This invention relates generally to pressurized gas-metal storage cells such as sealed nickel-hydrogen cells and, more particularly, to controlling the charging of such cells.
Rechargeable storage cells are electrochemical devices for storing an electrical charge and later delivering that charge as a useful current. A familiar example of the rechargeable storage cell is the lead-acid battery used in automobiles. Another type of cell having a greater electrical storage capacity per unit weight is the pressurized gas-metal storage cell, an important example of which is the nickel-hydrogen cell used in spacecraft applications. A nickel-hydrogen cell used in a satellite is typically periodically charged by electrical current produced by solar panels on the spacecraft, and then later discharged to supply electrical power, when the spacecraft is in shadow or the solar panels are otherwise unable to furnish electrical power.
The primary requirements of storage cells to be used in spacecraft are high energy capacity per unit weight of cell, reliability, and the ability to be recycled through many cycles of charging and discharging. Reliability and multiple cycle capability are influenced in part by the degree of charging of the cell. That is, it has been found that nickel-hydrogen cells tend to have a shorter life and greater incidence of failures if the cells are severely overcharged during the charging cycle.
It is therefore important to control the charging of pressurized gas-metal storage cells during charging, to optimize their long-life performance. It has been previously observed that the hydrogen partial pressure inside a sealed pressurized gas-metal storage cell at first increases linearly during charging, but that later some of the hydrogen combines with oxygen generated at the positive electrode as the cell nears its full charge. This observation has been used in controlling the charging of such cells by monitoring the value of pressure within the sealed cell. Unfortunately, the pressure corresponding with a particular degree of cell charging varies with age of the cell, temperature, and charging rate, as well as with the amount of electrical energy stored in the cell. Since spacecraft may be in orbit for a number of years with the storage cells constantly undergoing charge-discharge cycles, the value of gas pressure within the cell does not provide a fully reliable measure that can be used to control the charging of the cell.
There therefore exists a need for an improved approach to controlling the charging of pressurized gasmetal storage cells, which does not depend upon the value of the pressure of the gas within the sealed cell container. Such an approach should be dependable and reliable through many thousands of charge-discharge cycles. It should be compatible with existing cell designs, since the cells must be optimized for their electrical performance and weight, rather than control procedures, and should not add excessive weight or complexity to the storage cell. Desirably, the control procedure would allow the cells to be charged and discharged autonomously, without any outside contact such as intervention by a ground controller. The present invention fulfills this need, and further provides related advantages.