Battery Management Systems Battery Cell Diagnostics Directly in the Electric Vehicle—Laboratory Quality 

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Wuxi InfiMotion Technology Co., Ltd.

Until now, detailed insights into the condition of battery cells, including temperature gradients, ageing effects and internal short circuits, were only available in test laboratories and could not be measured directly in the vehicle. However, with the further development of battery management systems, it is becoming increasingly possible to implement these diagnostic capabilities in the vehicle architecture instead of just estimating them based on models, as was previously the case.

By integrating electrochemical impedance spectroscopy (EIS) into battery management systems (BMS), battery cells in electric vehicles can be analyzed by measurement using the same technology as during production and formation. This means that additional sensors no longer need to be used, nor is ongoing operation impaired.

The consistency of battery cell diagnostics from the laboratory, through production and into the vehicle, enables faster charging with integrated protection mechanisms, increases safety in daily operation and contributes to a longer battery life. At the same time, the technology is optimized from the outset for an integration and cost profile that meets the requirements of series production.

Take Targeted Measurements

While conventional methods derive the battery condition from estimates of the internal resistance, open-circuit voltage rest phases and temperature data from surface thermistors, EIS specifically excites the battery with an alternating current signal and measures the resulting feedback at selected frequencies. This provides information on higher-frequency physical effects that conventional systems are unable to obtain.

These measurements provide a characteristic signature of the electrochemical processes within the battery cell, from basic resistance effects to the mobility of the ions. Even the analysis of a limited frequency spectrum provides fast and reliable signals that support the determination of internal resistance and battery condition as well as safety-relevant analyses. EIS therefore goes beyond conventional methods, which are often unable to clearly separate different effects from one another.

In vehicle applications, a current-controlled excitation method is particularly suitable because modern lithium-ion cells have a low impedance and the voltage responses on the cells can be measured extremely accurately and transformed directly into the frequency range. Instead of collecting hundreds of measurement points over long periods of time, the BMS can focus on a few specifically selected frequency points to enable fast and accurate decisions without exceeding the computational capacity of modern embedded microprocessors.

More Safety During Fast Charging Thanks to Early Insights Into Cell Status

In fast charging, the risk lies not only in high currents, but above all in how and where these currents act within the cell. The EIS analysis makes it possible to detect early signs of lithium plating, where metallic lithium is deposited on the anode due to excessive current densities or an electrode surface that is too cold. On this basis, the BMS can adjust the charging process in a targeted and controlled manner before harmful deposits occur. In addition, characteristic patterns can be identified that indicate incipient internal defects or problems at cell connections. These cannot usually be detected by conventional resistance measurements. The result is robust fast charging with significantly fewer unexpected events.

The recorded EIS signals are also suitable for precise tracking of the internal cell temperature. This gives the BMS an insight into the heating and temperature distribution within the individual cell and the entire battery pack. Higher-frequency measurements also make it possible to measure areas of the cell that can only be detected to a limited extent or with a long time delay using surface thermistors. Under defined boundary conditions, the internal temperature distribution can be estimated and the occurrence of critical temperature gradients due to excessive cooling can be monitored. This enables fast and safe charging throughout the year at different ambient temperatures and across different application profiles.

Making EIS Ready for Series Production

Battery packs in vehicles present an environment with electrical and electronic interferences where wiring, connectors and PCB layout can influence measurement results. Nevertheless, robust and reliable measurement signals can be achieved through proven engineering practices such as precise calibration, thoughtful placement of measurement equipment and synchronized sampling. In the context of EIS, exact synchronization between current excitation and voltage measurement is particularly crucial. Cabling influences must also be taken into account and measurement results must be validated using defined reference values. This is where NXP's hardware-based synchronization comes in, making it possible to couple excitation and measurements precisely and stably down to the nanosecond range.

Energy efficiency also plays a key role. In practice, EIS implementations use existing power electronics paths so that the low AC overlay required for measurement does not significantly affect energy consumption or range. Systems with a well thought-out design allow measurements to be taken during the charging process and - if appropriate - also while the vehicle is parked.

An EIS-capable battery management system combines three key advantages. Firstly, it increases safety during fast charging by detecting early signs of lithium plating and allowing the charging current to be adjusted before irreversible damage occurs. Secondly, it provides a better understanding of thermal conditions by estimating internal temperatures and identifying critical temperature gradients. This would not be possible with conventional external sensors. Thirdly, early fault detection is possible, from abnormalities at cell connections to small internal short circuits. This allows countermeasures to be taken before they develop into major damage.

In combination with a secure, cloud-enabled data connection, fleet operators and charging infrastructure providers receive reliable information on battery status. This forms a sound basis for warranty coverage, residual value models and the planning of second use scenarios.

The Future of Battery Management Is Becoming Smarter

The use of EIS opens up new possibilities for more intelligent battery management. This is because the technology enables more intelligent thermal regulation, adaptive fast-charging limits, early safety measures and the detection of cell deviations and degradation trends.

As the technology becomes more widespread, EIS is evolving from a specialized tool to a firmly integrated component of modern BMS. This makes it possible to charge faster, operate batteries more safely and meet regulatory requirements based on reliable measurement data rather than estimates. (se)

*Wenzel Prochazka is Senior Product Manager BMS System Innovation at NXP Semiconductors.