<p>Designing a robust estimator to estimate the charge level of lithium-ion batteries has been one of the important challenges for researchers because the disturbances and uncertainties of the model affect the estimation accuracy and should be considered during the design of the estimator. On the other hand, the estimation convergence time is also one of the important parameters in some sensitive applications such as electric vehicles. This paper addresses these problems and presents a robust unknown input observe (UIO) estimator to estimate the charge level of Li batteries. During the estimator’s design, the disturbances and uncertainties of the model are considered to show robust performance. On the other hand, during the design of this estimator, the designer is able to predetermine the convergence time of the estimation. In other words, the designer determines the time of the state variables convergence to a certain range. To confirm the performance of the suggested approach, practical tests are done in two different scenarios. The results of these tests show that the method in question has greater accuracy and speed of convergence than another robust method such as the conventional sliding method. The results indicate that even at elevated frequencies, the proposed approach demonstrates 2% greater accuracy and converges 2&#xa0;s faster than the traditional sliding method. Additionally, at lower frequencies, it offers an accuracy improvement of up to 3% over the conventional SMO.</p>