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Drive systems of battery-powered vehicles may, under urban operating conditions, dissipate as losses up to 30% of the energy supplied by the battery. Hence, battery mass may be decreased by increasing the energy efficiency of the drive system, even though this may only be done at the expense of increasing the mass of the drive system. The combined mass of the battery and drive system may thus be minimised for a given operating duty and range. The theory which is presented allows the calculation of an optimum drive system design sensitivity of allowable mass increase per 1% efficiency increase. This sensitivity depends on the battery mass and operating duty of the vehicle and for a typical lead-acid battery-powered urban delivery van its value may be as much as 25 kg/1%, whereas for batteries with a lower mass the figure decreases in proportion to the battery mass. It also follows that the traction motors of drive systems which are optimised according to this principle, should be selected according to their efficiency characteristics over the required operating duty cycle, rather than their thermal capability alone.