As lithium ion batteries are adopted in electric vehicles and stationary storage applications, the higher number of cells and greater energy densities increases the risks of possible catastrophic events. This paper shows a definition and method to calculate the state of safety of an energy storage system based on the concept that safety is inversely proportional to the concept of abuse. As the latter increases, the former decreases to zero. Previous descriptions in the literature are qualitative in nature but don’t provide a numerical quantification of the safety of a storage system. In the case of battery testing standards, they only define pass or fail criteria. The proposed state uses the same range as other commonly used state quantities like the SOC, SOH, and SOF, taking values between 0, completely unsafe, and 1, completely safe. The developed function combines the effects of an arbitrary number of subfunctions, each of which describes a particular case of abuse, in one or more variables such as voltage, temperature, or mechanical deformation, which can be detected by sensors or estimated by other techniques. The state of safety definition can be made more general by adding new subfunctions, or by refining the existing ones.