What is the State of Safety (SoS)?
The State of Safety (SoS) is a metric that goes beyond the established State of Health (SoH) to determine whether a battery poses an actual safety risk – during operation or at the end of its first life. It is therefore a prerequisite for reliable safety qualification throughout a battery’s lifetime. A major challenge, however, remains the absence of a shared, internationally recognized understanding/definition of SoS.
As lithium-ion batteries from electric vehicles reach the end of their first life, repurposing them for stationary energy storage represents an attractive path towards greater sustainability. Yet a crucial question must be answered first: is an aged battery still safe enough for reuse?
It was precisely this issue that Dr Emanuele Michelini from Battery4Life – a COMET competence center based in Graz, Austria (https://battery4life.at/) – addressed in a presentation in January 2026, which was organized by the eWAVE partner SYRION. The entire eWAVE consortium was represented at this presentation – in many cases with several experts per partner – and took an active part in the subsequent discussion.
From Degradation to Qualification
Batteries degrade through a complex interplay of mechanisms — lithium plating, particle cracking, SEI layer formation, and others — ultimately manifesting in three measurable ageing modes: loss of lithium inventory (LLI), loss of active material (LAM), and increase in internal resistance (IRI), all of which may ultimately lead to safety issues. Michelini’s approach translates these degradation phenomena into a plenitude of electrical state indicators extractable from signals already available in standard battery management systems (BMS) – voltage, current, and temperature – meaning no additional specialized sensors are required.
Tests Under Critical Loads
To verify the safety relevance of these indicators, cells with different ageing histories were subjected to critical load scenarios identified through a Failure Mode and Effects Analysis (FMEA), including charging at high C-rates, operation at low temperatures, and mechanical shocks and indentations. Tests on standard NMC pouch cells showed that selected electric parameters shift consistently following mechanical stress – regardless of prior ageing history. This key finding forms the basis for an SoS estimator capable of classifying end-of-life batteries for second-life applications at high, medium, or low requirement levels, or flagging them for recycling.
Outlook
According to E. Michelini, no single parameter is sufficient: a multi-parameter approach is essential to capture the full complexity of battery ageing and assess safety holistically. The resulting framework aims to enable a real-time SoS estimator integrated into BMS hardware, contributing directly to Battery Passport initiatives and supporting safer second-life applications. Future R&D must address the lack of an internationally binding SoS definition, the transferability of findings across cell types, the mapping of ageing indicators to failure precursors and risk thresholds, and in-operando monitoring under diverse operating conditions.
eWAVE – SoS for Maritime Applications
In the maritime environment, SoS estimation faces specific challenges: distinct emergency response procedures, limited knowledge of maritime stressors on battery behavior, and the absence of clear requirements and standards from the IMO and classification societies, among other things.
Eventually, eWAVE aims to develop an SoS estimation algorithm for maritime batteries, drawing on thermal runaway test data, selected electrical monitoring parameters, and additional dedicated sensor data.
As of March 2026, Dr. Emanuele Michelini now is Programme Officer at the EC Joint Research Center.