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Lithium ion technology is state of the art for actual hybrid and electrical vehicles. It is well known that lithium ion performance and safety characteristics strongly depend on temperature. Thus, reliable temperature measurement and control concepts for lithium ion cells are mandatory for applications in electrical cars. Temperature sensors for all individual cells increase the battery complexity and cost of a battery management system. Normally, temperature is measured on module level in current battery packs, without observation of the individual cell temperature. Sensorless cell impedance-based temperature measurement concepts have been published and are validated in laboratory studies. Dedicated test equipment is usually applied, which is not useful for automotive series application. This work describes a practical approach to enable impedance-based sensorless internal temperature measurement for all individual cells using state-of-the art battery management system components.

Fast charging capability is one of the key requirements for the success of electric vehicles. Considering the growing energy storage capacity of automotive batteries, fast charging can only be achieved using high-power charging systems. This leads to increased power dissipation inside the battery cells. The resulting heat generation inside the battery cell is a critical effect, as cell safety, performance and life time strongly depend on cell temperature and current. This must be considered by a simultaneous current and thermal battery management strategy, which requires reliable information about the individual cell temperature. Sensorless cell temperature can be derived from the cell impedance, where the charging current profile is superimposed by an excitation current and the resulting cell voltages are observed by the battery management system (BMS). An efficient algorithm for the impedance and temperature calculation can be implemented in actual BMS.

Recent advances in energy density of Li-ion cells together with high-current fast charging ask for improved strategies for onboard safety and reliability observation of the cells. Potential degradation effects are stimulated by lithium plating and dendrite growth. The latter may ultimately cause an internal short circuit of the cell and can lead to serious damage. Increased self-discharge is an early indicator for safety-critical cell conditions. In this work, accelerated methods for self-discharge determination of Li-ion cells are presented. They are based on the analysis of cell voltage gradients during idle periods and can be applied in state-of-the-art battery management systems (BMS) performing low-drift measurement. However, transition into the idle state after driving requires a settling time of several hours before the voltage gradient can be extracted.

The performance of electrical vehicles strongly depends on characteristics of its energy storage system. A typical lithium-ion battery system is supervised by a battery management system to optimize operation and ensure safety over its whole lifecycle. Advanced battery management systems apply sophisticated fast charging procedures and active cell balancing. In future, impedance spectroscopy based on driving current stimulation for online estimation of the energy storage’s state of health can be expected. For efficient development and testing of such battery management systems it is impractical to use real lithium-ion cells in arbitrary condition of state of charge, temperature and state of health. Consequently, hardware in the loop cell emulators are state of the art. Most of them are limited to low frequency operation. In this paper, a novel modular wide bandwidth high performance lithium-ion cell emulator is introduced.

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 avoid the risk of thermal runaway. Battery temperature monitoring is already implemented in electric vehicles, however only cell surface temperature can be measured at reasonable cost using conventional sensors. The internal cell temperature may exceed the surface temperature significantly at high current due to the finite internal electrical and thermal cell resistance. In this work, a novel approach for internal cell temperature measurement is proposed applying on board impedance spectroscopy. The method considers the temperature coefficient of the complex internal cell impedance. It can be observed by current and voltage measurements as usually performed by standard battery management systems.

Battery aging is a main concern within hybrid and electrical cars. Determining the current state-of-health (SOH) of the battery on board of a vehicle is still a challenging task. Electrochemical Impedance Spectroscopy (EIS) is an established laboratory method for the characterization of electrochemical energy storages such as Lithium-Ion (Li-Ion) cells. EIS provides a lot of information about electrochemical processes and their change due to aging. Therefore it can be used to estimate the current SOH of a cell. Standard EIS methods require the excitation of the cell with a certain waveform for obtaining the impedance spectrum. This waveform can be a series of monofrequent sinusoidal signals or a time-domain current pulse with a dedicated Fourier spectrum. However, any form of dedicated perturbation is not generally applicable on board of an electric vehicle. This work presents a new passive spectroscopy method, which obtains the impedance spectrum directly out of real driving data.