Capacity estimation of nickel metal hydride backup batteries

A new capacity estimation method for nickel metal hydride (Ni/MH) backup batteries is described. It estimates the capacity of degraded batteries from the voltage drop seen after charging at a constant current. The voltage drop increases with the battery degradation caused by self-discharge at high temperature. A parabolic relationship is discovered between capacity C and the voltage drop /spl Delta/V after charging. The degradation temperature dependence of the parabolic function C = f(/spl Delta/V) is very small and the same function can be used to estimate the capacity of Ni/MH batteries degraded within a wide temperature range.