The use of electric scooters is being advocated by the government of Taiwan. An accurate indication of the state-of-charge (SoC) of the battery (lead-acid) is essential to let the driver of the electric scooter know exactly the residual amount what amount of usable capacity. This will result in greater confidence in using the electric scooter, and prevents the driver from becoming stranded with an inoperative machine. The basic idea of this study is to derive a mathematical model that describes the battery voltage as a function of discharge current and depth-of-discharge (DOD). The parameters of this model are identified in real time. Then, the SoC is estimated by means of the Newton-Raphson (N-R) method. The proposed algorithm enables the estimated SoC to be adjusted for changes in the behaviour of the battery. The effectiveness of the proposed method is verified using field-test data obtained from driving an electric scooter on various routes. The results show that the adopted method improves the accuracy in estimating the battery of SoC. (c) 2005 Elsevier B.V. All rights reserved.