Hydrogen gassing and the potential for cell rupture in aerospace nickel cadmium cells is directly related to loss of overcharge protection built into the cell during manufacturing. It is well known that cells having nylon separators contribute to this loss via a hydrolysis reaction of the nylon in the potassium hydroxide electrolyte environment in the cell. The hydrolysis reaction produces lower chain organics which are oxidized by the positive electrode and oxygen. Oxidation of the organics diminishes the overcharge protection. With introduction of the Super NiCdT" and the Magnum" nickel cadmium cells the nylon hydrolysis reaction is eliminated, but any reducing agent in the cell such as nickel or an organic additive can contribute to loss of overcharge protection. The present effort describes analyses made to evaluate the extent of overcharge protection loss in cells which do not have nylon hydrolysis and quantifies the diminished amount of overcharge protection loss as a result of eliminating nylon from aerospace cells.Overcharge Loss in Cells with Nylon Separator The loss of overcharge protection in nickel cadmium cells with nonwoven nylon separator materials has been thoroughly elucidated by Lim, et al., (1). In the accepted scheme of overcharge protection loss, nylon separator, usually in the form of 2505 ML (Pellon Division of Freudenberg, Inc.), is slowly hydrolyzed to adipic acid and other lower molecular weight products. The products formed from the hydrolysis are readily oxidized by the nickel oxide electrode or by oxygen evolved from the positive electrode on overcharge. This oxidation reaction upsets the balance of the cell, as originally established in the manufacturing process, by simultaneously reducing the cadmium hydroxide overcharge protection to elemental cadmium (cf., Figure 1). Additionally, carbonate and water are also formed increasing the electrolyte volume, raising the intemal resistance of the cell and impeding the oxygen recombination rate. Only a small percentage of the available nylon separator needs to be hydrolyzed in order to completely deplete the overcharge protection. The hydrolysis reaction follows a simple Arrhenius relationship with an activation energy of about 17 kcal/mole suggesting that the degradation process is chemical not physical and establishing the hydrolysis rate as a function of temperature. At 10" C. the hydrolysis of the nylon to lower molecular weight products, such as adipic acid, is rate determining step.The operational ramifications of overcharge protection loss are several fold. In a geosynchronous equatorial orbit (GEO) or medium altitude orbit (MAO) regimes where trickle charge storage at continuous rates between C/lOO to C/50 are used, the on-charge voltage must be continuously monitored and charge terminated before the hydrogen gassing potential occurs. Altematively, the battery must be left open circuit which will cause a more rapid degradation of the battery life and possibly limit the mission life (2). The above storage regimes, in addition to bein...
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