In a proposed method of controlling the electric current supplied for charging a nickel/hydrogen battery, the rate of evolution of heat would be taken into account along with the electrical quantities (voltage, current, and/or charge as functions of time) that are traditionally taken into account. This method might also prove useful for controlling charging currents in batteries based on different chemistries.

The parameters related to limitations on the lifetime of an Ni/H battery are depth of discharge, temperature, and overcharge. The major proximate cause of failure is gradual swelling of plates, associated with overcharge and with the generation of oxygen in the overcharge reaction sustained over thousands of charge/discharge cycles. In principle, the lifetime could be prolonged by minimizing overheating and precisely limiting the amount of overcharge.

The rate of heating cannot be measured directly but can be estimated from other quantities that can be measured. Because an Ni/H cell is a pressure vessel, its internal pressure can be used as an indication of the amount of hydrogen present and thus of the state of charge. Assuming that (1) the void volume in the battery remains constant; (2) the pressure, temperature, and number of moles of hydrogen are related by the ideal gas law; and (3) the charging efficiency is 100 percent, then the rate of increase of pressure during charging should be proportional to the rate of charging; that is, to the charging current.

Any deviation from this proportionality would be attributed to a decrease in efficiency associated with the overcharge reactions. The portion of applied current diverted to the overcharge reaction would not be available for the main charging reaction that generates hydrogen; as a consequence, for a given charging current and temperature, the rate of increase of pressure would decrease as the battery went into overcharge. The oxygen generated in part of the overcharge reaction is also quickly recombined in another part of the overcharge reaction; the net effect of the portion of the charging current diverted to the overcharge reaction is to generate heat. Thus, by use of basic equations of thermodynamics, it should be possible to determine the charging efficiency and the rate of generation of heat from measured values of pressure, temperature, and voltage as functions of time.

The proposed method would be implemented in control software. In this method, the rate of generation of heat computed as described above would be used as feedback in a control algorithm that would taper the charging current to maintain the lowest possible battery temperature and minimize the generation of oxygen. The maximum allowable rate of generation of heat would have to depend on the temperature of the battery because charging efficiency decreases with increasing temperature.

This work was done by Paul Timmerman of Caltech for NASA's Jet Propulsion Laboratory. NPO-20470


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Improved control of charging current for Ni/H battery

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This article first appeared in the October, 1999 issue of Electronics Tech Briefs Magazine.

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