Despite the battery cells itself, the electronics, especially sensors make about 10 % of the system costs and volume in automotive applications. As the cell price and volume is decreasing, the amount of cells will increase within a Battery Electric Vehicle (BEV). Accordingly the number of monitored cells increases as there is often a 100 % monitoring rate in regard to li-ion battery systems (BS) due to safety reasons. This means that each cell is employed with a voltage sensor and each stack with a current sensor, or if a balancing system is employed, there might be an additional current sensor for each cell. With a stack as big as 12 cells, this means 13 or even 24 (if a balancing system is employed) sensors per stack. The number of sensors increases when taking temperature sensors into account. The newly developed sensor minimal battery observer allows decreasing this number of voltage and current sensors by 90 % down to two sensors per stack by keeping up the 100 % monitoring rate. This means that the presented system affords only two sensors to monitor 12 battery cells individually. This remarkable decrease of the number of sensors leads to a commensurate decrease of costs, weight and volume. Despite the reduction of sensors, it means a decrease of cables and assembling as well as construction costs. In addition to the sensor minimal observer system the system includes a cell balancing opportunity, which makes additional systems abdicable. This balancing system ensures an equivalent energy level in all associated cells to allow maximal energy output and extended service live. The service live can be extended to five times the service live of an unbalanced system for automotive application (end of life capacity 80 %). Concomitant the traction range is increased well after a few recharges due to the application of balancing systems.
Autor: Philip Dost
Co-Autor: Constantinos Sourkounis
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– MOTIVATION-
Individual batteries have their own operational temperature ranges, which shall be respected to avoid both damaging of the cells and shortening of the cycle life. In terms of the Li-Ion cells, many of them do not function well above 60 °C. Therefore, a better understanding of the thermal behavior of the batteries has its significance during designing safe and robust battery packages for automotive applications.
-OBJECTIVE-
This study dedicates to analyze the thermal behavior of a 48 V high power battery module for automobile applications and seeks smart solutions for cooling purposes. In order to suppress self-discharge and control the capacity retention of the cells, it’s one of the primary goals to maintain the temperature of all cells not only below the maximal operational temperature, but also below app. 40 °C. The other objective of this study is to minimize the differences in cell temperature aiming at minimizing the differences of the cycle life of cells within the same battery module.
-APPROACH-
Simulative thermal analysis is employed in this study to gain knowledge of the heating of cells during operational conditions and study the cooling effect of different cooling principles. The study is carried out with following steps:
[1] Construction of the battery module in software environment of COMSOL Multiphysics. The construction of the cells and definition of the load profile are derived from the technical data of a suitable candidate for automotive applications. [2] Thermal analysis of the ground model, in which no cooling system is involved. [3] Employment and comparison of different internal cooling fin (ICF) concepts. [4] Employment of external water cooling systems on top of the ground model with one effective ICF concept. [5] Utilization of ICFs and external water cooling systems in a large and densely arranged 48 V battery module. [6] Combination of different cooling systems – ICF and external cooling systems (liquid and air) – to seek for smart solutions.-RESULT-
[1] The temperature distribution in the ground model is greatly uneven, which will lead to differences in cell cycle life within the same battery module in the long term and hence a shortened cycle life of the entire module. [2] By involving ICFs, the temperature of the cells stabilizes earlier in comparison to the ground model. [3] By employing one developed ICF concept, the ∆T between the hottest and coldest cell is successfully maintained below 3 K. The temperature of the hottest cell dropped to app. 40 °C at the stable state. [4] By involvement of external water cooling, the ∆T between the hottest and coldest cell is kept below 2 K and the temperature of the hottest cell dropped to below 37 °C at the stable stage. [5] The cooling effect of ICFs and external water cooling systems in the large and densely arranged 48 V battery module is not sufficient. [6] A smart solution – a combination of different cooling principles – is demonstrated in this study to maintain low operational temperatures for all cells and restrict ∆T between the hottest and coldest cell with in the module.-CONCLUSION-
Cooling systems for the battery module shall be considered as an indispensable component in high power battery systems for automotive applications. Combined cooling systems with different cooling principles shall be involved for large battery modules, in order to achieve a homogenous temperature distribution and ensure the function of all cells.
-ACKNOWLEDGEMENT-
The authors are thankful to the Ministry of Innovation, Science and Research of North Rhine-Westphalia for funding this study under the Project “ANFAHRT”.
Autor: M.Sc. Ziyi Wu
Co-Autor: Hans Kemper
The most widely used simulations of lithium-ion batteries are based on simple circuit diagrams und characteristics. Such models are only valid in specific ranges under specific operating conditions. For this reason typical challenges like the exact determination of state of charge (SOC) or state of health (SOH) cannot be simulated with the required accuracy. However, in order to be able to calculate realistic vehicle ranges for electric cars it is essential for ECUs to provide exact SOC calculations with respect to the current SOH. Otherwise calculations must be based on worst case assumptions in which the SOC shown is typically worse than the real SOC. The more exact the SOC values are, the more reliable is the possible range displayed for an electric car. Fraunhofer IWES has been working on the exact modeling of batteries for over twenty years. They have knowledge of almost any type of battery. In the last few years, Fraunhofer IWES has developed an electrochemical model for lithium-ion battery cells, the ISET-LIB. This model allows the design and parametrization of batteries and extremely realistic simulation of battery properties, enabling the user to simulate the internal behavior of the battery. All relevant quantities like voltage, temperature, influences of the aging of batteries, and so on can be simulated. A speed-optimized version of the model is available for fast HiL simulations. Because of the high level of details offered by the ISET-LIB, it was not possible to simulate the exact battery cells in the microsecond range. With over 20 years of experience in battery HiL-Simulators, MicroNova has developed a new generation of battery simulation cards that bring the resolution of a battery model down to step times in the range of a few microseconds. In order to achieve this, a number of innovations have been introduced. On the hardware side, all control of the cell simulation card has been made fully digital. In addition, new high-speed, high-precision hardware has been introduced to control cell voltages. This hardware functionality is combined with the Fraunhofer ISET-LIB in an innovative way. The ISET-LIB calculates the long-term behavior of battery cells very precisely every few milliseconds. Additionally, it delivers substitute parameters for the battery (e.g. R,L,C). These parameters are periodically updated in the battery simulation cards that model the short-time behavior. Thanks to this innovative approach, it is possible to achieve the simulation of a high-precision electrochemical battery model with a simulation step width at the microsecond level. Even applications like the simulation of RLC parameters for EIS (electrochemical impedance spectroscopy) are possible with this approach. This new and innovative combination of high-speed HiL and realistic battery simulation for the first time allows the development and testing of future battery functions like EIS and a realistic determination of vehicle ranges.
Autor: Matthias Puchta
Co-Autoren: Franz Dengler, MicroNova AG; Dr. rer. nat. Michael Schwalm, Fraunhofer IWES;