As stationary energy storage systems expand across Europe, the conversation is shifting from performance alone to long-term safety. Lithium-ion batteries used in grid storage and industrial applications are designed to operate for many years. Traditionally, their condition has been measured using the State of Health (SoH).
However, as these systems age, performance metrics do not always tell the full story. A battery can still deliver acceptable capacity while developing hidden safety risks. This is where the concept of State of Safety (SoS) becomes increasingly important.
With the introduction of the Digital Battery Passport under Regulation (EU) 2023/1542, there is a growing opportunity to track both performance and safety in a structured, lifecycle-based system.
Understanding State of Health in Battery Systems
State of Health is the most widely used indicator for assessing battery condition. It measures how a battery’s current performance compares to its original state, usually expressed as a percentage.
In practical terms, SoH reflects how much capacity remains, how efficiently the battery can deliver power and how internal resistance has changed over time. For example, a battery with 80% SoH typically delivers around 80% of its original capacity under standard conditions.
This metric is essential for planning operations, estimating remaining useful life and making decisions about replacement or repurposing. The International Energy Agency highlights the importance of tracking battery degradation to optimise performance and extend operational lifespan.
Yet, SoH is fundamentally a performance metric. It does not directly measure risk.
What is the State of Safety and Why Does it Matter
State of Safety focuses on the likelihood that a battery could experience a hazardous event. Instead of asking how well a battery performs, SoS asks how safely it operates.
A battery may appear healthy in terms of capacity while still being exposed to conditions that increase the risk of failure. These could include repeated exposure to high temperatures, internal faults, or stress caused by irregular charging patterns.
Safety risks such as thermal runaway can develop even when performance degradation is not yet severe. The U.S. Department of Energy explains that thermal runaway can be triggered by factors such as internal defects, overcharging or elevated temperatures, often progressing without obvious early signs in basic performance metrics.
This is why SoS is emerging as a separate and necessary indicator.
Why SoH Alone Cannot Capture Risk in Ageing Systems
In stationary storage systems, batteries often operate under fluctuating loads and environmental conditions. Over time, these variations can introduce subtle forms of degradation that are not immediately reflected in capacity loss.
For example, localised heating or internal imbalances may increase risk without significantly affecting overall performance. A battery could still meet its expected output while becoming more vulnerable to failure.
Relying only on SoH can therefore create gaps in risk awareness. Operators may continue using systems that meet performance thresholds but carry elevated safety risks.
A more comprehensive approach requires combining performance metrics with safety-focused indicators.
Combining SoH and SoS for Smarter Lifecycle Decisions
When SoH and SoS are considered together, decision-making becomes more reliable. SoH provides insight into how much useful life remains, while SoS indicates whether the battery can continue operating safely within its environment.
This combined perspective supports better judgment across the lifecycle. Operators can assess whether a system is suitable for continued use, whether it can be repurposed, or whether intervention is required. It also helps determine when a battery should be retired to avoid safety incidents.
For stationary energy storage, where systems are often deployed at scale and in critical infrastructure, this dual assessment is particularly valuable.
How State of Safety Can Be Measured
Unlike SoH, which has relatively established calculation methods, SoS is still evolving. However, it can be derived by combining multiple data sources and analytical approaches.
Battery Management Systems continuously monitor parameters such as voltage, current and temperature. These systems generate alerts when abnormal conditions occur. When this real-time data is combined with historical usage patterns, it becomes possible to identify trends that signal increasing risk.
Indicators that contribute to SoS assessment may include thermal anomalies, voltage imbalances, abnormal cycling behaviour and past safety events. Over time, advanced analytics can detect patterns that precede failure, enabling earlier intervention.
This aligns with broader digitalisation trends, in which predictive analytics are used to enhance performance and safety across energy systems.
The Role of the Digital Battery Passport in Safety Monitoring
The Digital Battery Passport provides the infrastructure needed to capture and share lifecycle data in a consistent and accessible way. The EU Battery Regulation requires that battery data be machine-readable, interoperable and available to authorised stakeholders.
While current requirements focus on sustainability and traceability, the same framework can support safety-related data. By integrating SoS indicators into the passport, stakeholders can access a more complete picture of battery condition.
This may include historical safety events, operational patterns and maintenance records. Over time, the passport becomes a reliable source of both performance and risk information, supporting safer handling across the lifecycle.
Challenges in Defining and Implementing SoS
Despite its importance, State of Safety is not yet standardised. Different battery chemistries, system designs and operating conditions make it difficult to define a universal metric.
There are also challenges related to data governance. Safety data can be sensitive, particularly in commercial or industrial settings. Organisations must balance transparency with confidentiality, ensuring that data is shared securely and only with authorised stakeholders.
In addition, integrating SoS into operational workflows requires investment in data infrastructure, analytics and interoperability. As the industry progresses, collaboration and standard-setting will be essential to establish common approaches.
How BASE Supports Safer Battery Lifecycles
At BASE, we recognise that understanding battery condition requires both performance and safety perspectives. Our Digital Battery Passport framework is designed to support structured lifecycle data that can incorporate both established metrics, such as SoH and emerging indicators like SoS.
BASE focuses on enabling interoperable data exchange, secure access control and integration of operational data across the battery lifecycle. By linking performance data with safety-related insights, we help stakeholders assess risk more accurately and make informed decisions.
Through pilot activities and collaboration with industry partners, BASE is exploring how data-driven approaches can enhance safety monitoring, improve lifecycle management and support the safe scaling of stationary energy storage systems.
Looking Ahead
As stationary energy storage continues to expand, ensuring long-term safety will become increasingly important. Monitoring the state of Health remains essential, but it does not provide a complete picture of battery condition.
Incorporating State of Safety introduces a risk-aware perspective that complements performance data. Together, these metrics enable a more comprehensive approach to lifecycle management, supporting safer operations and more reliable energy systems.
Digital Battery Passports provide the foundation for this transition. By enabling structured data sharing and advanced analytics, they allow stakeholders to move from reactive maintenance towards proactive risk management.
Organisations that adopt this combined approach will be better equipped to manage ageing assets, reduce operational risk and support the long-term resilience of energy storage infrastructure.
The BASE project has received funding from the Horizon Europe Framework Programme (HORIZON) Research and Innovation Actions under grant agreement No. 101157200.
References
EUR Lex - Regulation (EU) 2023/1542: https://eur-lex.europa.eu/eli/reg/2023/1542/oj
European Commission – Battery demand and policy context: https://environment.ec.europa.eu/topics/waste-and-recycling/batteries_en
International Energy Agency – EU Sustainable Batteries Regulation: https://www.iea.org/policies/16763-eu-sustainable-batteries-regulation
European Union - Regulation (EU) 2023/1542 on Batteries and Waste Batteries: https://circular-cities-and-regions.ec.europa.eu/support-materials/eu-regulations-legislation/regulation-eu-20231542-batteries-and-waste-batteries
U.S. Department of Energy – Energy Storage Safety Strategic Plan: https://www.energy.gov/sites/default/files/2024-05/EED_2827_FIG_SafetyStrategy%20240505v2.pdf
National Laboratory of the Rockies – Modelling Lithium Ion Battery Safety: https://research-hub.nlr.gov/en/publications/modeling-lithium-ion-battery-safety-venting-of-pouch-cells-nrel-n/