Battery safety is the most critical requirement for the energy storage systems in hybrid and electric vehicles. The allowable battery temperature is limited with respect to the battery chemistry in order to reduce the risk of thermal runaway. Battery temperature monitoring is mandatory, however only the cell surface temperature can be measured at reasonable cost using conventional temperature sensors. The internal cell temperature may exceed the surface temperature significantly at high current load due to the finite internal cell resistance. In this work, a novel approach to measure the internal cell temperature is proposed applying impedance spectroscopy. The method considers the temperature coefficient of the complex internal cell impedance. It can be observed by current and voltage measurements on the base of standard battery management systems. The observation frequency range considered for temperature measurements is optimized for high sensitivity and robust behavior and takes the influence of state of charge variant as well as aging effects into account. Transient temperature variations caused by various load profiles are conducted to characterize the static and dynamic thermal properties of the cell, resulting in a thermal equivalent circuit model that describes temperature changes inside the cell dependent on ambient temperature and load current. The temperature measurement approach and the thermal model are suitable for on board implementation in battery management systems. A dedicated excitation is not required, as signal components in the relevant frequency range are inherently present in typical load current profiles. Thereby, significant improvements in terms of on-board diagnostics and battery safety can be achieved without any significant hardware implementation effort.