Ageing inhomogeneity of long-term used BEV-batteries and their reusability for 2^nd-life applications

In this paper li-ion battery modules, which have been used in a battery electric vehicle (BEV) for several years and for up to 50,000 miles, have been investigated due to reusability in a 2^nd-life application. The aim was to choose modules with a similar state of health (SOH) and remaining useful life and to reassemble them to get an affordable battery pack for a stationary storage system. All in all 104 modules have been quick-tested. Since the modules show similar characteristics and in order to estimate the dispersion of the capacity and the ohmic resistance between the modules, 10 of these modules have been investigated in detail. Furthermore one module has been disassembled and the single cells have been measured to estimate the inhomogeneity on cell-level. 70 modules showing similar SOH have been put together in a battery pack, which has been integrated into the "Efficiency House Plus" in Berlin. This project demonstrates a possible 2^nd- life application of aged BEV-battery modules. On the one hand, the testing of the used modules needed a lot of time and testing capacity. On the other hand the testing results show limited validity, because they are just a snap-shot of the current attributes of each module but do not consider its history. So it is hard to determine the actual SOH and nearly impossible to estimate the remaining useful life of a used li-ion battery module without knowing its complete history, especially because of the non-linear aging characteristics of li-ion batteries. To provide a cost-efficient and automatic evaluation of the battery aging, this paper suggests a server based battery management system (BMS), which logs all relevant cell data and creates the history of each module. Thus more reliable conclusions about the remaining useful life and the actual value can be drawn and different options for servicing, repair and reuse arise.