CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Utility Application No. 13/106,246, filed May 12, 201 1 , and the benefit of U.S. Provisional Application No. 61 /334,709, filed May 14, 2010. The entire disclosures of the above applications are incorporated herein by reference.

FIELD

[0002] The present disclosure relates to a battery monitoring system that measures the float/discharge currents across an intertier or intercell of a battery string.

BACKGROUND

[0003] Uninterruptible power supply systems, such as those used for data centers, often utilize batteries as the source of back-up power. Each battery typically has multiple cells or multicell modules connected in series to provide the requisite voltage, commonly referred to as a battery string. The term "cell" will be used herein to refer to both individual cells and multicell modules of a battery string unless the context dictates otherwise. The individual battery cells adjacent to each other in a section of a battery string are connected to each other by a conductive connector, such as a copper bus bar, strap, cable or the like. This connector is commonly referred to as an intercell or intercell connector. Adjacent sections of a battery string are connected to each other by a longer conductive connector, such as a cable or group of cables (that are longer than cables used for intercell connectors), referred to as an intertier or intertier connector.

[0004] Since a battery has a finite life, it will eventually fail. Consequently, battery monitors are often used to monitor the batteries in UPS systems. By detecting battery problems at an early stage before they can cause abrupt system failure, system reliability is improved. [0005] One type of battery monitor used to monitor the batteries in UPS systems monitors the state of health of each cell in a battery string and depending on the configuration of the monitor, may monitor one or several batteries with each battery having a battery string of cells connected in series. In battery monitors available from Alber of Pompano Beach, Florida, such as the BDS series of battery monitors, the internal resistance of each cell in the battery string of each battery is measured as the internal resistance of a cell is a reliable indicator of that cell's state of health. The battery monitors also monitors other parameters, such as cell voltage, overall voltage, ambient temperature of the battery, intercell resistance, intertier resistance, discharge current, discharge events, float current, and the like. The battery monitors will alert a user if the monitored data shows a problem with the batteries being monitored. The battery monitors typically interface to a computer, local or remote, that is programmed to display the monitored data.

[0006] The battery monitors may for example utilize the teachings of

U.S. 4,707,795 for "Battery Testing and Monitoring System" issued November 17, 1987 and/or U.S. Pub. No. 2009/0224771 for "System and method for Measuring Battery Internal Resistance," published September 10, 2009, the entire disclosures of which are incorporated herein by reference.

[0007] Fig. 1 shows a prior art battery monitoring system 100 coupled to a battery string 102. Battery string 102 includes a plurality of battery cells 104 with adjacent battery cells connected to each other by an intercell 106. Battery string 102 may include a plurality of battery string sections 108 with adjacent battery string sections 108 connected to each other by an intertier 1 10. While battery string 102 is shown in Fig. 1 as having two battery string sections 108, it should be understood that battery string 102 may have more than two battery sections 108, with adjacent battery sections connected by an intertier 1 10, or just one battery section 108.

[0008] The positive and negative terminals of each battery cell 104 are connected to respective voltage sense leads 1 12. Voltage sense leads 1 12 are connected to appropriate voltage sense inputs of battery monitor 100, which are coupled to a voltage sense circuit of battery monitor 100, which measures the sensed voltage. To simplify the figure, only two such voltage sense leads 1 12 are shown. Illustratively, the inputs of battery monitor 100 to which voltage sense leads 1 12 are connected are coupled through a multiplexer to the voltage sense circuit, allowing these inputs to be switched between positive and negative inputs of the voltage sense circuit. The positive terminals of every 2, 3, 4, 5, or 6 battery cells 104 are also connected to respective test load inputs of battery monitor 100 by test load leads 1 14. Again to simplify the figure, only two such test load leads 1 14 are shown.

[0009] Battery monitor 100 measures, among other parameters, the internal resistance of the battery cells 104 and the intercell and intertier resistances. The flow chart of Fig. 2 shows in simplified form a method that battery monitor 100 uses to determine the resistance of intercell and intertiers. The method is described with reference to an intercell, but it should be understood that it is also applicable to intertiers. Illustratively, battery monitor 100 includes a controller, such as a microprocessor or microcontroller, that is programmed with software implementing the control of battery monitor 100.

[0010] At 200, battery monitor 100 applies a load across battery cell 104 and intercell 106 via test load leads 1 14 that are connected, respectively, to the positive terminal of the battery cell 104 and the positive terminal of the adjacent battery cell 104. This generates a test current, such as ten or twenty amps, that flows through battery cell 104 and intercell 106. At 202, battery monitor 100 measures, using voltage sense leads 1 12, the voltage drop across intercell 106 while the test current is flowing through the battery cell 104 and the intercell 106. Battery monitor 100 also measures the test current with an on- board current shunt in a known manner. At 204, using Ohm's law, battery monitor 100 then calculates the resistance across intercell 106. At 206, battery monitor 100 checks to see if a requisite sample size of resistances has been obtained. If not, it repeats steps 200 - 204. If so, it then averages the samples at 208 to arrive at a final resistance of intercell 106 (R _ic ). The requisite sample size is the number samples so that when averaged, the resulting final resistance of intercell 106 reflects the actual resistance of intercell 106. This sample size may illustratively be determined in any known fashion, such as heuristically and may be, by way of example and not of limitation, 1024 samples.

[0011] Float current is the current that flows through a battery string when the battery string is unloaded. The amount of float current that is flowing in a battery string is indicative of the health of the battery string. Consequently, it is desirable to measure the float current in a battery string. One technique for measuring the float current of a battery string uses a hall effect device that is clamped around the main power feed of the battery string. The hall effect device is coupled to an analog integrator with a large gain that integrates and low pass filters the signal from the hall effect device. The resulting signal is indicative of the float current and is measured and the float current determined therefrom. One such float current monitoring device utilizing the above technique is the FCCP Float Charging Current Probe available from Multitel of Quebec City, Quebec, Canada. It should be understood that such a float current monitoring device can be integrated with the above described battery monitoring system of Fig. 1 , such as by way of example, integrating the analog integrator in the battery monitoring system where the hall effect device is coupled to an appropriate input of the battery monitoring system. A disadvantage of the above type of float current monitor technique is that the hall effect devices used are fairly expensive.

SUMMARY

[0012] In accordance with an aspect of the present disclosure, a float current monitoring device determines the float current of a battery string by dwelling on voltage drop measurements across an intercell or intertier of the battery string when the battery string is unloaded. The float current monitoring device first determines the resistance of the intercell or intertier. The float current monitoring device then takes many samples of the voltage drop across the intercell or intertier when the battery string is unloaded and determines the float current from these measurements and the previously determined resistance of the intercell or intertier. Either the voltage drop measurement samples are averaged and the final float current then determined from this average, or a float current determined from each voltage drop measurement and the resulting float currents averaged to determine the final float current.

[0013] In one aspect, the float current monitoring device is a stand alone device. In another aspect, the float current monitoring device is incorporated into a battery monitor that monitors parameters of a battery string in addition to float current.

[0014] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

DRAWINGS

[0015] Fig. 1 is a simplified schematic of a prior art battery monitoring system;

[0016] Fig. 2 is a flow chart showing a program for the battery monitor of Fig. 1 to determine the resistance of an intercell;

[0017] Fig. 3 is a simplified diagram of a float current monitoring device in accordance to an aspect of the present disclosure coupled to a battery string; and

[0018] Fig. 4 is a flow chart showing an aspect of a program for the float current monitoring device of Fig. 3 that determines float current.

[0019] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

DETAILED DESCRIPTION

[0020] Example embodiments will now be described more fully with reference to the accompanying drawings. [0021] Referring to Fig. 3, a float current monitoring device 300 in accordance with an aspect of the present disclosure is shown coupled to battery string 102. Float current monitoring device 300 includes a pair of test load inputs 302 and a pair voltage sense inputs 304. One of test load inputs 302 is coupled by a test load lead 316 to a positive terminal of one of battery cells 104 and the other test load input 302 is coupled by another test load lead 316 to a positive terminal of an adjacent battery cell 104. Voltage sense inputs 304 are coupled by respective voltage sense leads 318 to opposed sides of intercell 106 joining the two adjacent battery cells 104, that is, to the negative side of a battery cell 104 and to the positive side of an adjacent battery cell 104. It should be understood, however, that voltage sense inputs 304 could alternatively be connected to opposed sides of an intertier 1 10 where the adjacent battery cells 104 are in adjacent battery strings. The above described connection of the test load leads 316 and voltage sense leads 318 is thus comparable to the connection of a pair of test load leads 1 14 and a pair of voltage sense leads 1 12 to battery string 102 as described above.

[0022] Float current monitoring device 300 further includes test load circuit 308 coupled to test load inputs 302 and voltage sense circuit 310 coupled to voltage sense inputs 304. Test load circuit 308 may illustratively be any type of known circuit for connecting a load across elements of a battery string, such as the test load circuits used in the BDS series of battery monitors discussed above. They may also be the test load circuits described in US 4,707,795 or U.S. Pub. No. 2009/0224771 referenced above. Voltage sense circuit 310 may be any type of circuit used in measuring voltage, such as the voltage sense circuits used in the BDS series of battery monitors discussed above. They may also be the voltage sense circuits described in U.S. 4,707,795 or U.S. Pub. No. 2009/0224771 referenced above. In this regard, voltage sense circuit 310 includes an analog-to-digital converter that digitizes the voltage signal at voltage sense inputs 304. It may also include an analog gain section that amplifies the voltage signal at voltage sense inputs 304 before that signal is digitized. Float current monitoring device further includes a controller 312, such as a microprocessor, microcontroller, application specific integrated circuit, or the like. [0023] Controller 31 2 is configured, such as by appropriate software programmed into it, to operate float current monitoring device 300 to determine float current. In operation, when battery string 1 02 is unloaded float current monitoring device 300 first determines the resistance of intercell 1 06 in the manner described above for battery monitor 1 00, and does so periodically thereafter. Float current monitoring device 300 then dwells on measuring a plurality of voltage drops across intercell 1 06 while battery string 1 02 is unloaded and determines the float current flowing through intercell 1 06, and thus through battery string 1 02. It does so based on the determined resistance of intercell 1 06 and the plurality of voltage drop measures across intercell 106 using Ohm's law, that is, If = V _ic /Ri _C where If is the float current, V _ic is the measured voltage drop across intercell 1 06 and Rj _C is the resistance of intercell 1 06 determined as described above.

[0024] The flow chart of Fig. 4 shows in simplified form that part of the program for controller 31 2 for determining float current. These steps take place when battery string 1 02 is unloaded, and are repeated periodically, such as every few hundred milliseconds. At 400, float current monitoring device measures the voltage drop across intercell 1 06. At 402, controller 31 2 calculates the float current using Ohm's law as discussed above. At 404, controller 312 checks to see if a requisite sample size has been obtained. If not, steps 400 and 402 are repeated until the requisite sample size is obtained. Once the requisite sample size is obtained, controller 312 then averages the samples to obtain a final float current that can then be used by float current monitoring device 300 such as determining the battery string state of health. It should be understood that the voltage drop measurements could alternatively be used as the samples. In which case, the voltage drop measurement samples are averaged at 406 by controller 312 and the average voltage drop measurement divided by the resistance of intercell 1 06 to obtain the final float current. It is further understood that only the relevant steps of the methodology are discussed in relation to Figure 4, but that other software-implemented instructions may be needed to control and manage the overall operation of the system. [0025] The requisite sample size is that number of samples that when averaged, accurately reflects the actual true float current flowing in battery string 102. This sample size may be determined in any known fashion, such as heuristically. In this regard, oversampling is used to obtain the requisite measurement resolution with more oversampling typically utilized when the intercell resistance is very low than when it is higher. The oversampling used is illustratively determined based on the expected resistance of intercell 106. This oversampling provides digital or processing gain, in known fashion, to the voltage measurement across intercell 106.

[0026] Float current monitoring device 300 may illustratively be a stand alone device where its inputs are connected to a battery string to be monitored. Float current monitoring device 300 may also be incorporated in a battery monitor, shown in phantom as battery monitor 314 in Fig. 3, such as battery monitor 100. In the latter case, the inputs of battery monitor 100 are used to provide the voltage sense inputs and test load inputs used to determine float current and a controller of the battery monitor programmed to implement the above described float current monitoring.

[0027] The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.