Abstract:
A probe assembly for use with a battery impedance meter which minimizes excitation pick-up voltages by routing the wires from the meter to the battery cell terminals without forming a loop within a changing magnetic field caused by current drawn from the battery cell.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a meter for determining the internal impedance of an individual battery cell within a battery backup system without disconnecting the battery cell from the backup system and, more particularly, to an improved probe assembly for use with the battery impedance meter which minimizes excitation pick-up voltages. 
   Large battery systems are commonly used to provide backup power in case there is a failure of the commercial power grid. Typically, such a backup system includes a single string, or a plurality of parallel strings, of serially connected rechargeable battery cells and a charger connected to the commercial power grid for maintaining the charge on the battery cells. An inverter is coupled between the strings of battery cells and the load, which inverter is enabled upon the detection of a failure of the commercial power grid. In some applications, the inverter may be continuously operational to power the load with energy from the charger during the time that commercial power is available. Many of these battery backup systems, called “uninterruptible power supplies” (UPS), are configured such that the load is never aware of any failure of the commercial power grid because the battery system immediately supplies the necessary energy upon detecting a failure of the commercial power grid. 
   A typical installation of such an uninterruptible power supply is between the commercial power grid and a large computer system used by financial, communications, manufacturing and other commercial industries. If the battery system is taken “off-line” for any reason, the necessary protection against a power outage is lost for the time that the battery system is not connected plus the time for recharging, if a significant amount of charge has been removed during the off-line period of time. However, such battery backup systems must be monitored on a regular basis to insure that protection from commercial power grid failure is always available. Therefore, systems have been developed to perform such monitoring while the battery backup system remains on-line. 
   Impedance measurement is a method by which the condition of a battery cell may be assessed without taking the battery system off-line. Impedance measurements typically impose a current (hereinafter called the “loading current”) on the battery cell to be evaluated and measure the resulting voltage. Various commercially available test instruments function this way. Using Kelvin connections, these instruments impose a current on just the battery cell to be measured. After a measurement has been made, the operator moves the Kelvin clips to the next battery cell, reads the value, moves the clips to the next cell, and continues in this manner until all the battery cells have been measured. Therefore, the loading current flows almost entirely through the battery cell being measured, it being thought that the parallel paths (if they exist) are generally of so much higher impedance that any loading current flowing through them is of little or no consequence. However,  FIG. 1  illustrates how conventional prior art measurement apparatus results in unavoidable errors. 
     FIG. 1  illustrates a typical two probe battery impedance meter  10  with a Kelvin connection to the battery cell  12 , meaning that there are separate contacts to the battery for current drawn and voltage sensed. The meter  10  draws current (i) from the battery cell  12 , symbolized by the current source  14 . The drawn current is at a predetermined amplitude and frequency. While drawing the current, the meter  10  measures the voltage drop across the battery cell  12 . The voltage measuring circuit in the meter is symbolized by the voltmeter  16 . The ratio of voltage drop to current drawn is the internal impedance of the battery cell  12 . Where the cables  18 ,  20  physically separate to the probes  22 ,  24  at the battery posts  26 ,  28 , a one-turn coil is formed. The current passing through the wires going to the current source  14  produces a magnetic field in the formed coil. The magnetic field is made up of flux lines symbolized in  FIG. 1  as circles with either a dot or a cross inside. The crosses signify that the flux lines are going into the page and the dots signify that the flux lines are coming out of the page. This magnetic field induces an excitation pick-up voltage in the wires, symbolized by v i . Faraday&#39;s law states that if a coil of N turns is placed in a region of changing flux (φ), a voltage (v i ) will be induced across the coil according to equation (1):
   v   i   =N ( dφ/dt ).  (1) 
The meter  10  will therefore measure a voltage (v) according to equation (2):
   v=v   i   +v   b ,  (2) 
where v b  is the actual voltage across the battery cell  12 .
 
   The term v i  thus causes an error in measuring the battery impedance because battery impedance should only use the voltage drop across the battery (v b ). It is difficult to compensate for v i  because v i  will change for different loop geometries. Since the spacing of the terminals  26 ,  28  differs from battery to battery, the loop geometries are different, and therefore v i  is different and unpredictable. 
   It would therefore be desirable to be able to minimize the excitation pick-up voltage while taking battery cell impedance measurements. 
   SUMMARY OF THE INVENTION 
   According to the present invention, a probe assembly for use with a battery impedance meter, which includes a voltage sensing device having two terminals and a current source having two terminals, comprises a first probe adapted for connection to a selected one of the positive and negative terminals of a battery cell, a second probe adapted for connection to the other one of the positive and negative terminals of the battery cell, and a cable assembly. The cable assembly includes four wires each adapted for connection at a first end to a respective one of the terminals of the current source and the voltage sensing device. At least three of the wires are constrained to extend in close proximity to each other from the battery impedance meter to the first probe. One of the at least three wires which is connected to the current source and one of the at least three wires which is connected to the voltage sensing device are each connected at their second ends to the first probe. At most one wire which is not constrained with the at least three wires is connected at its second end to the second probe. At least one bypass wire has a first end connected to the second probe and a second end insulatively held to the first probe and connected to the second end of one of the at least three constrained wires which is not connected to the first probe. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing will be more readily apparent from reading the following description in conjunction with the drawings in which like elements in different figures thereof are identified by the same reference numeral and wherein: 
       FIG. 1  is a simplified schematic circuit diagram illustrating a prior art arrangement for connecting a battery impedance meter to the positive and negative terminals of a battery cell; 
       FIG. 2  is a simplified schematic circuit diagram similar to  FIG. 1  illustrating a first embodiment of the improved probe assembly according to the present invention; 
       FIG. 3  illustrates the connection of a battery impedance meter to a selected battery cell of a battery backup system, utilizing equipment according to the first embodiment of the present invention; 
       FIG. 4  is an enlarged view of a portion of  FIG. 3  showing the connections of the inventive probe assembly to the terminals of the selected battery cell; 
       FIG. 5  is a simplified schematic circuit diagram similar to  FIG. 1  illustrating a second embodiment of the improved probe assembly according to the present invention; and 
       FIG. 6  is a simplified schematic circuit diagram similar to  FIG. 1  illustrating a third embodiment of the improved probe assembly according to the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 2–4 , the inventive probe assembly avoids the pickup of excitation voltages in the wires connected to the voltage sensing device  16  by eliminating the loop from those wires, in contrast to the prior art arrangement illustrated in  FIG. 1  and discussed above. As schematically shown in  FIG. 2 , a first embodiment of the inventive probe assembly includes the probes  30  and  32  and the cable assembly  34  extending from the battery impedance meter  10  to the probes  30 ,  32 . The probes  30 ,  32  are adapted for connection to respective terminals of the battery cell  12 . As shown in  FIGS. 3 and 4 , the probes  30 ,  32  are illustratively large, spring-loaded, alligator clips which clamp onto the battery cell terminals  26 ,  28  or the shunt bars extending between adjacent battery cells. It is understood that the present invention contemplates that other types of probes can be utilized to practice the invention. 
   The cable assembly  34  includes four wires  36 ,  38 ,  40  and  42 . The wire  36  is connected at a first end to a first terminal of the current source  14  and is connected at its other end to the probe  30 . The wire  38  is connected at a first end to a first terminal of the voltmeter  16  and is connected at its other end to the probe  30 . The wire  40  is connected at a first end to a second terminal of the voltmeter  16  and is attached at its other end to the probe  30  while being electrically insulated therefrom. The wire  42  is connected at a first end to a second terminal of the current source  14  and is connected at its other end to the probe  32 . The wires  36 ,  38  and  40  are constrained to extend in close proximity to each other from the meter  10  to the probe  30 , illustratively by being enclosed within a sheath  44  ( FIG. 3 ). The wire  42  exits the sheath  44  before reaching the probe  30  so that the probes  30  and  32  can be separated to reach different battery terminals. 
   Lastly, to complete the connection of the voltmeter  16  across the terminals  26 ,  28  of the battery cell  12 , there is provided a bypass wire  46  which is connected at a first end to the wire  40  at the probe  30 , while being electrically insulated from the probe  30 , and is connected at its other end to the probe  32 . Preferably, the bypass wire  46  is in the form of a wire coil, which is elastic so that it extends and contracts to accommodate the spacing between the battery cell terminals  26 ,  28 . 
   Since the wires  38  and  40 , which are connected across the voltmeter  16 , are within the sheath  44 , there is no loop formed by those wires which can pick up induced voltage from the current traveling through the wires  36  and  42 . There is still a magnetic field caused by the current flow in the wires  36  and  42 , but the voltage sense wires  38  and  40  are routed so as not to make a loop within that magnetic field. To summarize, the two voltage sense wires  38  and  40  are brought together to the probe  30 . When the wires  38  and  40  are close together, no loop exists and a voltage cannot be induced from the magnetic field. At the probe  30  the wires  38  and  40  are split up, with the wire  38  going to the battery terminal  26  and the wire  40  being connected to the bypass wire  46 . The magnetic field is bypassed, and therefore the voltmeter  16  will measure only v b . The wire coil  46  will extend and contract so as not to fall in the magnetic field. 
   A second embodiment of the present invention is shown in  FIG. 5 . That embodiment differs from the embodiment shown in  FIG. 2  by having the two wires connected to the current source  14  run together so that the induced magnetic field is cancelled out, as is the case where twisted pairs of wires are used. Thus, the cable assembly  48  includes four wires  50 ,  52 ,  54  and  56 . The wire  50  is connected at a first end to a first terminal of the voltmeter  16  and is connected at its other end to the probe  30 . The wire  52  is connected at a first end to a first terminal of the current source  14  and is connected at its other end to the probe  30 . The wire  54  is connected at a first end to a second terminal of the current source  14  and is attached at its other end to the probe  30  while being electrically insulated therefrom. The wire  56  is connected at a first end to a second terminal of the voltmeter  16  and is connected at its other end to the probe  32 . The wires  50 ,  52  and  54  are constrained to extend in close proximity to each other from the meter  10  to the probe  30 , illustratively by being enclosed within the sheath  44  ( FIG. 3 ). The wire  56  exits the sheath  44  before reaching the probe  30  so that the probes  30  and  32  can be separated to reach different battery terminals. 
   Lastly, to complete the connection of the current source  14  across the terminals  26 ,  28  of the battery cell  12 , there is provided a bypass wire  58  which is connected at a first end to the wire  54  at the probe  30 , while being electrically insulated from the probe  30 , and is connected at its other end to the probe  32 . Preferably, the bypass wire  58  is in the form of a wire coil, which is elastic so that it extends and contracts to accommodate the spacing between the battery cell terminals  26 ,  28 . 
   Since the wires  52  and  54 , which are connected across the current source  14 , are within the sheath  44 , there is no magnetic field caused by the current flow in the wires  52  and  54 . Therefore there is no induced voltage in the wires  50  and  56  and the voltmeter  16  will measure only v b . 
   A third embodiment of the present invention is shown in  FIG. 6 . That embodiment differs from the embodiment shown in  FIG. 2  by having the four wires connected to the meter  10  run together so that the induced magnetic field is cancelled out, as is the case where twisted pairs of wires are used, and also not having any loop of the wires connected to the voltmeter  16  which could pick up a voltage induced from a magnetic field. Thus, the cable assembly  60  includes four wires  62 ,  64 ,  66  and  68 . The wire  62  is connected at a first end to a first terminal of the current source  14  and is connected at its other end to the probe  30 . The wire  64  is connected at a first end to a first terminal of the voltmeter  16  and is connected at its other end to the probe  32 . The wire  66  is connected at a first end to a second terminal of the voltmeter  16  and is attached at its other end to the probe  30  while being electrically insulated therefrom. The wire  68  is connected at a first end to a second terminal of the current source  14  and is attached at its other end to the probe  30  while being electrically insulated therefrom. The wires  62 ,  64 ,  66  and  68  are constrained to extend in close proximity to each other from the meter  10  to the probe  30 , illustratively by being enclosed within the sheath  44  ( FIG. 3 ). 
   Lastly, to complete the connection of the current source  14  and the voltmeter  16  across the terminals  26 ,  28  of the battery cell  12 , there are provided bypass wires  70  and  72  which are connected at a first end to the wires  66  and  68 , respectively, at the probe  30 , while being electrically insulated from the probe  30 , and are connected at their other ends to the probe  32 . Preferably, the bypass wires  70  and  72  are in the form of wire coils, which are elastic so that they extend and contract to accommodate the spacing between the battery cell terminals  26 ,  28 . Alternatively, it is only necessary to provide a single bypass wire to replace the pair of bypass wires  70  and  72 , since the bypass wires  70  and  72  are connected together at the probe  32 . 
   Since the wires  62  and  68 , which are connected across the current source  14 , are within the sheath  44 , there is no magnetic field caused by the current flow in the wires  62  and  68 . Therefore there is no induced voltage in the wires  64  and  66  and the voltmeter  16  will measure only v b . 
   Accordingly, there has been disclosed an improved probe assembly for minimizing excitation pick-up voltages. While illustrative embodiments of the present invention have been disclosed herein, it will be appreciated by those of skill in the art that various adaptations and modifications to the disclosed embodiments are possible, and it is therefore intended that this invention be limited only by the scope of the appended claims.