The internal pressure variations of different Nickel-metal hydride batteries were studied. The relationship between the deflection of a battery shell and internal pressure can be determined by thin elastic shell theory, and also by experimental methods. A DNY-2 was specially designed to measure indirectly the pressure inside the battery shell without damage to any battery parts. The internal pressure variations were simulated by the Boltzmann function when the batteries were charged with a low current density. The parameters in the simulating function have definite physical meanings, and a special parameter k, which is the variability of internal pressure with the charge state or the charge time of the battery in the region of inflexion of this simulation function can be used to estimate the cycle life of batteries. The result shows that batteries with a smaller k value have a longer cycle life.