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This paper describes nonideal properties of batteries and how these properties may impact power-performance trade offs in wearable computing. The first part of the paper details the characteristics of an ideal battery and how these characteristics are used in sizing batteries and estimating discharge times. Typical nonideal characteristics and the regions of operation where they occur are described. The paper then presents results from a first-principles, variable-load battery model, showing likely areas for exploiting battery behavior in mobile computing. The major result is that, when battery behavior is nonideal, lowering the average power or the energy per operation may not increase the amount of computation that can be completed in a battery life.