Simulation

Individual materials

The Fraunhofer Battery Alliance is working on a crash-proof design for battery modules to be used in electric vehicles. Because of the danger of fire, batteries must not be damaged during a crash. In addition, batteries for electric motors are very heavy, and the optimization of crash-resistant fixings and protective casings should be combined with weight reduction. Within the core topic of energy storage technology the Battery Alliance will evaluate the crash-resistance of protective battery casings.

The simulation of material properties from the electronic structure of the material to its macroscopic behavior is one of the core competences of the Fraunhofer Battery Alliance. The experimental approach provides unique insights into the functioning of the material. These are used to better understand the different processes that occur when a battery system is charged / discharged. Particular emphasis is placed on the intercalation / insertion of lithium from the electrolyte into the anode or cathode materials. Understanding of these processes is not only useful for "tailoring" batteries. Their aging-resistance is directly linked to the decomposition of the electrolyte at these surfaces.

Microstructural simulation and simulation of porous media are also core competences of the Fraunhofer Battery Alliance. In the context of the project "Fraunhofer System Research: Electromobility" (FSEM), and in collaboration with industrial partners, mathematical models based on fundamental physical and electrochemical processes are developed. These enable transport and reaction processes in a battery cell to be described three dimensionally on two different macroscopic levels. With the transport model the microscopic structure of the electrodes can be spatially resolved and the ion transport in the electrolyte and individual active particles can be calculated explicitly. On the other hand the porous electrode model, calculated on the basis of microscopic equations, enables the simulation of a complete battery cell in any dimension, as the three-dimensional structure of electrodes and current collectors can be taken into account (see diagram). These approaches enable a simulation-enhanced optimization of material composition and geometry. The models were implemented in the "BEST" software (Battery and Electrochemistry Simulation Tool) developed by Fraunhofer. With the help of a model reduction process, simulation can be accelerated, enabling a combination of the detailed modeling of the transport processes to the system level. By this means, for example, the precision of the battery management system can be increased.

The investigation and evaluation of aging effects is a further challenge in the development of new lithium-ion secondary batteries for electromobile and stationary applications. The Battery Alliance offers aging models in order to reduce the duration and cost of trials in this field.

Connected battery systems

In the field of electric storage systems, work is being carried out on optimized algorithms for battery monitoring (charge and aging measurement), the development of optimized charging strategies, and energy and battery management systems. This includes current projects on the development of Kalman filters to measure the state of charge and aging of lithium batteries. Large battery banks of 60 kWh or more can be constructed and operated for use in PV hybrid systems (network-independent electricity supplies which can generate different characteristics for supply to technical systems or residential areas) . The Fraunhofer Battery Alliance also has a comprehensive battery laboratory, in which battery tests for different industrial branches (stationary and grid-connected, PV island systems, automobile applications) can be carried out.

In order to charge a battery with a particular chemistry, a precise voltage and current profile is necessary, or part of the capacity may be lost and the battery could be damaged or destroyed. To create this charging profile, special charge controllers are used which can generate the necessary voltage and current from one DC source. Precise information about the state of charge and aging of batteries is often needed. In high-voltage applications such as motors in electric or hybrid vehicles, a series of several battery cells is needed for energy supply. Production tolerances, self-discharge and uneven temperature distribution in the battery system means that the state of charge of the individual cells can diverge with time. This leads to a loss of capacity in the entire system which must be compensated by so-called cell balancing.

The Fraunhofer Battery Alliance is developing highly-integrated charge controllers, from simple analogue controllers to intelligent microprocessor-driven digital controllers. These devices are optimized for use with smaller external components and also for batteries with different chemistries.

Monitoring systems, and systems for the passive and active cell balancing of multi-celled battery systems, are a particular focus. Algorithms are being developed which use mathematical modeling to accurately predict the remaining capacity of batteries. This enables the typical battery parameters SOC (state of charge), SOH (state of health) and SOF (state of function) to be determined. A further topic is microelectronic devices with minimum power loss for application in battery management systems.

The Battery Alliance develops the necessary subcomponents such as charge controllers and ADCs, and can also use commercial products to construct complete energy supply systems.
Standards such as SBS (Smart Battery Standard), SMBus (System Management Bus) and PMBus (Power Management Bus) are used to achieve compatibility with commercial systems.

Application fields for the developed battery management systems include electric and hybrid vehicles and mobile electronic devices.

We also test battery management systems and batteries with a professional battery testing system, which is also used for battery modeling.