor  204  to measure the battery voltage. The capacitor  202  keeps the relay coil closed during this brief interruption. 
     Diodes  206  and  208  are connected in series to clip the voltage above 0.9 volt, with the diode  208  being a Schottky diode and preferably exhibiting a 0.2 voltage drop. The combination of the capacitor  202  and the voltage clipping circuit helps to prevent relay chattering when the fast charge signal converts to trickle charge. The trickle charge current preferably has the same 120 Hz current pulse as the fast charge mode, but a lower duty cycle of approximately 5%. 
     A resistor  210  is a voltage-dropping resistor and a diode  212  protects the reverse base-emitter transistor junction of a transistor  214  by limiting the base emitter voltage to −0.7 volt. The transistor  214  is a relay driver transistor that energizes the relay coil during fast charge and shuts down during trickle charge. 
     A diode  216  absorbs the relay coil energy when the relay coil is deenergized, and this helps to prevent voltage spikes. A diode  218  is an LED, which lights during fast charge of the second battery pack. A resistor  220  limits the LED current to a safe operating level. 
     The master start push button  182  energizes a field effect transistor (FET)  224 , which in turn energizes the DPDT relay  181  for about 3 seconds to allow the current sensing signal adequate time to continue driving the relay coil. A resistor  226  drops the 12-volt power supply voltage to preferably 5.1 volts and a diode  228  is a 5.1-volt zener diode. The energy is stored by a capacitor  230 . The FET  224  is kept on until the voltage falls exponentially below the gate-source threshold level. A resistor  232  depletes the remaining energy in the capacitor  230 . 
     While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims.