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The key for decentralized battery systems is a robust and communication-less control strategy for autonomous power sharing of parallel-connected DC-DC converters. Battery systems improve the reliability and quality of power supply in renewable energy systems and enable power supply for off-grid, mobile applications, including islanded grids, home storage, and electric vehicles. In many cases, components with different electrical properties require different voltage levels. An adaptation is consequently essential and is normally implemented in DC grids for the batteries via bidirectional DC-DC converters. The power flow in both directions can thus be ensured. To achieve a power distribution in parallel connected DC-DC converters, a droop control in the form of a virtual internal resistor can be used. This paper presents a novel approach of a DC-DC converter with a digitally parameterizable droop resistor, whose voltage regulation is based on an analog operational amplifier circuit to ensure low delays and robustness. The droop resistor is adjusted with a microcontroller, which offers the possibility to apply a higher-level control for load sharing via an interface. Mathematical correlations are used to clearly define the parameters of the control. Furthermore, the circuit was completely simulated and tested in the hardware setup. The shown results verify the functionality and indicate only minor deviations. Therefore, this circuit is important for future use in distributed battery systems.

Battery systems are used in a wide range of safety-relevant applications, such as electric vehicles, unmanned aerial vehicles and home storage systems. Safety, reliability and availability of the battery system therefore play a key role. In addition, the useful service lifetime of the batteries determines the environmental impact and economic efficiency of the overall system. One possible solution is to give batteries a second life in applications with lower requirements in terms of dynamic behavior or capacity. Heterogeneous battery systems consist of batteries with differences in cell technology, age, capacity, and optimal operating range. To meet the safety, reliability, and availability requirements a scalable, Decentralized Battery Management System (DBMS) based on a distributed control system is proposed. Batteries, generators, and loads have Local Control Units (LCUs) consisting of a microcontroller, a measurement unit, and a DC/DC converter with adjustable voltage and current limits. These LCUs are the basis for the communication-based, cooperative system control and enhance the reliability and scalability of the battery system compared to conventional centralized structures. They record and manage the operating parameters and provide the basis for predictive energy management and battery residual value estimation. As a fallback strategy, a droop-based control of the DC/DC converters is used in addition to the communication-based one. Transition conditions between the control modes are defined and the control methods are compared and differentiated. The performance and the resulting benefits of batteries are determined by the control strategies. In this paper, the requirements for the control strategies for different operating modes, including startup, severe fluctuations of the DC power line voltage, and safe shutdown, are analyzed. Keywords:Renewable energy sources • battery management system • second life battery • decentralized control • distributed control • control strategies • droop control • battery optimal operation

DC-DC converters control the power flow and thus the power distribution between the components on different voltage levels. They are essential for (dis)charging batteries and influence the safety and stability of the entire battery management system (BMS). Therefore, testing the functionality and the reliability of DC-DC converters is crucial. This is especially true for decentralized battery management systems (DBMS), where multiple nodes communicate to collectively control the system. The used DC-DC converters are modified to parameterize them during operation via microcontroller interfaces. Integrating the communication into the control loop requires an analysis of the control behavior due to additional delays. Therefore, this paper proposes a framework to test DC-DC converters considering the control and communication perspectives. The response time, the control accuracy and stability of these DC-DC converters, e.g., under continuous and abrupt load changes, are measured in automated tests. The dedicated software framework simulates the DBMS and stimulates the hardware components (e.g. electronic loads, data acquisition) via respective interfaces (CAN, RS232). This allows the test of various DC-DC converters with flexibly adaptable load and power generation profiles. An initial application validates the test framework by verifying the aforementioned aspects and thus the applicability of a DC-DC converter within the DBMS.