Calculation of the thermoneutral potential of NiCd and NiH2 cells

The thermoneutral potential of a nickel cadmium or nickel hydrogen cell is the potential at which the cell charge or discharge process puts out zero heat, and thus is the potential corresponding to the enthalpy change of the charge/discharge reaction, delta H. A relatively straightforward method for obtaining the thermoneutral potential E(sub tn), is based on the measured potential and temperature derivative of the cell reactions, which are related to the free energy change delta G, and entropy change delta S, respectively. Particularly in the nickel hydrogen cell, the pressure of hydrogen can often vary over an order of magnitude or more during the course of a charge or discharge. In a nickel cadmium cell, although significant changes in oxygen pressure can occur during charge or discharge, since oxygen does not enter into the charge/discharge reaction, these pressure changes are related to the heat generated from oxygen evolution and recombination. However, the entropy changes due to changes in hydrogen pressure relative to the 1 atm standard state must be included to apply this method to the nickel hydrogen cell.