Abstract:
For use with a battery plant coupled to a source of electrical power and having a power bus coupled to a load, the battery plant including a battery string coupled across a pair of rails of said power bus, a battery management system, method of operation therefor and battery plant employing the same. In one embodiment, the battery management system includes a DC/DC converter, couplable in series with the battery string, adapted to condition a voltage provided to the battery string as a function of a characteristic of the battery string. The battery management system also includes a switching circuit, coupled across the DC/DC converter, adapted to selectively decouple the DC/DC converter from the battery string thereby allowing the battery string to power the load.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to power plants and, more specifically, to a battery management system, method of operation therefor, and a battery plant employing the same. 
     BACKGROUND OF THE INVENTION 
     The traditional reliability of telecommunication systems that users have come to expect and rely upon is based, in part, on the reliance on redundant equipment and power supplies. Telecommunication switching systems, for example, route tens of thousands of calls per second. The failure of such systems, due to either equipment breakdown or loss of power, is unacceptable, since such failure may result in the discontinuation of millions of telephone calls and a corresponding loss of revenue. 
     Power plants, such as battery plants, address the power loss problem by providing the system with an energy reserve (e.g., a battery) in the event of the loss of primary power to the system. A battery plant generally operates as follows. The battery plant includes a number of batteries, rectifiers and other power distribution equipment. The primary power is produced by the rectifiers, which convert an AC main voltage into a DC voltage to power the load equipment and to charge the batteries. The primary power may, however, become unavailable due to an AC power outage or the failure of one or more of the rectifiers. In either case, the batteries then provide power to the load. Redundant rectifiers and batteries may be added to the battery plant as needed to increase the availability of the battery plant. 
     A battery plant that powers telecommunications systems, such as transmission and switching systems in wireless base stations, commonly employs valve-regulated lead-acid (VRLA) batteries as the energy reserve. The batteries are typically connected in strings (battery strings) and coupled directly to the output of the rectifiers to instantly provide power to the load in the event an AC power outage occurs. During normal operation, the batteries are usually maintained in a fully charged state to maximize a duration for which the batteries can provide energy to the load equipment. However, because all the battery strings in battery plants found in the prior art are charged simultaneously and for the same duration, the individual battery strings are typically not charged to their optimum potentials. 
     The batteries are typically float charged in multiple battery strings, with each battery string having multiple batteries or monoblocks. For example, four 12V monoblocks may be connected in series to form a 48V battery string. The battery strings are coupled across the output of the rectifiers and are charged by drawing current from the output bus of the rectifiers. As the batteries charge, the amount of current drawn from the rectifiers is reduced, until only a small float current, sufficient to keep the batteries fully charged, is drawn. A float voltage may be adjusted based on battery temperature. With multiple battery strings, however, the temperature of the battery strings may be different. However, since the voltage of the rectifiers&#39; output bus is common to all the battery strings, the float voltage of an individual battery string cannot be set at an optimal level. 
     Furthermore, the prior art methods of determining the individual capacities of each battery remain crude and imprecise. Current battery plants test the capacity of all the battery strings as a whole. Specifically, to determine the charge capacity of a battery string, the controller adjusts the overall battery plant voltage to allow the batteries in the battery string to discharge at a constant and desired current level. During this process the voltage of the battery string may be monitored to assess its capacity. 
     A problem occurs when multiple battery strings are employed in a single battery plant. In this situation, while the controller can still adjust the battery plant voltage to provide battery discharge at a constant desired current level, it is necessary to assess the capacities of all the battery strings at the same time. A defective battery string may, therefore, not be detectable. 
     Accordingly, what is needed in the art is a battery management system, and related method, employable with a battery plant having at least one battery string and, in many instances, a plurality of battery strings, that can individually assess and improve the performance of each battery string in the battery plant. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, the present invention provides, for use with a battery plant coupled to a source of electrical power and having a power bus coupled to a load, the battery plant including a battery string coupled across a pair of rails of said power bus, a battery management system, method of operation therefor and battery plant employing the battery management system or the method. In one embodiment, the battery management system includes a DC/DC converter, couplable in series with the battery string, adapted to condition a voltage provided to the battery string as a function of a characteristic of the battery string. The battery management system also includes a switching circuit, coupled across the DC/DC converter, adapted to selectively decouple the DC/DC converter from the battery string thereby allowing the battery string to power the load. 
     The present invention, in one aspect, introduces a battery management system for a battery string located in a battery plant. The battery management system is designed to be series-coupled to the battery string and adapted to condition a voltage to the battery string as a function of a characteristic thereof. The charging of the battery string can be customized by taking into account various parameters, such as the environment (e.g., temperature), state of charge of the battery and electrical characteristics (e.g., a voltage) of the battery string. The battery management system, therefore, provides additional functionality to the battery plant such as charging control and state of health assessment of the battery string. 
     In one embodiment of the present invention, the battery plant further includes at least one AC/DC rectifier coupled to the source of electrical power. The AC/DC rectifier transforms AC power from the source of electrical power to a substantially equivalent DC component to power the battery plant and, ultimately, the load. 
     In one embodiment of the present invention, the DC/DC converter is a bi-directional DC/DC converter. Of course, other power converter topologies may be used in accordance with the requirements of a particular application. 
     In one embodiment of the present invention, the battery plant further includes a controller that monitors the characteristic of said battery string. The controller is adapted to monitor any condition of the battery string such as a voltage, state of charge or environmental conditions. The controller then employs that information to regulate the battery management system. 
     In one embodiment of the present invention, the battery plant further includes a plurality of battery strings coupled across the rails of the power bus. A battery plant often includes a plurality of battery strings to accommodate higher load requirements or for backup protection. The battery management system is especially useful in such applications to customize the treatment of each battery string when there are multiple battery strings. In a battery plant including multiple battery strings, it is preferable to include a plurality of DC/DC converters and switching circuits adapted to be coupled to corresponding battery strings. 
     In one embodiment of the present invention, the battery string includes a battery selected from the group consisting of: (1) a valve-regulated lead-acid (VRLA) battery; (2) a flooded lead-acid battery; (3) a nickel-cadmium battery; and (4) a lithium battery. Such batteries are often employed in battery plants to power telecommunications systems such as transmission and switching systems, and in wireless base stations as the energy reserve. Of course, any type of battery may be employed in conjunction with the present invention. 
     The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a diagram of a prior art battery plant; and 
     FIG. 2 illustrates a diagram of an embodiment of a battery plant employing a battery management system constructed according to the principles of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Referring initially to FIG. 1, illustrated is a diagram of a prior art battery plant  100 . The battery plant  100  is coupled to a source of AC power  110  and provides DC power to a load  190 . The battery plant  100  is employable to power variable loads (i.e., loads having variable current draw) such as wireless base station equipment. The current draw of the load  190  may vary, for example, as a function of an amount of voice traffic carried by the base station. 
     The battery plant  100  includes a rectifier  120  coupled to the source  110 . Those skilled in the pertinent art realize, of course, that the battery plant  100  may include a number of parallel-coupled rectifiers  120 , depending on the power requirements of the load  190 . The battery plant  100  further includes first and second battery strings  130 ,  180  coupled to an output of the rectifier  120 . Of course, additional batteries or battery strings (employable for backup or supplemental power requirements) may be added as necessary to increase the availability of the battery plant  100 . The first and second battery strings  130 ,  180  may include VRLA batteries or any other type of battery, whether conventional or later-developed. 
     The battery plant  100  still further includes a system  140  for assessing the capacity of the first and second battery strings  130 ,  180 . The system  140  includes a controller  150 , coupled to the rectifier  120 , that, during the assessment interval, controls an output current of the rectifier  120  to set a discharge current of the first and second battery strings  130 ,  180  at a substantially constant level. The controller  150  controls the output current of the rectifier  120  by adjusting an output voltage of the rectifier  120  relative to the voltage of the first and second battery strings  130 ,  180 . The system  140  further includes a voltage sensor  160 , coupled to the first and second battery strings  130 ,  180 , that measures a voltage of the first and second battery strings  130 ,  180 . 
     The capacity of the first and second battery strings  130 ,  180  and their charge level determines a duration for which the first I and second battery strings  130 ,  180  can provide energy. As the batteries forming the first and second battery strings  130 ,  180  age, however, their individual capacities decrease, reducing the duration for which the first and second battery strings  130 ,  180  can provide energy, even when fully charged. The system  140 , therefore, detects when the actual capacities of the first and second battery strings  130 ,  180  have diminished beyond a threshold, such as 80% of their rated capacity. The first and second battery strings  130 ,  180  may then be considered to have failed and should be replaced to maintain the battery plant  100  at the desired level of availability. By treating the first and second battery strings  130 ,  180  as identical, the battery plant  100  may indicate that the first and second battery strings  130 ,  180  have failed when only one has actually failed. As a result, the full potential of first and second battery strings  130 ,  180  may not be achieved. 
     The system  140  operates as follows. The controller  150  employs a current sensor  170  to sense the discharge current of the first and second battery strings  130 ,  180 . Alternatively, the controller  150  can sense both the output current of the rectifier  120  and a current draw of the load  190  and derive therefrom the discharge current of the first and second battery strings  130 ,  180 . Even when using this method, the first and second battery strings  130 ,  180  are treated as identical and the actual characteristics of each individual battery of the first and second battery strings  130 ,  180  are not determined. 
     The controller  150  is coupled to the rectifier  120 . The controller  150  adjusts a reference voltage of the rectifier  120  to increase or decrease the output voltage of the rectifier  120  relative to the voltage of the first and second battery strings  130 ,  180 . By varying the output voltage of the rectifier  120 , the controller  150  may control the output current of the rectifier  120  and, indirectly, the discharge current of the first and second battery strings  130 ,  180 . The rectifier  120  may thus supply a varying amount of current to the load  190 , allowing the first and second battery strings  130 ,  180  to, together, maintain a substantially constant discharge current. In addition, the controller  150  can also adjust the rectifier  120  voltage to charge the first and second battery strings  130 ,  180  simultaneously. As discussed above, the simultaneous charging of the first and second battery strings  130 ,  180  may prevent either one from being charged to its full potential. 
     When assessing battery capacity, the controller  150  maintains the discharge current of the first and second battery strings  130 ,  180  at a substantially constant level for a predetermined assessment interval. The predetermined assessment interval may span, for example, 20 minutes. At the end of the predetermined assessment interval, the voltage sensor  160  measures the voltage of the first and second battery strings  130 ,  180 . The capacities of the first and second battery strings  130 ,  180  may then be determined with reference to a capacity graph or a battery capacity algorithm. 
     The predetermined assessment interval is preferably selected such that, with the discharge current of the first and second battery strings  130 ,  180  at a single substantially constant level, the charges of the first and second battery strings  130 ,  180  will decrease by a single predetermined discharge amount during the predetermined assessment interval. The predetermined assessment interval and the level of the discharge current of the first and second battery strings  130 ,  180  may be correlated such that the predetermined discharge amount is about 20% of a full charge levels of the first and second battery strings  130 ,  180 . The first and second battery strings  130 ,  180  may thus retain an adequate amount of energy for powering the load  190  in the event that an AC power outage occurs during or after the discharge test, but before the first and second batteries  130 ,  180  have been fully recharged. 
     As discussed above, because the prior art battery plant  100  treats both the first and second battery strings  130 ,  180  as identical, a predetermined discharge amount of 20% may not be adequate or beneficial therefor. As a result, one of the first and second battery strings  130 ,  180  may not maintain an adequate amount of energy for powering the load  190  if such a power outage does occur. 
     Referring now to FIG. 2, illustrated is a diagram of an embodiment of a battery plant  200  employing a battery management system constructed according to the principles of the present invention. The battery plant  200  is coupled to a source of AC power  205  and provides DC power to load a  255 . As with the battery plant  100  in FIG. 1, the battery plant  200  is employable to power variable loads (i.e., loads having variable current draw) such as wireless base station equipment. The current draw of the load  255  may also vary, for example, as a function of an amount of voice traffic carried by the base station. 
     The battery plant  200  includes first and second rectifiers  210 ,  215  coupled to the source  205 . Those skilled in the pertinent art realize, of course, that the battery plant  200  may include any number of parallel-coupled rectifiers, depending on the power requirements of the load  255 . The outputs of the first and second rectifiers  210 ,  215  are coupled to a pair of rails (first and second rails  217 ,  219 ), which form a power bus of the battery plant  200 . The battery plant  200  further includes first and second battery strings  220 ,  225  across the pair of rails  217 ,  219 . Of course, additional battery strings may be added as necessary to increase the availability of the battery plant  200 . The first and second battery strings  220 ,  225  may include VRLA batteries or any other type of battery, whether conventional or later-developed. 
     The battery plant  200  further includes a controller  250 , controlling the operation of the battery plant  200 . The controller  250  may be a single controller or, alternatively, may include a main controller that communicates with a number of sub-controllers to control the operation of the battery plant  200 . The controller  250  can charge the first and second battery strings  220 ,  225  all together or individually, conduct individual capacity assessments on the first and second battery strings  220 ,  225 , or simply operate the battery plant  200  in a normal manner (i.e., operate the battery plant in a conventional manner). 
     The battery management system includes first and second DC/DC converters  230 ,  235  having first and second outputs series-coupled with the first and second battery strings  220 ,  225 , respectively. The inputs of the first and second DC/DC converters  230 ,  235  are coupled across the pair of rails  217 ,  219  of the power bus. The first and second DC/DC converters  230 ,  235  may be any type of bi-directional converter. 
     The battery management system further includes first and second switching circuits  240 ,  245  coupled across the first and second DC/DC converters  230 ,  235 , respectively, and also coupled in series with respective first and second battery strings  220 ,  225 . 
     The battery management system further includes first and second sensors  260 ,  265 , respectively coupled to the first and second battery strings  220 ,  225 , that measure the individual characteristics of the first and second battery strings  220 ,  225  and provide this information to the controller  250 . Contrary to battery plants found in the prior art, the battery plant  200  preferably uses independent monitoring and separate sensors for each battery string so that the first and second battery strings  220 ,  225  are not treated as identical in temperature, voltage storage, voltage drop, and overall capacity. Those skilled in the art understand that this is important because the first and second battery strings  220 ,  225  are rarely, if ever, identical to one another. Thus, by not treating the first and second battery strings  220 ,  225  as identical, the full potential of the first and second battery strings  220 ,  225  may be realized by the battery plant  200 . The battery management system still further is adapted to employ first and second rectifier control signals  270 ,  275  from the controller  250  to control the first and second rectifiers  210 ,  215 , respectively. The controller  250  uses the first and second rectifier control signals  270 ,  275  to vary the outputs of the respective first and second rectifiers  210 ,  215 , depending upon whether the battery plant  200  is individually charging the first and second battery strings  220 ,  225 , assessing the individual capacities of the first and second battery strings  220 ,  225 , or simply operating normally. 
     When charging the first and second battery strings  220 ,  225 , the battery management system operates as follows. Once some or all of the first and second battery strings  220 ,  225  have been used to power the load  255 , or have otherwise lost a portion of its charge, the battery management system optimizes the charging of the first and second battery strings  220 ,  225  by providing individual battery charging control. First, the controller  250  closes the first and second switching circuits  240 ,  245  to allow the first and second battery strings  220 ,  225  to charge at a constant level directly from the rectifiers  210 ,  215 . During this initial charging stage, the first and second sensors  260 ,  265  detect “worst case” characteristics from one of the first and second battery strings  220 ,  225 , and the controller  250  reacts by treating the first and second battery strings  220 ,  225  as having these “worst case” characteristics. 
     For example, if the current of the first battery string  220  rises to a predetermined value before that of the second battery string  225 , which is likely indicative of the first battery string  220  reaching its charge capacity, the first sensor  260  passes this information to the controller  250 . The controller  250  then opens the first and second switching circuits  240 ,  245  to prevent further constant level charging to the first and second battery strings  220 ,  225  at once. At this point, the first and second battery strings  220 ,  225  have been charged simultaneously. 
     Then, to insure optimal charging of the first and second battery strings  220 ,  225 , the controller  250  employs the first and second sensors  260 ,  265  to sense the individual characteristics of the first and second battery strings  220 ,  225 . These characteristics can include battery temperature, voltage, voltage drop, or any other characteristics advantageous to determining the optimum charge of a battery string. By opening the first or second switching circuits  240 ,  245 , the controller  250  uses the first and second DC/DC converters  230 ,  235  to individually charge the respective first and second battery strings  220 ,  225 . The controller  250  regulates the outputs of the first and second DC/DC converters  230 ,  235  based on characteristics detected therefrom in association with the respective first and second battery strings  220 ,  225 . By regulating a single converter for each battery string, the controller  250  insures that each battery string obtains its optimum level of charge so as to guarantee peak performance from the first and second battery strings  220 ,  225 , especially, in the event of a power outage of the battery plant  200 . The first and second DC/DC converters  230 ,  235  may thus supply varying outputs to the respective first and second battery strings  220 ,  225  while the first and second rectifiers  210 ,  215  maintain a constant amount of current to the load  255 . 
     In addition, the controller  250  may open all but one of the first and second battery strings  220 ,  225  during this charging interval. By maintaining at least one of the first and second battery strings  220 ,  225  directly coupled to the first and second rectifiers  210 ,  215  and the load  255 , the controller  250  can insure that the load  255  will have a power source in the event of a power outage during the charging interval, even while the remaining batteries are receiving individual peak charging through the converters. Alternatively, when more than one battery string is used in the battery plant  200 , the controller  250  may maintain a direct connection to that battery string receiving an optimum charge, while maintaining a connection through the respective ones of the first and second DC/DC converters  230 ,  235  between the remaining battery strings, the first and second rectifiers  210 ,  215 , and the load  255 . By optimally charging one battery string at a time in this manner, the controller  250  further insures that the load  255  will have a sufficient power source in the event of a power outage. 
     In addition to individual battery string charging, the battery management system disclosed by the present invention also allows the charge capacity of each of the first and second battery strings  220 ,  225  to be assessed. Assessing the charge capacity of each of the first and second battery strings  220 ,  225  ensures that each battery string has not fallen below a minimum threshold (such as 80% of rated capacity) and requires maintenance or replacement. When testing the capacity of the first and second battery strings  220 ,  225 , the battery management system operates as follows. During a capacity assessment function, the controller  250  uses the first and second rectifier control signals  270 ,  275  to control the output voltage of the first and second rectifiers  210 ,  215 , respectively, to discharge designated battery strings at a substantially constant current level. The controller  250  designates which battery string&#39;s capacity will be assessed by closing the switch located proximate a respective battery string. By closing the switch, the battery string to be assessed is directly coupled to the first and second rectifiers  210 ,  215 . In the illustrated embodiment, the controller  250  controls the output voltage of the first and second rectifiers  210 ,  215  by adjusting their output voltage relative to the individual characteristics of the one of the first and second battery strings  220 ,  225  designated for capacity assessment. These individual characteristics are detected by and transmitted through the first and second sensors  260 ,  265  to the controller  250 . 
     The capacity of each of the first and second battery strings  220 ,  225  and their individual charge levels still determine the duration for which each of the first and second battery strings  220 ,  225  can provide energy. However, as the first and second battery strings  220 ,  225  age, their individual capacities will unevenly decrease, reducing the duration for which each of the first and second battery strings  220 ,  225  can provide energy, even when fully charged. The system, therefore, detects when the actual respective capacities of the first and second battery strings  220 ,  225  has diminished beyond the above-mentioned threshold of 80% (or any other threshold) of their rated capacity. Thus, the first and second battery strings  220 ,  225  may be considered to have failed, and thus replaced, individually to maintain the battery plant  200  at an optimum level of availability. 
     Because the first and second battery strings  220 ,  225  are not treated identically, the battery plant  200  will indicate the failure of only that battery string which has actually failed and requires replacement, rather than indicating failure of both of the first and second battery strings  220 ,  225  (when only one has actually failed). As a result, the full potential and useful life of the first and second battery strings  220 ,  225  may be achieved. 
     Finally, the battery plant  200  can also operate normally, similar to conventional battery plants. Since the first and second switching circuits  240 ,  245  are coupled across the respective first and second DC/DC converters  230 ,  235  and in series with the respective first and second battery strings  220 , 225 , the controller  250  can activate one or both of the first and second switching circuits  240 ,  245  in the event of a loss of the power source  205 . By closing the first and second switching circuits  240 ,  245 , the controller  250  closes the circuit between the first and second battery strings  220 ,  225  and the load  255 , allowing the load  255  to be driven by the electrical current of the first and second battery strings  220 ,  225 . Once the power source  205  has been restored, the controller  250  can deactivate the first and second switching circuits  240 , 245 , thus opening the circuit between the first and second battery strings  220 ,  225  and the load  255 , thus bypassing the first and second battery strings  220 , 225 . Bypassing the first and second battery strings  220 , 225  allows the load  255  to once again be driven by the power source  205  via the first and second rectifiers  210 ,  215 . Then, the controller  250  can regulate the outputs of the first and second DC/DC converters  230 ,  235  to recharge the first and second battery strings  220 ,  225  to peak capacity individually, based on the respective characteristics of each of the first and second battery strings  220 ,  225 , as discussed above. The bulk of the initial recharge, however, will typically be performed by the rectifiers  210 ,  215 . 
     Also, the controller  250  can decouple one or more of the battery strings  220 ,  225  found in the battery plant  220 ,  225 . For example, the controller  250  can deactivate the first switching circuit  240 , thus decoupling the first battery string  220  from both the first and second rectifiers  210 ,  215  and the load  255 . In addition, the controller  250  could prevent the first DC/DC converter  230  from transferring any electrical current to the first battery string  220 . 
     Although the present invention has been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.