Abstract:
An electronic battery tester, comprising first and second connectors configured to electrically couple to terminals of the battery, a microprocessor configured to test the battery using the first and second connectors, a memory containing a set of locked instructions for the microprocessor, an input configured to receive a software unlocking key, and the microprocessor configured to execute the set of locked instructions in response to the software unlocking key corresponding a predetermined software unlocking key.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to storage batteries. More specifically, the present invention relates to a battery system tester for testing storage batteries. 
   Storage batteries, such as lead acid storage batteries of the type used in the automotive industry, have existed for many years. These storage batteries usually consist of a plurality of individual storage cells electrically connected in series. Typically, each cell has a voltage potential of about 2.1 volts. By connecting the cells in series, the voltage of the individual cells are added in a cumulative manner. For example, in a typical automotive storage battery, six storage cells are used to provide a total voltage when the battery is fully charged up to 12.6 volts. 
   Several techniques have been used to test the condition of storage batteries. These techniques include a voltage test to determine if the battery voltage is below a certain threshold, and a load test that involves discharging a battery using a known load. A more recent technique involves measuring the conductance of the storage batteries. This technique, which has been pioneered by Dr. Keith S. Champlin and Midtronics, Inc. of Burr Ridge, Ill., is described in a number of U.S. patents, for example, 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. Pat. No. 6,323,650, issued Nov. 27, 2001, entitled ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,329,793, issued Dec. 11, 2001, entitled METHOD AND APPARATUS FOR CHARGING A BATTERY; U.S. Pat. No. 6,331,762, issued Dec. 18, 2001, entitled ENERGY MANAGEMENT SYSTEM FOR AUTOMOTIVE VEHICLE; U.S. Pat. No. 6,332,113, issued Dec. 18, 2001, entitled ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,351,102, issued Feb. 26, 2002, entitled AUTOMOTIVE BATTERY CHARGING SYSTEM TESTER; U.S. Pat. No. 6,359,441, issued Mar. 19, 2002, entitled ELECTRONIC BATTERY TESTER; U.S. Pat. No. 6,363,303, issued Mar. 26, 2002, entitled ALTERNATOR DIAGNOSTIC SYSTEM, U.S. Ser. No. 09/595,102, filed Jun. 15, 2000, entitled APPARATUS AND METHOD FOR TESTING RECHARGEABLE ENERGY STORAGE BATTERIES; U.S. Ser. No. 09/703,270, filed Oct. 31, 2000, entitled ELECTRONIC BATTERY TESTER; 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. 09/816,768, filed Mar. 23, 2001, entitled MODULAR 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, untitled 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, U.S. Ser. No. 10/042,451, filed Jan. 8, 2002, entitled BATTERY CHARGE CONTROL DEVICE; U.S. Ser. No. 10/042,451, filed Jan. 8, 2002, entitled BATTERY CHARGE CONTROL DEVICE, U.S. Ser. No. 10/073,378, filed Feb. 8, 2002, entitled METHOD AND APPARATUS USING A CIRCUIT MODEL TO EVALUATE CELL/BATTERY PARAMETERS; U.S. Ser. No. 10/093,853, filed Mar. 7, 2002, entitled ELECTRONIC BATTERY TESTER WITH NETWORK COMMUNICATION; U.S. Ser. No. 60/364,656, filed Mar. 14, 2002, entitled ELECTRONIC BATTERY TESTER WITH LOW TEMPERATURE RATING DETERMINATION; U.S. Ser. No. 10/101,543, filed Mar. 19, 2002, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 10/112,114, filed Mar. 28, 2002; U.S. Ser. No. 10/109,734, filed Mar. 28, 2002; U.S. Ser. No. 10/112,105, filed Mar. 28, 2002, entitled CHARGE CONTROL SYSTEM FOR A VEHICLE BATTERY; U.S. Ser. No. 10/112,998, filed Mar. 29, 2002, entitled BATTERY TESTER WITH BATTERY REPLACEMENT OUTPUT; U.S. Ser. No. 10/119,297, filed Apr. 9, 2002, entitled METHOD AND APPARATUS FOR TESTING CELLS AND BATTERIES EMBEDDED IN SERIES/PARALLEL SYSTEMS; U.S. Ser. No. 10/128,790, filed Apr. 22, 2002, entitled METHOD OF DISTRIBUTING JUMP-START BOOSTER PACKS; U.S. Ser. No. 10/143,307, filed May 10, 2002, entitled ELECTRONIC BATTERY TESTER; U.S. Ser. No. 10/207,495, filed Jul. 29, 2002, entitled KELVIN CLAMP FOR ELECTRICALLY COUPLING TO A BATTERY CONTACT, which are incorporated herein in their entirety. 
   Battery testers are available in different models, with the most basic models just indicating whether the battery is “good” or “bad”, and the more developed models providing information such as the state of charge of the battery, the state of health of the battery, etc. Currently, when a user needs to upgrade to a more advanced model, the user may, for example, have to place an order with the vendor, and the battery tester must be shipped to the user from inventory. This series of transactions creates delays that may frustrate the user, while adding significant overhead to the vendor&#39;s operations. Further, the vendor may be obligated to inventory and support nonrevenue producing models of battery testers. 
   SUMMARY OF THE INVENTION 
   An electronic battery tester, comprising first and second connectors configured to electrically couple to terminals of the battery, a microprocessor configured to test the battery using the first and second connectors, a memory containing a set of locked instructions for the microprocessor, an input configured to receive a software unlocking key, and the microprocessor configured to execute the set of locked instructions in response to the software unlocking key corresponding a predetermined software unlocking key. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a very simplified block diagram showing a battery tester in accordance with an embodiment of the present invention. 
       FIG. 2  is a block diagram illustrating different software modules stored in battery tester memory in accordance with an embodiment of the present invention. 
       FIG. 3  is a flow chart illustrating the operation of battery tester key recognition routine in accordance with an embodiment of the present invention. 
       FIG. 4  is a simplified block diagram showing details of battery tester measurement circuit in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention, described below, all relate to a battery testing system which includes locked battery testing instructions that can be executed upon receipt of a software unlocking key that corresponds to a predetermined software unlocking key. 
     FIG. 1  is a very simplified block diagram of a battery tester  10  in accordance with an illustrative embodiment of the present invention. The same reference numerals are used in the various figures to represent the same or similar elements. Note that  FIG. 1  is a simplified block diagram of a specific type of battery tester. However, the present invention is applicable to any type of battery tester including those which do not use dynamic parameters. Other types of example testers include testers that conduct load tests, current based tests, voltage based tests, tests which apply various conditions or observe various performance parameters of a battery, etc. Battery tester  10  includes a test circuit  14  that directly couples to vehicle battery  12 , an input  18  and an output  28 . Test circuit  14  includes measurement circuit  22 , processor  24  and memory  26 . Measurement circuit  22  can be any circuit configuration which measures a dynamic parameter of battery  12 . As used herein, a dynamic parameter is one which is related to a signal having an alternating current (AC) component. The signal can be either applied directly or drawn from battery  12 . Example dynamic parameters include dynamic resistance, conductance, impedance, admittance, etc. This list is not exhaustive, for example, a dynamic parameter can include a component value of an equivalent circuit of battery  12 . Operation of measurement circuit  22  is controlled by processor  24  which, in turn, carries out different battery testing functions based upon battery testing instructions stored in memory  26 . In accordance with the present invention, the battery testing instructions are locked in memory  26  when tester  10  is delivered to a customer. Tester  10  can be utilized to test battery  12  only when it is enabled by unlocking the battery testing instructions by entering a software unlocking key that corresponds to a predetermined software unlocking key that is stored in memory  26 . The software unlocking key can be entered by the user through input  18 . Input  18  may be a keyboard, a reader through which a card including the software unlocking key can be swiped, etc. Input  18  can also include a keyboard that is a touchscreen or integrated with a touchscreen, a unit capable of radio frequency (RF) communication with a personal computer (PC) or a personal digital assistant (PDA), a RF identification (ID) tag, a unit capable of infrared (IR) communication with a PC or a PDA, a serial interface cable, a parallel interface cable, a universal serial bus (USB) interface cable, a bus using the Institute of Electrical and Electronics Engineers (IEEE) 1394 standard, an Ethernet interface cable, a Transmission Control Protocol/Internet Protocol (TCP/IP) internet connection, a plug-in device (such as a memory card, a memory chip, an ID button, etc.), etc. Upon receiving the software unlocking key from input  18 , microprocessor  24  utilizes a key recognition module stored in memory  26  to validate the entered software unlocking key. If the software unlocking key is recognized (or found to be valid) by the key recognition routine, the key is stored in memory and the battery testing instructions are unlocked to thereby enable tester  10  for use in testing battery  12 . 
   In preferred embodiments of the present invention, multiple locked battery testing instruction sets are stored in memory  26  and a different unique software unlocking key is required to unlock each different instruction set of the multiple battery testing instruction sets. The use of such multiple locked battery testing instruction sets to upgrade battery tester  10  is described below in connection with  FIGS. 2 and 3 . 
     FIG. 2  is a block diagram illustrating different software modules stored in battery tester memory  26  in accordance with an embodiment of the present invention. As can be seen in  FIG. 2 , memory  26  includes multiple battery testing instruction sets, such as, first instruction set  30 , second instruction set  31  and third instruction set  32 . Encrypted codes or predetermined software unlocking keys  33 - 35 , each corresponding to a respective battery testing instruction set of instruction sets  30 - 32 , are also stored in memory  26 . Memory registers  36 - 38  can each store a software unlocking key that matches a respective one of encryption codes  33 - 35 . As mentioned above, the software unlocking keys are entered by the battery tester user. Key recognition routine  40  can compare the user entered software unlocking keys, which can be stored in registers  36 - 38 , with respective encrypted codes  33 - 35  and can enable one or more of corresponding instruction sets  30 - 32  if matches are detected between one or more unlocking keys and one or more encrypted codes  33 - 35 . The operation of an example key recognition routine is described further below in connection with FIG.  3 . 
   In an example embodiment of the present invention, first instruction set  30  contains basic functions which, when executed, only provide an output indicating whether battery  12  is “good” or “bad”. Second instruction set  31  contains more advanced functions which, when executed, provide an output indicating the state of charge, state of health, etc., of battery  12 . Third instruction set  32  contains functions which, when executed, provide an output related to a replacement battery or replacement options for battery  12 . Each instruction set is in a locked condition when battery tester  10  is delivered to the user. The price of tester  10  is tied to the number of software unlocking keys that the user purchases. For example, the user may purchase battery tester  10  and only one software unlocking key to enable first instruction set  30  at a relatively low price. Subsequently, if the user desires an upgrade of the functions, the user may, for example, call a 1-800 number and, upon providing a credit card number, obtain an unlocking key for second instruction set  31  and/or third instruction set  32 . Thus, the upgrade of battery tester  10  takes place locally after the appropriate software unlocking key is input into tester  10 . 
   In some embodiments of the present invention, additional instruction sets may be provided to battery tester  10  by coupling input  18  to a separate device or remote system via a telephone line, for example, and downloading the additional instruction sets into memory locations  42  and corresponding encrypted codes or predetermined software unlocking keys into memory locations  44  form the remote system. Additional unlocking keys entered by the battery tester user for unlocking the additional instruction sets can be stored in memory registers  46 . 
   In embodiments of the present invention, key recognition routine  40  can be executed by microprocessor  24  each time it receives a start test command, which may be provided by the tester user through input  18 .  FIG. 3  is a flow chart illustrating the operation of key recognition routine  40  in accordance with an embodiment of the present invention. The routine begins at step  60  and proceeds to step  62  at which a determination is made as to whether the software unlocking key for first instruction set  30  is valid. This step may involve comparing the contents of register  36  with encrypted code  33 . If the comparison indicates that the software unlocking key is invalid, a message is displayed to the user requesting the user to enter an activation code (or unlocking key) at step  64  and the routine waits for the user input at step  66 . Upon receipt of the user input, control returns to step  62 . If the comparison indicates that the software unlocking key is valid, first instruction set  30  is enabled at step  68  by setting a status flag in register  48  to a predetermined value. At step  70 , a determination is made as to whether the software unlocking key for second instruction set  31  and/or the unlocking key for third instruction set  32  is valid. This step involves a comparison of encrypted codes  34  and  35  and unlocking keys in registers  37  and  38 . If no matches are detected between the unlocking keys and the encrypted codes, the routine ends at step  74 . If one or more matches are detected, second instruction set  31  and/or third instruction set  32  are enabled by appropriately setting status flags in registers  49  and  50  to predetermined values at step  72  before the routine ends at step  74 . 
   In some embodiments of the present invention, key recognition routine  40  first checks if the status flags in registers  48 - 50  are set to an appropriate predetermined value that indicates that the instruction sets are enabled and only carries out a comparison of software unlocking keys and encryption codes for instruction sets that are not in an enabled status. Status flags for instruction sets that are subsequently downloaded into memory locations  42  are stored in registers  52 . 
   In some embodiments of the present invention, all instruction sets are enabled for trial use (for example, one-time use) when tester  10  is delivered to the user. This may be carried out by setting status flags in registers  48 - 50  to appropriate predetermined values that indicate one-time activation of instruction sets  30 - 32 . 
   In some embodiments of the present invention, encrypted codes  33 - 35  for a particular battery include the encrypted serial number of the particular battery tester unit. Thus, each predetermined software unlocking key or encrypted code of encrypted codes  33 - 35  for a particular battery tester unit can consist of a combination of the encrypted serial number for the battery tester unit and one or more characters and/or digits that render each encrypted code of encrypted codes  33 - 35  unique. 
   As used herein, “the microprocessor executing the set of locked instructions in response to a software unlocking key corresponding to a predetermined software unlocking key” includes the microprocessor being configured to execute the set of locked instructions if the software unlocking key corresponding to the predetermined software unlocking key is stored in the memory, the microprocessor being configured to execute the set of locked instructions if a status flag is set to a predetermined value in the memory, the microprocessor being configured to execute the first set of locked instructions upon receipt of a start test command, etc. 
     FIG. 4  is a simplified block diagram of electronic battery tester circuitry  10  in accordance with a specific embodiment of the present invention. Apparatus  10  is shown coupled to battery  12  which includes a positive battery terminal  13  and a negative battery terminal  15 . Circuitry  10  includes current source  76 , differential amplifier  78 , analog-to-digital converter  80  and microprocessor  24 . Amplifier  78  is capacitively coupled to battery  12  through capacitors C 1  and C 2 . Amplifier  78  has an output connected to an input of analog-to-digital converter  80 . Microprocessor  24  is connected to system clock  82 , memory  26  and analog-to-digital converter  80 . Microprocessor  24  is also capable of receiving an input from input device  18 . Microprocessor  24  also connects to output device  28 . 
   In operation, current source  76  is controlled by microprocessor  24  and provides current I in the direction shown by the arrow in FIG.  4 . In one embodiment, this is a square wave or a pulse. Differential amplifier  78  is connected to terminals  13  and  15  of battery  12  through capacitors C 1  and C 2 , respectively, and provides an output related to the voltage potential difference between terminals  13  and  15 . In a preferred embodiment, amplifier  78  has a high input impedance. Circuitry  10  includes differential amplifier  84  having inverting and noninverting inputs connected to terminals  13  and  15 , respectively. Amplifier  84  is connected to measure the open circuit potential voltage (V BAT ) of battery  12  between terminals  13  and  15 . The output of amplifier  84  is provided to analog-to-digital converter  80  such that the voltage across terminals  13  and  15  can be measured by microprocessor  24 . 
   Circuitry  10  is connected to battery  12  through a four-point connection technique known as a Kelvin connection. This Kelvin connection  86  allows current I to be injected into battery  10  through a first pair of terminals while the voltage V across the terminals  13  and  15  is measured by a second pair of connections. Because very little current flows through amplifier  78 , the voltage drop across the inputs to amplifier  78  is substantially identical to the voltage drop across terminals  13  and  15  of battery  12 . The output of differential amplifier  78  is converted to a digital format and is provided to microprocessor  24 . Microprocessor  24  operates at a frequency determined by system clock  82  and in accordance with programming instructions stored in memory  26 . 
   Microprocessor  26  determines the conductance of battery  12  by applying a current pulse I using current source  76 . The microprocessor determines the change in battery voltage due to the current pulse I using amplifier  78  and analog-to-digital converter  80 . The value of current I generated by current source  76  is known and is stored in memory  26 . In one embodiment, current I is obtained by applying a load to battery  12 . Microprocessor  24  calculates the conductance of battery  12  using the following equation: 
             Conductance   =       G   BAT     =       Δ   ⁢           ⁢   I       Δ   ⁢           ⁢   V                 Equation   ⁢           ⁢   1             
 
where ΔI is the change in current flowing through battery  12  due to current source  76  and ΔV is the change in battery voltage due to applied current ΔI.
 
   Based upon the battery conductance G BAT  and the battery voltage, the battery tester  10  determines the condition of battery  12 . A temperature sensor  88  can be thermally coupled to battery  12  and used to compensate battery measurements. Temperature readings can be stored in memory  26  for later retrieval. 
   As mentioned above, microprocessor  24  operates at a frequency determined by system clock  82  and in accordance with programming instructions stored in memory  26 . In accordance with the present invention, the programming instructions stored in memory  26  are locked and can be executed by microprocessor  24  upon receipt of a software unlocking key that corresponds to a predetermined software unlocking key. 
   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.