New Method for Designing Robust External Short-Circuit Protection Systems at Different Scales

Abstract

External short circuit is one of the failure modes of Li-ion and other batteries that potentially leads to serious damage such as, overheating and/or thermal runaway. Therefore in this work we test and analyse the short circuit behaviour of different system sizes from coin cells, 10Ah pouch cells and upto large scale automotive high voltage battery packs. It was found that the complex short circuit behavior can be described by 3 regions independently of the size of the battery. In the first region 274C-rate is observed which is mainly governed by the cell's double and diffusion layer discharge. In the second region, the current drops significantly to 50–60C-rate where mass transport becomes the current limiting factor. The maximum temperature is reached and cell rupture, venting and electrolyte leakage may occur. Then the current stabilizes at the maximum mass transport region independently of the external short circuit. This complex time behavior need to be covered by different fuses, circuit breaker and intelligent battery management system. The protection strategy depends on several factors, which makes the design process complex. We introduce the normalized resistance ratio which allows the comparison of external shorts with different external resistance and cells and with different capacities. We demonstrate that the estimation of the maximum external short circuit current derived in this work is more accurate than other calculations at different levels. Furthermore we developed and present a generalized visualisation of the full time-current window which potentially help scientists and engineers to design more robust external short circuit protection systems. With this method we also demonstrate that the currently used external short circuit design methods may potentially lead to unsafe regions especially in the soft short region. We also propose a testing protocol which can be applied in standards, regulations without jeopardizing the safety of testing personnels and infrastructure.