Qualification and Verification of High-Power Battery Systems for Traction Application


Prof. Dr.-Ing. Johannes Teigelkötter and Dipl.-Ing. Klaus Lang at HBM Test and Measurement
Johannes Büdel, M.Eng. at University of Applied Sciences Aschaffenburg

Battery storages for use in mobile traction applications must meet high requirements. Criteria such as energy storage capacity and size, characterized by the energy or power density, as well as the implemented safety concepts, serve as a basis for the first valuation of the storage systems and, hence, as a selection criteria for the planned application.

Introduction

Therefore, already during the development process, qualified testing of the necessary, application-specific requirements is inevitable. To achieve meaningful results, there is a compelling need to execute all these test procedures as realistically as possible and in an application-oriented manner. Only then can statements be made about the behaviour of the battery system in the later-intended application.

High-Power Traction Battery

The following considerations only refer to the lithium-ion battery, because this technology has the potential to fulfil the demands of energy and power density today for its application in electric vehicles [1]. As figure 1 reveals, a battery system consists of various complex components.

Figure 1: Components and Typical Setup of a Traction Battery

Besides the interconnection of the single cells, a battery system consists of further mechanical and electrical components which permit the operation of the system. These components have to be closely coordinated with each other and with the intended application. The complexity of individual components leads to a complex interaction, and therefore, to a highly sophisticated battery system. Thereby exists the absolute necessity for a realistic verification of the interaction for the intended application. Altogether, which tests are performed at each step is a different matter and depends on the specifics of the process and the device as well as the intended application. Only after conducting this qualification and verification process, a battery system is enabled for use in the application. 

High-Performance Test Bench

In accordance with the high and individual requirements of high-power traction batteries, a corresponding test bench has to fulfil the high requirements for the test procedures. The main focus of a test bench is to provide the most flexible test execution and a high power range of the future batteries. The execution of the test procedures has to be as realistic and application-oriented as possible. Considering the requirements for the process of testing a high-power traction battery, a unique high-performance test bench for these systems is presented in this section.

The power electronic interface of the test bench consists of two two-level converters, connected via the shared dc-link. The three half-bridges of the output dc-dc converter are controlling the battery current, while the grid side converter controls the dc-link voltage and the grid current. With this configuration, the test bench allows a bidirectional power flow, with a maximum operating power of 250 kVA, a output voltage range of 0-750 V and a maximum output current up to ± 800 A. This ensures high-power and energy-efficient experiments. Figure 2 presents the principle wiring diagram of the output power connections.

Figure 2: Output Wiring Diagram and Measured System Quantities

To ensure accurate investigation results and to not strain the DUT unnecessarily, it has to be charged with a low ripple current. Thus, it is guaranteed that the DUT is solely charged with the predefined and standardized load profiles, so the reaction of the DUT is obviously attributable to them. Therefore, the three output half-bridges are built to a multiphase interleaved current-sharing converter. As a result of the approach to switch the three phases interleaved, for a phase shift of exactly 120 °, the inductor ripple currents tend to cancel each other, resulting in a smaller ripple current [2]. Under certain conditions, it is possible to eliminate the ripple current at the output node. The phase relationship in Figure 3 shows how ripple current cancellation works. Furthermore, to minimize the ripple current for all operating points, an individual filter network is developed. As a result, the output ripple current over the entire voltage range of the test bench is minimized below 1 A.

Along with the low ripple current and the high power range, the test bench offers the possibility to execute realistic and application-oriented test procedures. The test bench can flexibly adapt to the specified test conditions and battery systems. Also, it offers the possibility to execute common test procedures like determining the capacity or cycle tests, as well as more complex investigations like the determination of the internal DC and AC resistance. Due to the high degree of flexibility, the test bench is also suitable for other DC test applications.

Instrumentation

Another important aspect of setting up a test environment for the qualification and verification process of traction batteries is the selection and optimisation of appropriate measurement equipment. This is necessary to achieve reliable results, based on which, precise statements about the behaviour of the battery system in the later-intended application can be made. So the reaction of the DUT during a test procedure has to be recorded in a highly-precise and dynamic manner. For understanding and verifying the battery behaviour under different conditions, the discrete signals have to be synchronized. According to the different signal types and ranges, a suitable data acquisition system has to flexibly adapt to the specific conditions and signal ranges. Just as important as the adaptation to the different signal levels, is to ensure that the huge number of different signals can be recorded simultaneously.

The HBM-GEN3i is especially suited for this high requirement of data acquisition and transient recording. The GEN3i data recorder enables synchronous acquisition of all important quantities in energy-related systems with a large number of channels and high sampling rates [3]. With this data recorder, the commissioning of the test bench as well as the data acquisition during test procedures can be accomplished. Post process, the data can be analysed and further processed. Figure 2 shows the measurement acquisition of system quantities which are sent to the GEN3i. 

Exemplary Measurements

Methods for Ripple Current Reduction

The measurement in Figure 8 shows the principle of the interleaved switching method and the effects of the low-pass filter on the ripple current. Due to the ripple current cancellation effect for interleaved switching the ripple currents tend to eliminate each other. This results in a reduced current ripple in the node point. Furthermore, the filter network is final damping this ripple current below 1 A.

Figure 3: Interleaved Waveforms, Measured with GEN3i, D=0.85

Determination of the Internal DC Resistance

Besides the battery capacity, internal resistance is also one of the major battery parameters. The lower the resistance, the lesser the restriction the battery encounters in delivering the needed power spikes. Figure 4 presents a technique to measure the DC resistance of a battery. This method is based on the voltage change during a current pulse. Ideally, the current jumps from a small value (e.g. 0.1 C-Rate) to a high value (e.g. 2 C-Rate). After a defined duration, the voltage drop is measured. Following this, the voltage change is divided by the current change. The result is the internal resistance of the DUT. [4]

Figure 4: DC-Resistance Test Method, Measured with GEN3i

 

[1]
International Electrotechnical Commission (IEC): Electrical Energy Storage, White Paper, 12/2011.
[2]
Linear Technology Corporation: High Efficiency, High Density, PolyPhase Converters for High Current Applications. http://cds.linear.com/docs/en/application-note/an77f.pdf., 1999
[3]
Eberlein; Lang; Teigelkötter; Kowalski: Electromobility in the fast lane: increased efficiency for the drive of the future. Proceedings of the 3rd conference of Innovation in Measurement Technology, 14.5.2013
[4]
Jossen; Weydanz: Moderne Akkumulatoren richtig einsetzen. Untermeitingen: Inge-Reichardt-Verlag, 2006. ISBN: 3-939359-11-4