HK1024058B - Electronic battery tester - Google Patents

Description

Battery testing device

Technical Field

The present invention relates to a battery testing technique, and more particularly, to a battery testing apparatus for testing a battery.

Background

Batteries have been known for a long time, such as lead-acid batteries widely used in automotive vehicles or in industrial fields requiring auxiliary power. Efforts are constantly being made to understand the characteristics of such batteries, to study how the batteries operate, and in particular to accurately test the condition of the batteries, but such efforts have proved far from satisfactory. The battery comprises a plurality of battery cells electrically connected in series, each battery cell having a voltage of about 2.1V, and by connecting the battery cells in series, their voltages will add up to form a total voltage. For example, in the case of a typical motor vehicle battery, a total voltage of 12.6V is formed by the series connection of six battery cells when the battery is fully charged.

For many years, efforts have been made to accurately test the condition of batteries. A simple test is to measure the voltage of the battery and if the measured voltage is below a certain threshold, the battery is considered bad. However, this measurement is inconvenient because it requires that the battery must be charged prior to measurement, and if the battery is discharged, the voltage on it must be reduced, and a good cell is considered bad. Furthermore, such measurements do not show how much energy is still present in the battery. Another test technique is to consider the battery test as a load test. In a load test, the battery is discharged with a known load, and as the battery discharges, the voltage across the battery is monitored to determine the current state of the battery. This measurement technique requires that the battery must be sufficiently charged to be able to supply current to the load.

More recently, a method for testing batteries by measuring the electrical conductivity of the batteries was pioneered by doctor Keith s.champlin and Midtronics, Burr Ridge, Illinois, and relevant teachings of this method can be found in various U.S. patent documents, such as U.S. Pat. No. 3-3,873,911 to Champlin, published 3-25 1975, entitled "battery testing apparatus"; U.S. Pat. No. 5,3,909,708 to Champlin, entitled "Battery test device", published on 9/30/1975; U.S. Pat. No. 3, 4,816,768 to Champlin, entitled "Battery test device", published 3,28, 1989; U.S. Pat. No. 5, 4,825,170 to Champlin, published 25/4/1989, entitled "Battery test device with automatic Voltage Scan"; U.S. Pat. No. 5, 4,881,038 to Champlin, published 11/14/1989, entitled "Battery test device with automatic Voltage Scan to determine its dynamic status"; U.S. Pat. No. 5, 4,912,416 to Champlin, published 3/27 in 1990, entitled "Battery test device with State of Charge Compensation"; U.S. Pat. No. 8, 5,140,269 to Champlin, published on 8/18 1992, entitled "Battery test apparatus for determining Battery/cell Capacity"; US-5,343,380, published on 8/30/1994, entitled "method and apparatus for suppressing a time varying signal during charging and discharging of a battery"; US-5,572,136, published 11/5/1996, entitled "battery test device with automatic compensator for low state of charge"; US-5,574,355, published 12.12.1996, entitled "method and apparatus for monitoring and controlling thermal runaway during battery charging"; US-5,585,728, published 12, 17, 1996, entitled "battery test device with automatic compensator for low state of charge"; U.S. Pat. No. 3, 5,592,093, published 1997, 1/7, entitled "Battery test device for testing loose terminal connections by means of a comparison Circuit"; U.S. Pat. No. 5,5,598,098, entitled "Battery test device with Strong noise resistance", published on 28.1.1997; US-5,757,192, published 26.5.1998, entitled "method and apparatus for detecting bad cells in a battery"; US-5,821,756, published 10/13/1998, entitled "battery test set with a low state of charge tailored automatic compensator"; US-5,831,435, published on 3.11.1998, entitled "battery test device suitable for japanese industrial standards".

Disclosure of Invention

The invention includes apparatus and methods for battery testing or battery operating condition monitoring. One measure of the invention is to employ an inductance cancellation circuit for a Kelvin detector in a battery testing apparatus that reduces the inductive coupling between the probes of the Kelvin detector. Another measure of the invention is to include a DC-coupled AC amplifier for amplifying the AC response signal of the battery test apparatus. Other measures of the invention include a critically damped bandpass filter, a DC-DC converter isolation circuit, operator programmable test standards, a battery temperature sensing element, and an automatically adjusting gain stage, and use of a self-calibrated internal reference.

Drawings

FIG. 1 is a simplified block diagram showing a battery test apparatus of the present invention;

FIG. 2 is a schematic diagram of an inductance cancellation circuit of the present invention employing a Kelvin detector;

FIG. 3 is a simplified schematic diagram of the DC-coupled AC amplifier of FIG. 1;

FIG. 4A is a simplified schematic diagram of one stage of the critically damped filter of FIG. 1;

FIG. 4B is a graph representing the magnitude and frequency of the response signal of the filter of FIG. 4A;

FIG. 5 is a simplified schematic diagram of the adjustable gain amplifier of FIG. 1;

FIG. 6 is a simplified block diagram of a DC-DC converter according to yet another aspect of the present invention;

FIG. 7 is a simplified circuit diagram of a circuit interrupting circuit in accordance with yet another aspect of the present invention;

fig. 8 is a simplified circuit diagram of a self-calibrating normalization circuit according to yet another aspect of the present invention.

Detailed Description

There is still a need for improved battery testing techniques, and the present invention is therefore directed to a battery testing apparatus for testing a battery in which current can flow into or out of the battery (i.e., a current exchanger), said current alternating at a mains ac frequency, using a load of high impedance, with the advantages of:

has a characteristic of resisting a change in contact resistance,

high capacity-1-6 cells can be tested without damage with a single circuit,

the detection leads for connecting the battery have a stable and/or predictable galvanic coupling effect,

according to the formula G (conductivity) ═ I/E, where I denotes a substantially constant current, so that only the variable E needs to be measured, and it is quantitatively in a simple reciprocal relationship to G,

the amount of electrical fuse in the device can be reduced.

Fig. 1 is a simplified block diagram of a battery detection device 10 illustrating an embodiment of a detection device for monitoring the condition of a battery 12 in accordance with an aspect of the present invention. The battery test apparatus 10 connects the two terminals 14A and 14B of the battery 12 through a four-point Kelvin circuit formed by the inductance cancellation circuit 20 and the self-calibration circuit 21 connected by the cables 16A, 16B, 18A and 18B. The switchable current source 22 includes a switch 22A and a current source 22B connected in series with the cables 18A and 18B. In one embodiment, switch 22A operates at 5HZ and 500 HZ. According to one measure of the invention, the cables 16A and 16B are connected to a high impedance DC-coupled AC amplifier 24. In accordance with yet another measure of the present invention, the DC-coupled AC amplifier 24 provides an amplified output 26 to a critical damping filter 28 which outputs a filtered signal 30 to a self-regulating amplifier 32. The output signal 34 from the self-regulating amplifier 32 is fed to an analog-to-digital converter 36, the output signal 38 of which is in turn fed to a microprocessor 40 which is connected to a display output device 42 and a keyer, keyboard or other input device 44. Other input and output devices I/O46 may also be connected to microprocessor 40 as shown. For example, microprocessor 40 may be connected to an external printer device, data communication device-modem, or external storage device, and I/O46 may be connected using physical connections or through non-physical connections, such as through far infrared, ultrasonic, or radio frequency. According to the present invention, the battery test apparatus 10 is powered by a DC-DC converter 48.

Fig. 2 is a circuit diagram of a simplified inductance cancellation circuit 20 that connects the battery 12 to a current source 22 and a DC-coupled AC amplifier 24 through a Kelvin connection, a problem with prior art battery testing devices is that mutual inductance is caused by the Kelvin connection often being connected to the battery, which results in inductively coupled interference of the current in the sensing probes because the probes are very close together. However, with the transformer coupling connection according to the invention as shown in the figure, the above-mentioned undesired cross-coupling interferences can be effectively avoided. The present invention thus allows the use of multiple, replaceable cables and is adaptable to large capacity batteries. The coils 52 and 54 of the transformer 50 are connected in series with the Kelvin wires 16B and 18B, respectively, and the transformer coils 52 and 54 are wound around the core 56 in opposite directions.

The current flowing in conductors 18B and 18A comes from a current source 22, which is inductively coupled to conductors 16B and 16A. In the existing battery test apparatus, the above phenomenon may be a source of measurement errors. However, transformer 50 couples in conductors 16B and 16A to produce an opposing counteracting current. The coupling between coils 52 and 54 can be controlled by adjusting the position of core 56, which can be achieved when manufacturing the transformer. In addition, the cables 16A, 16B, 18A and 18B may be removably connected to the battery testing apparatus 10, such that if the cables are damaged or otherwise need to be replaced, installation of a new cable pair is easily accomplished. The new cable pair comprises its own transformer 50 which has been "tuned" at the time of manufacture by means of the tuning core 56, so that disturbing currents can be excluded. It should be noted that in less critical applications, a fixed transformer finish can be used, and the transformer coupling does not need to be tuned. Moreover, the use of the inductance cancellation circuit 20 allows for longer cable lengths without excessive inductive coupling between the cables. For example, the cable test circuit may be spaced a distance from the battery for testing operations by an operator at an operator station or while seated in the vehicle. Various cables of different lengths or different configurations may be changed by simply removing the cable pairs from the test apparatus 10 and connecting the newly replaced cable pairs. If the cable has been properly countertuned beforehand, the operator does not need to make any further adjustments. The transformer shown in fig. 2 is only a simple embodiment of this concept and other techniques for generating the cancellation signal may be used, such as by active devices or other techniques without departing from the principles of the invention.

Fig. 3 is a simplified schematic diagram of a DC-coupled AC amplifier according to yet another embodiment of the present invention, such a DC-coupled AC amplifier 24 comprising a differential amplifier 60 and a differential amplifier 62. In the embodiment shown in fig. 3, the non-inverting input of amplifier 60 is connected to cable 16A as shown in fig. 1 by a resistor 64, and the inverting input of amplifier 60 is connected to cable 16B by coil 52 and resistor 66 as shown in fig. 2. The output of amplifier 60 is connected to the inverting input of amplifier 60 through resistor 68 to provide negative feedback. The output of amplifier 60 is connected to the non-inverting input of amplifier 60 through resistors 72 and 74 and integrator circuit 70. The inverter 70 is constructed such that the output of the differential amplifier 62 is negatively fed back to its inverting input through a capacitor 76 and the non-inverting input of the differential amplifier 62 is grounded. In a preferred embodiment, the DC-coupled AC amplifier 24 has an overall gain.

In the prior art, battery test devices commonly employ AC-coupled amplifiers, but such amplifiers produce common mode errors and vary in impedance due to variations in the capacitors used to couple the sense signals. Furthermore, the coupling capacitor is required to have a large capacitance in order to avoid damaging the incoming induced signal, the presence of such a capacitor having a negative effect on the reduction of the set-up time necessary to obtain an accurate measurement result. However, the use of AC coupling in prior art battery test devices may avoid voltage doubling of the DC voltage of the battery due to the high gain of the amplifier. The use of a DC-coupled AC amplifier overcomes the above-mentioned drawbacks and does not require large coupling capacitors.

In the embodiment shown, the DC-coupled AC amplifier 24 receives a DC error signal 80 from the output of the amplifier 60 representing a DC signal, this DC error signal 80 being generated by an integrating circuit 70, the time constant of the latter being determined by a capacitor 76, the capacitance being selected according to the application of the operating frequency of the switch 22A for switching the current source 22. Thus, the output 26 of the DC-coupled AC amplifier 24 is a pure alternating current signal generated in response to the application of the switched current source 22. The illustration shown in fig. 3 is only one specific embodiment of the invention showing a preferred embodiment of a DC-coupled AC amplifier according to the invention. It should be noted that the present invention includes any coupling technique for inducing an AC signal from the battery under test that does not require the installation of a large AC coupling capacitor for cutting off the DC voltage of the battery.

Fig. 4A is a simplified circuit diagram of the second stage filtering stage 200 of the critically damped filter 28 shown in fig. 1, and fig. 4B is a frequency-amplitude plot showing the characteristics of the filter 28 according to the invention. In the preferred embodiment, filter 28 includes four such stages 100, stage 100 including a differential amplifier 110 with negative feedback through resistor 102 and capacitor 104, with the input signal coupled through resistor 106 and capacitor 108 to the inverting input of amplifier 110, and the non-inverting input of amplifier 110 coupled through resistor 112 to ground 78.

The filter 100 forms the second stage of the band pass filter and serves as a signal stage for the critically damped filter 28 shown in fig. 1. The critically damped filter 28 is comprised of four stages connected in series with one another.

Critically damped filter 28 provides a critically damped bandpass filter as shown in fig. 4B, which also shows the curves for an over-damped filter and an under-damped filter in fig. 4B. Preferably, filter 28 has a Q equal to 1, and its bandpass center frequency (F0) should be the same as the frequency of switch 22A shown in FIG. 1. If the filter is over-damped the response of the system will be slow and if the filter is under-damped the signal of the system will "ring" which will essentially only allow the components on the battery 12 in the response signal to pass through the self-adjusting amplifier 30, the frequency of the battery 12 being the same as the current source 22. Note that additional filter stages beyond the preferred embodiment, for example eight stages, could also be used, although this would increase the manufacturing cost. In a preferred embodiment, the overall gain of amplifier 28 is 16. Any kind of filter can be applied in the present invention, and a simple preferred embodiment shown in the figure is a dedicated analog filter, however digital filters can also be used.

Fig. 5 is a simplified circuit diagram of a self-adjusting amplifier (programmable gain or selectable gain) 32. In the embodiment shown in fig. 5, amplifier 32 is a two-stage amplifier having selectable gains 1, 2,4, and 8 in a first stage 140 and selectable gains 1, 10, 100, and 1000 in a second stage 142. Amplifier 140 is connected to output 30 of critical damping filter 28 through coupling capacitor 144. The amplifier 140 includes a selectable gain amplifier 146 and a resistor 148. The amplifier 146 receives input signals a0 and a1 controlled by the amplification of the amplifier 146, the amplifier stage 142 comprises an amplifier 150 whose input is connected to the output of the amplifier 146 via a resistor 152 and a coupling capacitor 154, the gain of the amplifier 150 is controlled by the input signals a2 and A3, the amplifiers 146 and 150 are connected to the microprocessor 40 via control inputs a0-A3, the microprocessor 40 selectively varies the input signals a0-A3 according to the table belowControlling the gain of amplifier 32: TABLE 1

A3	A2	A1	A0	Gain of	A3	A2	A1	A0	Gain of
										0	0	0	0	1	1	0	0	0	100
0	0	0	1	2	1	0	0	1	200
										0	0	1	0	4	1	0	1	0	400
0	0	1	1	8	1	0	1	1	800
										0	1	0	0	10	1	1	0	0	1000
0	1	0	1	20	1	1	0	1	2000
										0	1	1	0	40	1	1	1	0	4000
0	1	1	1	80	1	1	1	1	8000

During operation, the amplifier 32 provides a programmable gain of between 1 and 8000, and the gain of the amplifier 32 is adjustable under the control of the microprocessor 40 so that the battery testing apparatus 10 is capable of testing batteries over a wide range of resistances or conductances (i.e., between 10 ohms and 10000 ohms). Microprocessor 40 increases the gain of amplifier 32 by controlling input signals a0-A3 until a maximum signal is received through analog-to-digital converter 36. The adjustment is accomplished automatically without operator intervention. This demonstrates that the device 10 can be easily used and that the possibility of inaccurate measurements due to operator error can be reduced. The adjustable gain amplifier 32 shown in fig. 5 is a simple adjustable amplifier and any amplifier configuration may be used with the present invention. Furthermore, the amplifier may be placed anywhere in the signal path and need not be placed between a critically damped filter, such as filter 28, and analog-to-digital converter 36.

Fig. 6 is a simplified schematic diagram of a DC-DC converter circuit 48 according to yet another embodiment of the present invention. The circuit 48 includes a switching type DC-DC converter 170 that includes positive and negative input terminals, positive and negative output terminals, and a synchronization input terminal. The voltages of the DC-DC converter of fig. 6 are denoted VCC, V' CC, + VSS and-VSS, where VCC is either supplied by an internal voltage source, such as an internal battery, or by the battery 12 if the battery is sufficiently large in capacity. VCC is connected to the positive input of the converter 170, V 'CC is connected to the positive input of the converter 170 through a resistor 172, a capacitor 174 is connected to VCC ground 78, and a capacitor 176 is connected to V' CC ground 78. The combination of resistor 172 and capacitor 174 and the combination of resistor 172 and capacitor 176 provide the filter with signal noise on V' CC and VCC. In one embodiment of the present invention, VCC is used to drive very low noise circuits, such as amplifier 24, filter 28, amplifier 32, and the like. However, V' CC is used to drive power supply circuits that may generate noise, such as microprocessor 40 and other digital and logic circuits, which are themselves very insensitive to noise. The voltages + VSS and-VSS are used to power certain analog devices that require multiple power supplies, such as plus and minus 15V. Inductors 178 and 180 block noise from converter 170, which is powered by + VSS and-VSS. In one embodiment, the transducer 170 is a switching transducer having a frequency of about 400KHZ and the microprocessor 40 operates at a frequency of about 4MHz, and the noise isolation provided in the embodiment of FIG. 6 reduces noise in critical components of the battery testing apparatus 10, thereby improving measurement accuracy.

Converter 170 also includes power reduction techniques and the signal provided by microprocessor 40 includes synchronization automatic coordination (SYNCIN). In this case, the analog test circuit does not need to operate. During these time periods, microprocessor 40 may control the SYNCIN to converter 170 such that converter 170 is off and no voltage supplies + VSS and-VSS are generated, thus reducing power requirements.

Another aspect of the invention can improve internal (power) battery life. The present invention can be used to test batteries using the 1.75V voltage of the internal battery as the open circuit voltage (if the capacity of the battery 12 is large enough, the device 10 can be powered by the battery 12). A mobile portable 9V battery could also be used as an internal battery if the rechargeable battery device is considered too expensive. Such a battery can provide a relatively low amount of stored power because the present invention is constructed of a plurality of circuits that require less power, thereby extending the life of the battery. These circuits include a two-stage analog power shutdown circuit 200 (fig. 7), a TOP-DOWN self-adjusting circuit to minimize test time, and an automatic power DOWN circuit for power supply 48 (as described above).

In one embodiment, the amplifier 32 automatically adjusts between 1-8000, and if engaged, the amplifier 32 consumes 10 times the internal battery power as if it were not engaged, thus advantageously limiting the on-time of the amplifier. Increasing the number of cells can reduce conductivity because of the cumulative effect of series resistance, and as a result, low conductivity measurements require much lower gain than is the case with high conductivity measurements.

In particular, the number of cells of the battery 12 may be known by the microprocessor 40, which has been input by the operator, whereupon the microprocessor 40 controls the initial gain of the amplifier 32 required for the measurement. Based on the initial gain, the microprocessor may adjust the gain of the amplifier up or down to achieve the appropriate gain for the particular battery configuration, which may greatly reduce the amount of time required to obtain measurement data and reduce power consumption. In one embodiment, the initial gain of amplifier 32 is selected as follows:

number of batteries Initial gain

1 8000

2 4000

3 4000

4 2000

5 1000

6 800

Shutdown circuit 200 as shown in fig. 7 may be used to power down all analog circuits in supply 10, including internal battery 202, transistors 204 and 206, diode 208, and bias resistors 210, 212, 214, and 216. If the device 10 is connected to the battery 12, the transistor 204 is turned on, which in turn activates the transistor 206, so that the VDD of the internal battery 202 supplies the output of the circuit 200, which voltage VDD supplies the analog device power in the device 10. However, if the battery 12 is removed, the transistor 204 will be turned off, and the supply of the voltage VDD is terminated. In operation, microprocessor 40 may selectively terminate the supply of voltage VDD by turning off transistor 206 via a signal provided by diode 208. One aspect of the present invention therefore includes disconnecting certain devices that do not use the internal power supplied by the internal battery 202 of the apparatus 10. The various techniques for implementing this aspect of the invention are shown in a simplified example, and those skilled in the art will be able to use other similar techniques to terminate power to or shut down various devices upon initiation of this example.

Yet another aspect of the present invention is the use of calibration circuit 21 to achieve self-calibration, which is illustrated in fig. 8. circuit 21 includes a mnico shunt conductor standard 240, shunt 240 being calibrated according to the NIST standard, switches 240,242 and 244 connected to microprocessor 40 and adapted with selectively switched shunt 240 connected in series with an amplifier 24. The microprocessor measures the conductivity of shunt 240 using amplifier 24, filter 28, amplifier 32 and analog-to-digital converter 36, compares the measured value with a read calibration standard that is already present in memory 40A of microprocessor 40, generates a calibration factor based on the difference between the measured value and the calibration value, and continuously measures, maintaining self-calibration of battery test apparatus 10. This calibration may occur automatically if the testing device 10 is initially connected to the battery 12, or by the user through the keypad 44.

In yet another aspect of the present invention, the memory 48 of the microprocessor 40 includes various predetermined reference criteria. The appropriate reference criteria for a particular battery 12 are selected by the operator via input device 44, although the operator may recall criteria already stored in memory 40A and present them on display 42, and if the criteria have changed, or it is desired to modify the criteria, the operator may change the criteria using a keypad input device, and the criteria already stored in memory 40A may also be printed out, for example using input/output port 46.

The use of an automatic temperature compensator is another feature of the present invention. The temperature sensor 250 shown in fig. 1 may input a temperature signal to the microprocessor 40, for example, the sensor 250 may comprise a thermocouple, thermistor, or an infrared temperature sensor directed toward the battery 12. An analog-to-digital converter (not shown) provides a digital representation of temperature signal 250 to microprocessor 40, and microprocessor 40 calibrates the test results based on the measured temperature based on information stored in memory 40A to compensate for temperature variations.

The present invention has been described with reference to the various embodiments shown above, and 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 functions of the various circuits shown in fig. 1 may be implemented using any suitable technique and are not limited to the description herein. Moreover, the various aspects of the present invention may be implemented in no particular order, nor limited to the order shown in FIG. 1. The circuit functions can be realized by analog circuits, digital circuits or a mixture of the two. Furthermore, current source 22B may be selectively switched using switch 22A, and the present invention also includes the application of a voltage and measuring the resultant current response. The invention may be used for measurements of conductivity, admittance, impedance or resistance associated with battery test circuits. Furthermore, certain aspects of the present invention may be applied to any type of battery testing device, including load testing devices, simple voltage testing devices, and testing devices that require the placement of batteries through a number of conditioning steps.

Claims (16)

1. An apparatus for monitoring the operating condition of a battery, comprising:

an electronic battery test circuit interconnected to the battery through a first KELVIN connection and a second KELVIN connection;

first and second electrical leads connected to a first terminal of the battery test circuit, configured to connect to a first KELVIN connection of a battery;

third and fourth electrical leads connected to a second terminal of the battery test apparatus arranged to connect to a second KELVIN connection of a battery; and

the induction cancellation circuit connects the first electrical conductor to the second electrical conductor.

2. The apparatus of claim 1, wherein the inductive cancellation circuit provides a reverse current in the second conductor as a function of the current in the first conductor.

3. The apparatus of claim 2, wherein the current in the first conductor comprises a supply current and the second conductor provides a voltage sensing connection to the battery test circuit.

4. The device of claim 3, wherein the first conductor carries an electrical signal having a frequency between 5HZ and 500 HZ.

5. The apparatus of claim 1, wherein the inductive cancellation circuit is a transformer.

6. The apparatus of claim 5, wherein a first winding of the transformer is connected in series with the first conductor and a second winding of the transformer is connected in series with the second conductor.

7. An apparatus according to claim 6, wherein the first and second coils are of opposite polarity.

8. The apparatus of claim 1 wherein the cancellation circuit comprises a circuit that provides a reverse current in the second conductor as a function of the current in the first conductor.

9. The apparatus of claim 1, wherein the first sense current in the first conductor flows into the third conductor as the second sense current, and the sense cancellation circuit couples the second sense current to the second conductor.

10. The apparatus of claim 1, wherein the first wire is laid adjacent to the second wire.

11. The apparatus of claim 1, wherein the first, second, third and fourth conductors are removably connected to the battery test circuitry, and the induction cancellation circuit is housed in a housing in which the first, second, third and fourth conductors are mounted, such that the induction cancellation circuit is matingly connected to conductors of different gauges during manufacture.

12. The apparatus of claim 1, wherein the first, second, third and fourth wires are of sufficient length to enable the battery test apparatus to be installed in an interior of a vehicle, the wires being extendable to a battery located in an engine compartment of the vehicle.

13. A method of reducing inductive coupling current in a KELVIN probe of a battery test device, comprising:

the KELVIN detector is coupled to a sensing current flowing in a first lead; and

an opposite current is provided in the second conductor of the KELVIN probe corresponding to the sense current, which is in a direction opposite to the direction of the inductively coupled current flowing in the second conductor.

14. The method of claim 13, wherein the step of coupling comprises the steps of flowing the sense current directly into a first coil of a transformer, and connecting a second coil of the transformer in series with said second conductor.

15. A method according to claim 14, comprising the step of adjusting said current in opposition to the sense current so as to cancel said inductive coupling current.

16. The method of claim 15, wherein the step of adjusting comprises adjusting a coupling state between the first and second windings of the transformer.