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.