Electricity: battery or capacitor charging or discharging – Serially connected batteries or cells – Having variable number of cells or batteries in series
Reexamination Certificate
2000-08-29
2001-05-08
Wong, Peter S. (Department: 2838)
Electricity: battery or capacitor charging or discharging
Serially connected batteries or cells
Having variable number of cells or batteries in series
C257S665000
Reexamination Certificate
active
06229282
ABSTRACT:
BACKGROUND OF THE INVENTION
Rechargeable cells or batteries are electrochemical energy storage devices for storing and retaining an electrical charge and later delivering that charge as useful power. Familiar examples of the rechargeable electrical storage cell are the lead-acid cell used in automobiles and the nickel-cadmium cell used in various portable electronic devices. Another type of electrical storage cell having a greater storage capacity for its weight and longer life is the nickel oxide/pressurized hydrogen electrical storage cell, an important type of which is commonly called the nickel-hydrogen electrical storage cell and is used in spacecraft applications. The weight of the spacecraft electrical storage cell must be minimized while achieving the required performance level, due to the cost of lifting weight to an earth orbit and beyond.
The nickel-hydrogen electrical storage cell includes a series of active plate sets which store an electrical charge electrochemically and later deliver that charge as a useful current. The active plate sets are packaged within a hermetic pressure vessel that contains the plate sets and the hydrogen gas that is an essential active component of the electrical storage cell. A single nickel-hydrogen electrical storage cell delivers current at about 1.3 volts, and a number of the electrical storage cells are usually electrically interconnected in series to produce current at the voltage required by the systems of the spacecraft.
Although the electrical storage cells are designed for excellent reliability, there is always the chance of a failure. One failure mode of the electrical storage cell is an open-circuit failure, in which there is no longer a conducting path through the electrical storage cell. In the event of an open-circuit failure of a single electrical storage cell in a series-connected array of cells, all of the storage capacity of the array is lost.
A bypass around a potentially failed cell is required to prevent loss of the storage capacity of the entire array. The bypass must not conduct when the electrical storage cell is functioning properly, but it must activate to provide an electrically conductive bypass when the electrical storage cell fails in the open-circuit mode. The use of bypass rectifier diodes and relays to provide this bypass function is known, but these bypass devices add a considerable amount of weight to each of the electrical storage cells, and a separate bypass is required for each of the 24 or more storage cells in a typical battery system. Additionally, the bypass diode has a relatively high voltage drop that dissipates power when it functions as a bypass, and the relay itself has the potential for failure.
There is a need for an improved technique for achieving an electrical bypass of electrical storage cells. The present invention fulfills that need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an electrical bypass for a failed-open electrical storage cell. The bypass is extremely light in weight. It passes no current when the electrical storage cell operates normally, but is activated as the electrical storage cell fails to the open-circuit state. When activated, the bypass has a low electrical resistance, so that it does not dissipate much power as the remainder of the battery is charged and discharged. Any significant dissipated power tends to overload the heat-dissipation structure of the spacecraft and is a drain on the power supply of the system.
In accordance with the invention, a battery system comprises an electrical storage cell having a positive terminal and a negative terminal. A normally open bypass circuit path comprises a first electrical conductor connected to the positive terminal of the electrical storage cell, a second electrical conductor connected to the negative terminal of the electrical storage cell, and a shorting gap between the first electrical conductor and the second electrical conductor. A mass of a fusible material is positioned at an initial mass location. At this initial mass location, the mass of the fusible material does not close or short the shorting gap. A heat source, activatable upon the occurrence of an open-circuit condition of the electrical storage cell, is operable to melt at least a portion of the mass of the fusible material. A biasing mechanism, which preferably comprises a spring, is positioned to force the mass of the fusible material into the shorting gap, when the mass of the fusible material is at least partially molten (and preferably nearly completely melted), thereby closing the shorting gap so that the first electrical conductor is in electrical communication with the second electrical conductor.
The heat source is preferably at least one diode. In one embodiment, the diode has a cathode and an anode. The cathode of the diode is electrically connected to the positive terminal of the electrical storage cell, and the anode of the diode is electrically connected to the negative terminal of the electrical storage cell. The diode and its electrical resistance are sized such that, as the electrical storage cell begins to fail and a large electrical current passes through the diode, the diode heats to a sufficiently high temperature to melt at least some (and preferably all or nearly all) of the fusible material, leading to closure of the shorting gap as the biasing mechanism rapidly drives the molten fusible material into the gap. As the shorting gap is quickly closed, its electrical resistance rapidly falls and the bypassing current flows through the fusible material and the first and second electrical conductors rather than through the diode. Consequently, the electrical resistance, and thence heat generation, of the bypass circuit falls.
The closure of the shorting gap by the fusible material may be accomplished in any of several ways. The fusible material may be a metallic electrical conductor such as a solder that flows into the shorting gap upon melting and provides a good electrical conduction path. The flow may be aided by providing a partially or fully tinned flow path from the initial mass location to the shorting gap. To ensure that the molten fusible material flows into the shorting gap, the biasing mechanism forces the molten fusible material toward the shorting gap. Completion of the flow of the molten fusible material into the shorting gap is aided by capillary action, but the biasing mechanism provides the primary driving force. When the fusible material enters the gap and the resistance of the bypass circuit path falls, the heat produced by resistance heating also falls, and the metal re-solidifies to firmly fix the electrical conductor in the gap and permanently electrically short the gap.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.
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Rogers Howard H.
Stadnick Steven J.
Gudmestad T.
Hughes Electronics Corporation
Toatley Jr. Gregory J
Wong Peter S.
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