Algorithm for the detection of a single cell contact loss within parallel-connected cells based on continuous resistance ratio estimation

Large-scale battery applications like electric vehicles (EV), hybrid electric vehicles (HEV) and stationary energy storage systems have high energy and power demands. In order to provide these requirements these battery systems mostly consist of parallel-connected cells and/or strings. These parallel configurations enable closed electric circuits even if a single cell or string gets disconnected, which can be caused by, e.g mechanic stress and chemical degradation. This type of connection error has rarely been investigated in present articles and is therefore the subject of this article. The detachment of one or more cells has to be detected in order to prevent electrical or thermal overloading of the remaining former parallel-connected cells. Otherwise in the worst case non detection can lead to hazardous safety issues like thermal runaway and propagation. This article provides an algorithm detecting the detachment of a cell within a few seconds based on continuous resistance ratio estimation. The algorithm is validated by measurements on a module with three serial-connected cell groups, each consisting of two parallel-connected 50 Ah pouch cells. Under typical driving loads a cell detachment is detected in less than 14 s. Furthermore, the estimated resistance ratios are compared to the pulse response approach with a mean deviation of xi = 0.69% for the reference, respectively xi = 0.9% for the measurements with one detached cell.