One of the key challenges in the development of electric vehicles is the limitations of current battery technology. As technology progresses, battery capacity is expected to increase significantly. In the near future, battery life will no longer be a major concern, but issues such as cost, reliability, and safety will become more prominent.
To address these challenges, Rudolf and the University of Applied Sciences in Zwickau have developed an innovative hybrid energy storage system (HESS). This system combines lithium-ion batteries with double-layer capacitors (also known as ultracapacitors). By integrating a low-impedance EDLC with a higher-impedance battery through a novel topology, this system provides an optimal solution for energy management.
The design utilizes ultra-fast power MOS switches for full digital control, allowing for highly adaptive performance. With this system, the lifespan of lithium-ion batteries can be extended by up to 100%. Additionally, the power management system can be adapted for use in light electric vehicles (LEVs), offering improved reliability and lower development costs.
Through practical implementation, it has been confirmed that existing battery systems can effectively integrate with supercapacitors. This combination allows for optimal energy distribution: while the battery supplies steady power for continuous operation, the supercapacitor handles short-term peak currents and voltages. The battery’s discharge current remains within its rated range, ensuring it operates within its optimal conditions at all times. This “protective operation†method extends battery life and reduces internal temperature rise, further enhancing longevity.
The system also enables rapid charging of the battery-supercapacitor combination without damaging the battery, even after months of inactivity. Supercapacitors have minimal self-discharge, so they can be quickly recharged in seconds. Their robust structure ensures stable performance even at sub-zero temperatures, significantly improving overall system reliability.
Supercapacitors store energy in a Helmholtz layer formed by a two-layer electrolyte. Their high capacitance results from the thinness of this layer and the large surface area of the electrode material. Common materials include metal oxides, activated carbon, and conductive polymers. Under typical voltages (2.7–3 V), supercapacitors can reach thousands of farads. Unlike batteries, they can charge and discharge large amounts of energy in seconds. With a 10-year lifespan and over 500,000 charge cycles, they outperform lithium or lead-acid batteries.
Their wide operating temperature range (-40°C to 70°C) makes them less sensitive to temperature fluctuations than batteries. However, their energy density is relatively low. Double-layer capacitors offer better discharge characteristics; for example, they can handle 75% discharge depth compared to a lithium battery's 25%, without causing permanent damage.
To combine the best features of both energy storage devices, a hybrid buck/boost converter is used to measure and equalize the charging and current characteristics of the battery and supercapacitor. This approach is based on the characteristic curves of each component.
There are various topologies for such systems, including parallel circuits, bidirectional transformers, and combinations of unidirectional and bidirectional designs. However, these often result in complex structures, longer development cycles, and higher costs.
To simplify the design, the R&D team adopted a unidirectional DC-DC converter topology, resulting in a compact and efficient circuit. This reduces development time, cost, and component count. A digital solution offers flexible parameter settings, making the system adaptable and user-friendly.
Inverter voltage can vary across a wide range, and the supercapacitor can dynamically couple to the inverter to manage peak currents. The only limitation is that peak current must pass through a controlled diode (MOSFET). To optimize voltage matching, the supercapacitor voltage can be set at twice the battery voltage, maximizing energy utilization.
A demonstration device was developed for professional battery-powered tools, such as screwdrivers, to meet industrial needs. The topology includes a combined buck or MOS boost structure, fully digitally controllable with software-configurable parameters. This improves battery life, current regulation, and overall performance.
In addition to connecting supercapacitors and lithium batteries, the new power circuit controllers play a crucial role. They monitor analog signals from both components to ensure efficient energy usage. High-performance microcontrollers generate PWM timing for power MOSFETs, enabling precise control.
When no peak current is needed, the battery directly powers the motor through a special switch. During idle periods, the supercapacitor can be recharged by the battery at any time.
The control algorithm was developed using extensive pre-testing and simulation. It uses model-based design, including modeling in VHDL-AMS and automated code generation for target hardware. This ensures accurate representation of the controller, battery, and supercapacitor performance.
To maintain stability, ultra-fast logic circuits are required to meet security and real-time demands. Hardware components like high-speed comparators are used to enhance performance. The challenge lies in accurately modeling and simulating the actual behavior of the system components.
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