Peter Keil

Hybrid Energy Storage Systems for Electric Vehicles: An Experimental Analysis of Performance Improvements at Subzero Temperatures

Electric vehicles based on high-energy lithium-ion batteries often exhibit a substantial loss in performance at subzero temperatures: Due to slower electrochemical kinetics, the internal resistances of the batteries rise and diminish available power and capacity. Hybrid energy storage systems (HESSs) can be used to overcome these weaknesses. In this paper, the performance of two HESSs, combining a high-energy lithium-ion battery with either a high-power lithium-ion battery or a lithium-ion capacitor, has been investigated experimentally for a driving scenario at various temperatures. Both configurations enable driving at -20 °C, which was not possible without hybridization. The HESS using the high-power lithium-ion battery provides a substantially higher driving range due to its higher energy density. An analysis of different operating strategies has helped to maximize the driving range: Discharging the high-energy battery with a constant current and keeping the high-power cell at a higher state of charge (SoC) extend the driving duration, as the requested driving power can still be provided at a lower SoC of the high-energy battery. In addition to the HESSs, two energy storage systems without hybridization, consisting of different generations of high-energy lithium-ion cells, have been examined to disclose improvements in battery technology. These improvements narrow the benefits of HESSs, as the high-energy batteries have become less reliant on the support of an additional high-power device. Although HESSs lose importance for current lithium-ion battery systems, they can be a valuable option for next-generation lithium batteries, which are expected to provide higher energy densities but exhibit reduced rate capability.

Improving the Low-Temperature Performance of Electric Vehicles by Hybrid Energy Storage Systems

Electric vehicles based on high-energy Li-ion batteries often show a substantial loss in performance at cold temperatures: Due to slower electrochemical kinetics, internal resistances of the battery rise and available power and capacity diminish. In order to overcome these weaknesses, a selection of hybrid energy storage systems (HESS) is investigated here: Different hybrid systems combine a high-energy Li-ion battery with either a double-layer capacitor or a Li-ion capacitor or a high-power Li-ion battery. For these three types of HESS, experimental studies performed at various temperatures reveal available energy under realistic driving conditions. At temperatures of 0 oC and below, an increased driving range can be achieved with two of the three HESS combinations. Depending on the available space for the energy storage system, either the HESS utilizing a Li-ion capacitor or the HESS utilizing a high-power Li-ion battery is found to be the most promising solution for electric vehicle applications.

Thermal impedance spectroscopy for Li-ion batteries with an IR temperature sensor system

Thermal impedance spectroscopy (TIS) is a non-destructive method for characterizing thermal properties of entire battery cells. Heat capacity, thermal conductivity and heat exchange with environment are determined by an evaluation of the heat transfer behavior of the battery. TIS measurements are usually conducted with contact-based temperature sensors, such as thermocouples or thermistors, which show drawbacks at higher convection rates and higher temperature differences between battery and environment. To elude drawbacks in these kinds of sensors, an infrared-based temperature sensor system for battery surface temperature measurements is implemented. TIS measurements are conducted with this sensor system and with conventional, contact-based temperature sensors. Accuracy and reliability of thermal parameter identification is analyzed for the different sensor systems. Moreover, thermal parameters are identified for different cylindrical 18650 Li-ion cells with capacities between 1.1 Ah and 2.7 Ah. The comparison of different types of temperature sensors shows that contact-based sensors underestimate surface temperatures even at low temperature differences to environment. This causes an error in thermal parameter identification. The TIS measurements performed with contact-based sensors show divergence of 20-60 % for heat capacity, 30-70 % for thermal conductivity and 20-60 % for convective heat exchange with environment. With our IR temperature sensor system, parameter identification is performed for different batteries. Resulting values for specific heat capacity are in a range between 900 and 1020 J/kgK and thermal conductivities in radial direction lies between 3.1 and 3.6 W/mK. Our investigations show that IR-based temperature sensors are an effective progression for TIS measurements and improve quality of parameter identification at low cost. Moreover, discrepancies mentioned in TIS literature can be explained by our findings.