We present a Battery and Motor Circuit Model (BMCM) for electric vehicles, followed by a systematic and comprehensive interval observer design aimed at estimating all the states of the BMCM, including internal capacitor voltages. Traditional designs have often depended on transformations that impose certain limitations when converting the model into a Metzler form. We demonstrate that these can be effectively replaced by a linear time-invariant transformation, which can be easily precomputed offline. Our method, grounded in the Kazantzis-Kravaris/Luenberger (KKL) paradigm, offers a simplified solution requiring only a single constant transformation matrix.