Cold charge nickel hydrogen geosynchronous satellite batteries

It has been well recognized that the capacity of nickel hydrogen batteries is improved if the batteries are operated at relatively cool temperatures (~0°C to 10°C). This is somewhat contra-intuitive relative to other batteries and is generally ascribed to the competition between the useful nickel electrode recharge reaction. At Space Systems/Loral we have recognized that the inverse capacity vs. temperature dependency of the nickel hydrogen battery is a result of different kinetic parameters. This in turn leads to the realization that in the absence of other effects the charge capacity of the nickel hydroxide electrode should continue to increase as the temperature in decreased. The narrow conventional ideal temperature range for operation is due to limitations of the discharge kinetics rather than the charge acceptance of the positive electrode. The cold charge concept exploits the difference in charge vs. discharge kinetics and the fixed rates and periods of geosynchronous operation to increase the name plate capacity of IPV Ni/H₂ by forcing discharge to occur at a higher temperature than charge. In practice battery recharge is completed (~90%→100% SOC) under taper charge conditions (C/10→C/100) with a battery temperature ~-20°C. Prior to high rate discharge the temperature of the battery is artificially increased (via the battery heaters) to -10°C to 0°C. The net result is a 15% increase in practical battery capacity and hence reduction in battery weight