The self-discharge rate of aerospace flight-quality nickel-hydrogen batteries is limited by hydrogen diffusion within the nickel electrode active material. A diffusion-limited reaction model is developed which accounts for the observed self-discharge behavior. Effective hydrogen diffusion coefficients were calculated from the open-circuit self-discharge data on Hubble space telescope flight-quantified nickel-hydrogen cells and ranged from 1.12×10−10 cm2/sec at 25°C to 7.45×10−12 cm2/sec at 0°C.