Electric power conversion systems – Current conversion – With condition responsive means to control the output...
Reexamination Certificate
1999-06-03
2001-10-16
Wong, Peter S. (Department: 2838)
Electric power conversion systems
Current conversion
With condition responsive means to control the output...
C320S132000, C324S427000
Reexamination Certificate
active
06304471
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention is directed, in general, to power plants and, more specifically, to a system and method for assessing the capacity of a backup battery in a power plant and a power plant employing the system or method.
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 systems' operation with 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 it would result in a loss 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, a backup battery, in the event of the loss of primary power to the system. A battery plant generally operates as follows. The battery plant generally includes a number of backup batteries, rectifiers and other power distribution equipment. The primary power is produced by the rectifiers, which convert an AC mains voltage into a DC voltage to power the load equipment and to charge the backup 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 backup batteries then provide power to the load. Redundant rectifiers and backup 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 backup batteries are typically coupled directly to the output of the rectifiers and may instantly provide power to the load in the event the AC power outage occurs. During normal operation, the backup batteries are usually maintained in a fully charged state to maximize a duration for which the backup batteries can provide energy to the load equipment.
As a backup battery ages, its capacity or energy-storage capability decreases, reducing the duration for which the backup battery can provide energy, even when fully charged. In many telecommunications applications, a backup battery is considered to have failed when its actual capacity has fallen below a threshold, such as 80% of its rated capacity. A failed backup battery should be replaced in an orderly fashion to maintain the availability of the battery plant. It is crucial, therefore, to be able to assess whether the capacity of a particular backup battery has fallen below the threshold.
The capacity of a backup battery may be assessed when the backup battery is on-line or off-line. One straightforward approach is to take the backup battery off-line and couple it to a dissipative-resistive load. The load can then completely discharge the backup battery at a constant current thus providing an accurate indication of the backup battery's capacity. The off-line method, however, requires that the backup battery be temporarily removed from the battery plant, decreasing the availability thereof. Therefore, to maintain the battery plant at the desired availability level, the capacity of the backup battery should be assessed on-line.
The complete discharge method, however, has some major disadvantages. If an AC power outage occurs during or after the discharge test, but before the backup battery has been fully recharged, the full energy reserve provided by the backup battery will not be available, thus jeopardizing the availability of the battery plant and the reliability of the telecommunications system powered thereby. Further, since a backup battery may only be charged and discharged a finite number of times, each cycle of complete discharge and charge necessarily reduces the overall life span of the battery.
Accordingly, what is needed in the art is a system and method for assessing the capacity of a backup battery that provides an accurate measurement of the backup battery's capacity yet maintains the availability of the battery plant at a satisfactory level.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the prior art, the present invention provides, for use with a power plant having a rectifier and a backup battery coupled to a variable load, a system for, and method of, assessing a capacity of the backup battery. In one embodiment, the system includes: (1) a controller, coupled to the rectifier, that controls an output current of the rectifier to maintain a discharge current of the backup battery at a substantially constant level and (2) a voltage sensor, coupled to the backup battery, that measures a voltage of the backup battery.
The present invention therefore introduces the broad concept of employing a rectifier to supply varying amounts of current to a varying load to allow a backup battery to maintain a substantially constant discharge current. Maintaining the discharge current of the backup battery at a substantially constant level allows an accurate assessment of the backup battery's capacity.
In one embodiment of the present invention, the controller senses the discharge current of the backup battery. Alternatively, the controller can sense the output current of the rectifier and a current draw of the load. Either embodiment allows the discharge current of the battery to be determined and held substantially constant during the time the backup battery is being capacity-tested (the “assessment interval”).
In one embodiment of the present invention, the controller maintains the discharge current of the backup battery at the substantially constant level until the charge of the backup battery decreases by a predetermined discharge amount. While suitable for determining capacity, it is disadvantageous to discharge a backup battery completely, since it compromises the availability of the backup battery in the event of an AC power outage. An embodiment of the present invention avoids this disadvantage by discharging the backup battery only by a predetermined amount, which may be expressed in terms of a percentage of the backup battery's full charge level. In a specific embodiment of the present invention, the predetermined discharge amount is about 20% of the full charge level of the backup battery.
In one embodiment of the present invention, the capacity is a function of the voltage of the backup battery at a termination of a predetermined assessment interval. Representative capacity graphs will be set forth in the Detailed Description that follows to illustrate how the voltage of the backup battery at a termination of a predetermined assessment interval can be employed to determine the capacity of the backup battery.
In one embodiment of the present invention, the predetermined assessment interval spans at least 20 minutes. Those skilled in the pertinent art will understand, however, that the predetermined assessment interval may be shorter or longer than 20 minutes.
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.
REFERENCES:
patent: 4952862 (1990-08-01), Biagetti et al.
Chalasani Subhas Chandra
Thottuvelil Vijayan Joseph
Patel Rajnikant D.
Wong Peter S.
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