StaTrak – Second Life of Lithium-ion Traction Batteries in Mobile and Stationary Applications

In context to energy and traffic transformation towards electrified vehilces, concepts for optimizing the use of vehicle batteries for electric vehicles are increasingly gaining in importance. The Lithium ion batteries batteries used in these applications are characterized by a high energy density, thus, high energy content at little mass.  However, they are still relatively expensive. These batteries age during their use so that they can store less energy. At the same time, the power, they can provide, reduces as well. The objective of the »Statrak« project is the investigation of aged energy storage devices with a now lower energy density, not usable anymore for mobile applications, and to check their suitability for other applications.

Approach

Suitable aging models were developed for lithium ion batteries to meet the project objective. These models are the basis for the subsequent investigations of business models and the derivation of requirements on the BMS (battery management system).

Battery Aging

Battery cells were stored and cycled in the lab under controlled conditions and capacitance and internal resistance measured at regular intervals. The results showed that high temperatures lead to faster aging of the used battery cells (in calendar days) than medium and low temperatures. Cyclic conditioning at low temperatures leads to severe capacitance losses, while medium temperatures enable significantly higher service lives. Influences of currents and discharge depths can be observed as well. These influences decrease at high temperatures. Heuristic models were developed based on the results. These can be used to determine the expected remaining battery service life.

Residual Battery Value

Degressive depreciation was used as template for determining the residual battery value. For this purpose, the previously determined remaining service life is entered the degressive depreciation model and a residual value determined.

Business Model

The »battery replacement station« business model was evaluated.Battery replacement would replace charging and enable significantly higher ranges of electric vehicles. Based on the relatively large number of users, the infrastructure to be built represents marginal cost components, while the battery costs (approx. 40 %) and energy costs (approx. 50 %) represent the overall costs.

Requirements on the BMS (battery management system)

As battery cells already age with their first use, it should not be differentiated between a BMS for new and second systems. In addition to safety management, a state determination to derive extended services, as well as thermal management and data storage are important components.