Thermal runaway of single cells within a large scale lithium-ion battery is a well-known risk that can lead to critical situations if no counter measures are taken in today’s lithium-ion traction batteries for battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEV) and hybrid electric vehicles (HEVs). The United Nations have published a draft global technical regulation on electric vehicle safety (GTR EVS) describing a safety feature to warn passengers in case of a thermal runaway. Fast and reliable detection of faulty cells undergoing thermal runaway within the lithium-ion battery is therefore a key factor in battery designs for comprehensive passenger safety. A set of various possible sensors has been chosen based on the determined cell thermal runaway impact. These sensors have been tested in different sized battery setups and compared with respect to their ability of fast and reliable thermal runaway detection and their feasibility for traction batteries.

The increasing need for high capacity batteries in plug-in hybrids and all-electric vehicles gives rise to the question of whether these batteries should be equipped with a few large capacity cells or rather many low capacity cells in parallel. This article demonstrates the possible benefits of smaller cells connected in parallel because of discharge effects. Measurements have been conducted proving the beneficial influence of a lower SoC on the thermal runaway behaviour of lithium-ion cells. A second test series examines the short circuit currents during an ongoing thermal propagation in parallel-connected cells. With the help of a developed equivalent circuit model and the results of the test series two major system parameters, the ohmic resistance of a cell during thermal runaway $R_{\mathrm{tr}}$ and the resistance post thermal runaway $R_{\mathrm{ptr}}$ are extracted for the test set-up. A further developed equivalent circuit model and its analytical description are presented and illustrate the great impact of $R_{\mathrm{ptr}}$ on the overall discharged capacity. According to the model, cells with a capacity of no more than $C_{\mathrm{cell}}$ = 10–15 Ah and a parallel-connection of 24 cells show the most potential to discharge a significant amount.