Electrical Energy Storage Technology

Battery Systems

Battery systems encompass everything from individual cells to battery packs, including the connection, sensors, casing and tests for energy storage solutions as well as battery management.

Battery systems are designed based on their objective which is shaped by the power, energy, and grid connection requirements.

The cell chemistry is equally as important as the battery’s application. It can have implications on the permissible voltage, current, temperature and aging of the equipment.

Second-life batteries

Second-life batteries refer to reusing cells obtained from mobile electric devices like cellphones, notebooks or automobiles.

This involves retrieving information about different cell types and their operational history and designing tests that can determine their state of health and correlate this to the service life that these cells can have in a second use. The conditions of the second use must be carefully selected to allow use according to the degradation of those used cells. Performance must be also tested to prove their lifetime and compare their condition to new cells. Being able to relate information on the history, current state of health, and performance in a second life will determine if the cost of these used cells can be lower than using new batteries. In addition, the ecological impact is also to be considered when examining whether cells can be reused.


Technische Universität Berlin
Electrical Energy Storage Technology
Institute of Energy and Automation Technology
Faculty IV
Office code EMH 2
Einsteinufer 11
D-10587 Berlin


Office EMH 2
Building EMH
Room EMH 255

Hybrid energy storage systems

Energy storage system hybridization is characterized by the beneficial combination of two or more energy storage technologies with supplementary operating characteristics such as energy and power density, self-discharge rate, cycle efficiency, lifetime, set-up cost, etc. The hypothesis is that the total energy throughput of a storage device is significantly reduced and the thermal stresses caused by high discharge rate responses are mitigated. In the case of high-performance storage systems with low energy density, the total energy throughput can be greatly reduced by adding a storage system with a high capacity, and in turn the thermal load on the possibly lower-performance storage system caused by reactions with a high discharge rate can be kept low.
Defining the best size and structure for hybrid energy systems (HESS) requires a thorough understanding of all their components. To achieve this, an actual prototype of the system is built and all possible configurations tested. Computational models of the individual storage technologies and the required power and control electronics are required in order to run multiple simulations and investigate the impacts of different technologies and other influencing factors and compare relevant results.

Hybrid energy storage systems or HESS are typically coupled with power converters through a DC or AC network. Different converters are used depending on the application. Power converters are used to control the power flow among the different storage elements. There are different ways of coupling different batteries using power converters. Both series and parallel converter arrangements are available. Depending on the complexity of the control strategies, the use of power converters and microcontrollers can be expensive. As a result, the trade-off between economic feasibility and technical advantages is crucial in determining the financial and technical viability and implementation of HESS.

One major challenge in HESS is designing the energy management controllers for real-time implementation to yield a good power split performance. An imbalance often develops between the DC link bus and the battery bank voltage as a result of the battery change in SOC. This change occurs even if the battery voltage sizing was previously carefully determined. A fixed structure of an HESS bank seems insufficient to solve this issue. Although a run time reconfiguration of the energy storage banks has been proposed in the past, a comparative study in terms of energy efficiency, capacity utilization, scalability, flexibility, hot swap capability, cost as well as the overall systems enhancement has yet to be established. Other critical areas in the study of HESS include charge allocation, charge replacement and charge migration within the energy storage systems.