The effect of cell design and storage conditions on the cycle life performance silver-zinc cells

The Mars Pathfinder spacecraft is expected to utilize a single eighteen cell, forty ampere-hour, silver-zinc battery. The calendar life clock for the battery begins with the addition of KOH electrolyte to the constituent cells by the vendor. A period of battery inactivity begins with shipment of the battery to JPL and then on to Florida. This inactivity continues while the battery is inverted for four months prior to launch. Following launch, the battery will be inactive for 7 months except for several shallow discharges associated with trajectory correction maneuvers. The battery is expected to deliver forty ampere-hours during entry, descent and landing at Mars. Thirty shallow depth of discharge cycles are planned during Mars surface operations. A study was undertaken to examine the impact of various cell design and storage conditions on the cycle life performance of silver-zinc cells. Eight 4 cell groups were assembled and tested. Variations in the number of separator layers, operating temperature and state of charge were examined. As expected, a greater number of separator layers, lower states of charge and lower temperatures tend to yield better cycle life performance following eleven months of inactivity. This study was undertaken with “off the shelf” cells purchased from Eagle Picher Inc., of Joplin, Missouri