Abstract:
A charge control device for providing a constant charge voltage with temperature compensation to a battery being charged by a constant current charger is provided. The device includes a first electrical connector that couples to a positive terminal of the battery and a second electrical connector that couples to a negative terminal of the battery. A current bypass circuit electrically couples to the positive and negative terminals of the battery through respective first and second electrical connectors. The current bypass circuit includes a bypass path for a portion of a charge current from the constant current charger to flow, thereby maintaining a substantially constant voltage across the battery terminals at a particular temperature.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to rechargeable storage batteries. More specifically, the present invention relates to a charge control device used for such storage batteries. 
     Chemical batteries which create electricity from chemical reactions have been known for many years. Such batteries are becoming increasingly important and have found uses throughout industry. These uses include automobiles, UPS systems, etc. 
     One advantage of chemical batteries, such as lead-acid storage batteries, is that they can be charged and the chemical process reversed by forcing electricity through the battery. Charging systems are widely known in the art and are widely available in the consumer market. One of the most common techniques for recharging storage batteries is simply placing a voltage source across the battery having a voltage which is greater than the battery voltage. The voltage difference will cause a charging current to flow through the battery causing a reversal of the chemical reaction. The charging current decreases as the voltage difference between the charging voltage and the battery voltage decreases. Typically, the charging voltage is selected to be greater than the nominal battery voltage in order to cause a slight overcharge of the battery. The battery is deemed to be “charged” when the battery will accept no additional current. Frequently, this is through a simple visual inspection of an amp meter on the battery charger by the user of the battery charger. The battery charger may then be switched off. This constant voltage charging technique is relatively safe since as the charging process progresses, the charging current decreases until it is just a trickle. 
     A constant current charger is another type of charger used to charge rechargeable batteries. Constant current chargers vary the voltage they apply to the battery to maintain a constant current flow. As the current drops during the charging process, the charger automatically rises its voltage to keep the same current amplitude flowing. When the battery is fully charged, there must be some mechanism for stopping the constant current charger, otherwise, the battery would continue to charge and may lead to excessive overcharging of the battery that can permanently damage the battery and even lead to the boiling of the battery electrochemicals. On the other hand, undercharging of a battery results in a battery that is not capable of providing its full potential output. Thus, if a constant current charger is not shut off as soon as the battery is charged to an optimum level, one or more of the above-described problems could occur. 
     Another problem with battery charging is that the temperature of the battery typically rises during the recharging cycle. As the temperature of the battery increases, the chemical reactivity increases; the reactivity doubles approximately every 10 degrees Centigrade (or Celsius) for lead-acid batteries. Further, as the temperature of the battery increases, the internal resistance decreases so that the battery accepts a larger charging current at a given charging voltage. The increased current flow generates additional heating of the battery, further reducing its internal resistance. This cycle of battery heating followed by an increase in battery charging current results in a run-away condition which can damage the battery and cause it to fail. 
     Various types of battery testing and charging techniques are shown in U.S. Pat. No. 3,873,911, issued Mar. 25, 1975, to Champlin, entitled ELECTRONIC BATTERY TESTING DEVICE; U.S. Pat. No. 3,909,708, issued Sep. 30, 1975, to Champlin, entitled ELECTRONIC BATTERY TESTING DEVICE; U.S. Pat. No. 4,816,768, issued Mar. 28, 1989, to Champlin, entitled ELECTRONIC BATTERY TESTING DEVICE; U.S. Pat. No. 4,825,170, issued Apr. 25, 1989, to Champlin, entitled ELECTRONIC BATTERY TESTING DEVICE WITH AUTOMATIC VOLTAGE SCALING; U.S. Pat. No. 4,881,038, issued Nov. 14, 1989, to Champlin, entitled ELECTRONIC BATTERY TESTING DEVICE WITH AUTOMATIC VOLTAGE SCALING TO DETERMINE DYNAMIC CONDUCTANCE; U.S. Pat. No. 4,912,416, issued Mar. 27, 1990, to Champlin, entitled ELECTRONIC BATTERY TESTING DEVICE WITH STATE-OF-CHARGE COMPENSATION; U.S. Pat. No. 5,140,269, issued Aug. 18, 1992, to Champlin, entitled ELECTRONIC TESTER FOR ASSESSING BATTERY/CELL CAPACITY; U.S. Pat. No. 5,343,380, issued Aug. 30, 1994, entitled METHOD AND APPARATUS FOR SUPPRESSING TIME VARYING SIGNALS IN BATTERIES UNDERGOING CHARGING OR DISCHARGING; U.S. Pat. No. 5,572,136, issued Nov. 5, 1996, entitled ELECTRONIC BATTERY TESTER WITH AUTOMATIC COMPENSATION FOR LOW STATE-OF-CHARGE; U.S. Pat. No. 5,574,355, issued Nov. 12, 1996, entitled METHOD AND APPARATUS FOR DETECTION AND CONTROL OF THERMAL RUNAWAY IN A BATTERY UNDER CHARGE; U.S. Pat. No. 5,585,416, issued Dec. 10, 1996, entitled APPARATUS AND METHOD FOR STEP-CHARGING BATTERIES TO OPTIMIZE CHARGE ACCEPTANCE; U.S. Pat. No. 5,585,728, issued Dec. 17, 1996, entitled ELECTRONIC BATTERY TESTER WITH AUTOMATIC COMPENSATION FOR LOW STATE-OF-CHARGE; U.S. Pat. No. 5,589,757, issued Dec. 31, 1996, entitled APPARATUS AND METHOD FOR STEP-CHARGING BATTERIES TO OPTIMIZE CHARGE ACCEPTANCE; U.S. Pat. No. 5,592,093, issued Jan. 7, 1997, entitled ELECTRONIC BATTERY TESTING DEVICE LOOSE TERMINAL CONNECTION DETECTION VIA A COMPARISON CIRCUIT; U.S. Pat. No. 5,598,098, issued Jan. 28, 1997, entitled ELECTRONIC BATTERY TESTER WITH VERY HIGH NOISE IMMUNITY; U.S. Pat. No. 5,656,920, issued Aug. 12, 1997, entitled METHOD FOR OPTIMIZING THE CHARGING LEAD-ACID BATTERIES AND AN INTERACTIVE CHARGER; U.S. Pat. No. 5,757,192, issued May 26, 1998, entitled METHOD AND APPARATUS FOR DETECTING A BAD CELL IN A STORAGE BATTERY; U.S. Pat. No. 5,821,756, issued Oct. 13, 1998, entitled ELECTRONIC BATTERY TESTER WITH TAILORED COMPENSATION FOR LOW STATE-OF-CHARGE; U.S. Pat. No. 5,831,435, issued Nov. 3, 1998, entitled BATTERY TESTER FOR JIS STANDARD; U.S. Pat. No. 5,914,605, issued Jun. 22, 1999, entitled ELECTRONIC BATTERY TESTER; U.S. Pat. No. 5,945,829, issued Aug. 31, 1999, entitled MIDPOINT BATTERY MONITORING; U.S. Pat. No. 6,002,238, issued Dec. 14, 1999, entitled METHOD AND APPARATUS FOR MEASURING COMPLEX IMPEDANCE OF CELLS AND BATTERIES; U.S. Pat. No. 6,037,751, issued Mar. 14, 2000, entitled APPARATUS FOR CHARGING BATTERIES; U.S. Pat. No. 6,037,777, issued Mar. 14, 2000, entitled METHOD AND APPARATUS FOR DETERMINING BATTERY PROPERTIES FROM COMPLEX IMPEDANCE/ADMITTANCE; U.S. Pat. No. 6,051,976, issued Apr. 18, 2000, entitled METHOD AND APPARATUS FOR AUDITING A BATTERY TEST; U.S. Pat. No. 6,081,098, issued Jun. 27, 2000, entitled METHOD AND APPARATUS FOR CHARGING A BATTERY; U.S. Pat. No. 6,091,245, issued Jul. 18, 2000, entitled METHOD AND APPARATUS FOR AUDITING A BATTERY TEST; U.S. Pat. No. 6,104,167, issued Aug. 15, 2000, entitled METHOD AND APPARATUS FOR CHARGING A BATTERY; U.S. Pat. No. 6,137,269, issued Oct. 24, 2000, entitled METHOD AND APPARATUS FOR ELECTRONICALLY EVALUATING THE INTERNAL TEMPERATURE OF AN ELECTROCHEMICAL CELL OR BATTERY; U.S. Pat. No. 6,163,156, issued Dec. 19, 2000, entitled ELECTRICAL CONNECTION FOR ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,172,483, issued Jan. 9, 2001, entitled METHOD AND APPARATUS FOR MEASURING COMPLEX IMPEDANCE OF CELL AND BATTERIES; U.S. Pat. No. 6,172,505, issued Jan. 9, 2001, entitled ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,222,369, issued Apr. 24, 2001, entitled METHOD AND APPARATUS FOR DETERMINING BATTERY PROPERTIES FROM COMPLEX IMPEDANCE/ADMITTANCE; U.S. Pat. No. 6,225,808, issued May 1, 2001, entitled TEST COUNTER FOR ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,249,124, issued Jun. 19, 2001, entitled ELECTRONIC BATTERY TESTER WITH INTERNAL BATTERY; U.S. Pat. No. 6,259,254, issued Jul. 10, 2001, entitled APPARATUS AND METHOD FOR CARRYING OUT DIAGNOSTIC TESTS ON BATTERIES AND FOR RAPIDLY CHARGING BATTERIES; U.S. Pat. No. 6,262,563, issued Jul. 17, 2001, entitled METHOD AND APPARATUS FOR MEASURING COMPLEX ADMITTANCE OF CELLS AND BATTERIES; U.S. Pat. No. 6,294,896, issued Sep. 25, 2001; entitled METHOD AND APPARATUS FOR MEASURING COMPLEX SELF-IMMITANCE OF A GENERAL ELECTRICAL ELEMENT; U.S. Pat. No. 6,294,897, issued Sep. 25, 2001, entitled METHOD AND APPARATUS FOR ELECTRONICALLY EVALUATING THE INTERNAL TEMPERATURE OF AN ELECTROCHEMICAL CELL OR BATTERY; U.S. Pat. No. 6,304,087, issued Oct. 16, 2001, entitled APPARATUS FOR CALIBRATING ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,310,481, issued Oct. 30, 2001, entitled ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,313,607, issued Nov. 6, 2001, entitled METHOD AND APPARATUS FOR EVALUATING STORED CHARGE IN AN ELECTROCHEMICAL CELL OR BATTERY; U.S. Pat. No. 6,313,608, issued Nov. 6, 2001, entitled METHOD AND APPARATUS FOR CHARGING A BATTERY; U.S. Pat. No. 6,316,914, issued Nov. 13, 2001, entitled TESTING PARALLEL STRINGS OF STORAGE BATTERIES; U.S. Ser. No. 09/293,020, filed Apr. 16, 1999, entitled AUTOMOTIVE BATTERY CHARGING SYSTEM TESTER; U.S. Ser. No. 09/544,696, filed Apr. 7, 2000, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 09/304,315,filed May 3, 1999, entitled MIDPOINT BATTERY MONITOR”; U.S. Ser. No. 09/280,133, filed Mar. 26, 1999, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 09/560,920, filed Apr. 28, 2000, entitled MULTI-LEVEL CONDUCTANCE TESTER; U.S. Ser. No. 09/431,446, filed Nov. 1, 1999, entitled ALTERNATOR DIAGNOSTIC SYSTEM; U.S. Ser. No. 09/388,501, filed Sep. 1, 1999, entitled METHOD AND APPARATUS FOR EVALUATING STORED CHARGE IN AN ELECTROCHEMICAL CELL OR BATTERY; U.S. Ser. No. 09/703,270, filed Oct. 31, 2000, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 09/564,740, filed May 4, 2000, entitled ENERGY MANAGEMENT SYSTEM FOR AUTOMOTIVE VEHICLE; U.S. Ser. No. 09/575,629, filed May 22, 2000, entitled VEHICLE ELECTRICAL SYSTEM TESTER WITH ENCODED OUTPUT; U.S. Ser. No. 09/780,146, filed Feb. 9, 2001, entitled STORAGE BATTERY WITH INTEGRAL BATTERY TESTER; U.S. Ser. No. 09/575,627, filed May 22, 2000, entitled METHOD AND APPARATUS FOR CHARGING A BATTERY; U.S. Ser. No. 09/577,421, filed May 22, 2000, entitled METHOD AND APPARATUS FOR CHARGING A BATTERY; U.S. Ser. No. 09/816,768, filed Mar. 23, 2001, entitled MODULAR BATTERY TESTER; U.S. Ser. No. 09/662,401, filed Sep. 14, 2000, entitled TESTING PARALLEL STRINGS OF STORAGE BATTERIES; U.S. Ser. No. 09/654,715, filed Sep. 5, 2000, entitled APPARATUS FOR CALIBRATING ELECTRONIC BATTERY TESTER; U.S. Ser. No. 09/756,638, filed Jan. 8, 2001, entitled METHOD AND APPARATUS FOR DETERMINING BATTERY PROPERTIES FROM COMPLEX IMPEDANCE/ADMITTANCE; U.S. Ser. No. 09/862,783, filed May 21, 2001, entitled METHOD AND APPARATUS FOR TESTING CELLS AND BATTERIES EMBEDDED IN SERIES/PARALLEL SYSTEMS; U.S. Ser. No. 09/483,623, filed Jan. 13, 2000, entitled ALTERNATOR TESTER; U.S. Ser. No. 09/870,410, filed May 30, 2001, entitled INTEGRATED CONDUCTANCE AND LOAD TEST BASED ELECTRONIC BATTERY TESTER; U.S. Ser. No. 09/960,117, filed Sep. 20, 2001, entitled IN-VEHICLE BATTERY MONITOR; U.S. Ser. No. 09/908,389, filed Jul. 18, 2001, entitled BATTERY CLAMP WITH INTEGRATED CIRCUIT SENSOR; U.S. Ser. No. 09/908,278, filed Jul. 18, 2001, entitled BATTERY CLAMP WITH EMBEDDED ENVIRONMENT SENSOR; U.S. Ser. No. 09/880,473, filed Jun. 13, 2001; entitled BATTERY TEST MODULE; U.S. Ser. No. 09/876,564, filed Jun. 7, 2001, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 09/878,625, filed Jun. 11, 2001, entitled SUPPRESSING INTERFERENCE IN AC MEASUREMENTS OF CELLS, BATTERIES AND OTHER ELECTRICAL ELEMENTS; U.S. Ser. No. 09/902,492, filed Jul. 10, 2001, entitled APPARATUS AND METHOD FOR CARRYING OUT DIAGNOSTIC TESTS ON BATTERIES AND FOR RAPIDLY CHARGING BATTERIES; and U.S. Ser. No. 09/940,684, filed Aug. 27, 2001, entitled METHOD AND APPARATUS FOR EVALUATING STORED CHARGE IN AN ELECTROCHEMICAL CELL OR BATTERY; U.S. Ser. No. 09/977,049, filed Oct. 12, 2001, entitled PROGRAMMABLE CURRENT EXCITER FOR MEASURING AC IMMITTANCE OF CELLS AND BATTERIES; U.S. Ser. No. 10/047,923, filed Oct. 23, 2001, entitled AUTOMOTIVE BATTERY CHARGING SYSTEM TESTER, U.S. Ser. No. 10/046,659, filed Oct. 29, 2001, entitled ENERGY MANAGEMENT SYSTEM FOR AUTOMOTIVE VEHICLE; U.S. Ser. No. 09/993,468, filed Nov. 14, 2001, entitled KELVIN CONNECTOR FOR A BATTERY POST; U.S. Ser. No. 09/992,350, filed Nov. 26, 2001, entitled ELECTRONIC BATTERY TESTER, which are incorporated herein by reference in their entirety. 
     SUMMARY OF THE INVENTION 
     A charge control device for providing a constant charge voltage with temperature compensation to a battery being charged by a constant current charger is provided. The device includes a first electrical connector that couples to a positive terminal of the battery and a second electrical connector that couples to a negative terminal of the battery. A current bypass circuit electrically couples to the positive and negative terminals of the battery through respective first and second electrical connectors. The current bypass circuit includes a bypass path for a portion of a charge current from the constant current charger to flow, thereby maintaining a substantially constant voltage across the battery terminals at a particular temperature. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a simplified block diagram of a charge control device in accordance with one example embodiment of the present invention. 
     FIG. 2 is a charge control float curve for an automobile battery. 
     FIG. 3 is an example of a detailed implementation of a charge control device in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a simplified block diagram of a basic implementation of a charge control device  100  in accordance with one example embodiment of the present invention. Device  100  can physically mount to a battery  102  (shown by a one-cell battery symbol for simplification) and includes electrical connectors  104  and  106  that electrically couple to terminals of battery  102 . As can be seen in FIG. 1, a constant current charger  108  also electrically couples to terminals of battery  102  via electrical connectors  104  and  106 . Device  100  is designed to provide a constant charge voltage with temperature compensation to battery  102  being charged by constant current charger  108 . In the absence of charge control device  100 , current I T , supplied by constant current charger  108 , would flow directly into battery  102  during the entire recharging cycle, which could lead to excessive overcharging of battery  102 . The inclusion of charge control device  100  causes a portion of current I T , which is designated by I 2 , to flow through a current bypass circuit  110  and thereby maintain a substantially constant voltage across the terminals of battery  102 . Device  100  also includes a voltage sense and temperature compensation circuit  112 , described below, through which a negligible amount of current flows. Thus, current I T  is essentially equal to the sum of currents I 1  and I 2 . Voltage sense and temperature compensation circuit  112  senses changes in voltage across terminals of battery  102  and also senses changes in the temperature of battery  102  and accordingly provides an output  114  to current bypass circuit  110 . The magnitude of current I 2 , flowing through current bypass circuit  110 , is adjusted as a function of output  114  provided by voltage sense and temperature compensation circuit  112 . Voltage sense and temperature compensation circuit  112  may be divided into a separate voltage sense circuit  116  and a temperature compensation circuit  118 . 
     Temperature based voltage regulation across battery  102  is carried out by device  100  in accordance with a voltage vs. temperature graph of “compensating” or “float” voltages for the type of battery employed. Voltage vs. temperature graphs vary (have different slopes) for different types of batteries. FIG. 2 shows an exemplary charge control float curve for a nominal automobile lead-acid battery. The nominal float voltage, plotted along the vertical axis, changes as a function of battery temperature that is plotted along the horizontal axis in degrees Centigrade. The relationship between the nominal float voltage and the temperature in degrees Centigrade is given according to Equation 1 below. 
     
       
           Y=− 0.024 X+ 14.32  Equation. 1  
       
     
     where Y is the nominal float voltage and X is the temperature in degrees Centigrade. 
     As mentioned above, device  100  is designed to provide a constant charge voltage with temperature compensation to battery  102  being charged by constant current charger  108 . Thus, at a particular battery temperature, device  100  maintains a substantially constant voltage across the terminals of battery  102 . In response to changes in the temperature of battery  102 , during its recharging cycle, device  100  changes the voltage applied to the battery  102 . Device  100  achieves temperature-based voltage regulation with the help of a temperature-sensing element (not shown in FIG. 1) included in voltage sense and temperature compensation circuit  112 . Circuit  112  outputs a voltage which is proportional to the sensed temperature. This output voltage is provided to the current bypass circuit  110  which causes the magnitude of current I 2  to change, thereby changing the voltage applied across battery  102 . Details of the components and operation of device  100  are provided in connection with FIG. 3, described below. 
     FIG. 3 is an example of a detailed implementation of charge control device  100  in accordance with an embodiment of the present invention. In FIG. 3, voltage sense and temperature compensation circuit  112  includes an operational amplifier U 1 A with a temperature stable voltage applied to its non-inverting input at node  304  and a temperature variable voltage applied to the inverting input at node  302 . The temperature stable voltage is maintained by Zener diode D 1 . The temperature variable voltage is provided by precision temperature sensor U 2  which senses the battery temperature and produces a voltage output proportional to the temperature sensed. For example, when the temperature of battery  102  increases, temperature sensor U 2  senses this increase in temperature and outputs a change in voltage proportional to the sensed temperature increase, which causes the temperature variable voltage at the inverting input of U 1 A to increase. This increase in voltage is amplified by operational amplifier U 1 A and appears as an inverted amplified output at node  306  because the temperature variable voltage is connected to the inverting input of U 1 A. This inverted amplified output is applied to the non-inverting input of second operational amplifier U 1 B. The output of U 1 B, which is the output  114  of voltage sense and temperature compensation circuit  112 , is input to current bypass circuit  110 . In this example, a drop in voltage at the base of transistor Q 1 , as a result of the output from U 1 B, causes transistor Q 1 , which operates in linear mode, to allow an increase in magnitude of bypass current I 2 . An increase in current I 2  results in decrease in voltage across battery  102 . This decrease in voltage across battery  102  is proportional to the increase in temperature of battery  102 . In general, the increase or decrease in voltage applied to battery  102  is carried out in accordance with the float curve shown in FIG.  2 . 
     As battery  102  charges, the difference between the charging voltage, applied by device  100 , and the battery voltage decreases. When battery  102  is charged, current I 1  is just a trickle and almost all of current I T  flows through bypass circuit  110 . Thus, when battery  102  is charged, bypass current I 2  is large and substantially equal to I T . Since I 2  is the sum of the current through branch  308  and branch  310 , an increase in I 2  is accompanied by corresponding increases in current in branches  308  and  310 . Branch  310  includes an LED D 3  which lights to indicate that the battery  102  is charged when the current through branch  310  increases above a predetermined threshold. 
     In addition to the components described above, device  100  also includes resistor R 5  and diode D 2  that provide a path for leakage current during the initialization of device  100 . Resistors R 1  and R 7  in branch  308  and resistor R 2  in branch  310  are a part of the bypass current path. Resistors R 3  and R 4  are used to provide a proper voltage drop to ensure that operational amplifier U 1 A operates within its optimum voltage range. Resistors R 8  and R 11  provide a voltage divider for the non-inverting input of operational amplifier U 1 A. Similarly, resistors R 6 , R 10  and R 13  form a voltage divider for the inverting input of operational amplifier U 1 B. Capacitor C 1  is included in circuit  106  for noise suppression. Resistor R 9 , which is in series with the inverting input of amplifier U 1 A, and feedback resistor R 12  are selected based on the gain required for operational amplifier U 1 A. 
     As mentioned above, the increase or decrease in voltage applied to battery  102  is carried out in accordance with the charge control float graph, which is a straight line, shown in FIG.  2 . The slope of the straight line is different for different types of batteries. By setting the ratio of resistors R 12  and R 9 , circuit  100  (FIG. 3) is configured to operate in accordance with a particular slope for a particular battery type. An offset of provided by resistor R 10 . When a different type of battery is employed, the ratio of resistors R 12  and R 9  is changed for circuit  100  to operate in accordance with a charge control graph having a different slope. 
     A list of the various components that may be used in the circuit of FIG. 3 are provided in Table 1 below. 
     
       
         
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 C1 
                 0.047μ 
                 R3 
                  18 KΩ 
                 R10 
                  20 KΩ 
               
               
                 D1 
                 LM4040 
                 R4 
                  62 KΩ 
                 R11 
                  1 MΩ 
               
               
                 D2 
                 3.3 V 
                 R5 
                  10 KΩ 
                 R12 
                  1 MΩ 
               
               
                 D3 
                 LED 
                 R6 
                 287 KΩ 
                 R13 
                 200 KΩ 
               
               
                 Q1 
                 MJD45H11 
                 R7 
                  68 KΩ 
                 U1A 
                 LM2904M 
               
               
                 R1 
                  68 Ω 
                 R8 
                 150 KΩ 
                 U1B 
                 LM2904M 
               
               
                 R2 
                 300 Ω 
                 R9 
                  1 MΩ 
                 U2 
                 LM355M 
               
               
                   
               
             
          
         
       
     
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. The invention is not limited to the specific physical implementation shown herein. Any appropriate hardware, software or other combination can be employed to provide the current bypass circuit of the invention.