Rechargeable batteries are a key component for sustainable mobility. The last years showed a significant reduction of price and increase in energy density of Li-ion batteries for electric vehicles. Unfortunately, batteries with high energy density can be source of hazard. Recently, burning Li-ion batteries of mobile-phones got a lot of negative attention in the media. In the first part of the paper we review the hazards of conventional Ni-MH and sealed lead-acid batteries. In the second part we focus on Li-ion batteries: we introduce different cell geometries and electrode types, we show a test-stand designed to measure thermal runaway characteristics in our laboratory and finally we conclude with results of a real thermal runaway experiment.