How is the safety of the cells ensured?

‚Thermal runaway‘ describes a self-reinforcing process. In such instances, decomposition reactions which occur at a particular temperature generate so much energy that, in the worst of cases, a violent bursting results. This process cannot be stopped once it has started. The occurrence of a critical condition – caused, for example, by an external short circuit or overcharging – is prevented effectively by the battery management system. But even the cell itself prevents these uncertainties due to various measures:

Positive Temperature Coefficient (PTC)
A thermistor / PTC is a heat-dependent resistor which is triggered in the event of excessive current and reversibly minimises the current flow. 

Current Interrupt Device (CID)
In the event of misuse, a CID works by irreversibly deforming a current-carrying jumper, so that the current flow is interrupted.

Fusing separator
A rather new method involves a fusing separator. The separator consists of two different plastic materials which are both porous. In the event of excess temperature, one of the two plastic materials melts, thus filling the gaps of the other plastic material, which in turn leads to the current flow being restricted or a complete stop (shutdown) occurs.

Safety valve
A safety valve is a predetermined breaking point which, in the event of an increase in pressure caused by a rise in temperature, bursts open and enables a controlled discharge of electrolyte gas. This prevents a violent bursting.