BESS safety and thermal runaway prevention start with early cell detection

Thermal runaway in a battery energy storage system does not begin with fire. Changes inside a lithium-ion cell can develop long before temperatures reach a critical level. Detecting these changes early can provide more time to intervene before a cell failure develops into a larger safety event.

A battery cell can become unstable for many reasons, including internal damage, electrical problems or excessive heat. But an abnormal cell does not suddenly enter thermal runaway. Its condition changes over time. Electrical characteristics can change, gases can form and pressure inside the cell can rise before a significant increase in temperature becomes visible. These changes create a window in which the problem can be detected before the cell reaches thermal runaway. If that happens, the heat and gases released can affect neighbouring cells and cause the failure to spread.

The safety in storage systems therefore depends not only on containing an event after thermal runaway has started. It also depends on identifying the electrical and physical changes that precede it.

LONGi addresses this through its intelligent cell contact system, or iCCS, which forms part of the safety architecture of its battery energy storage systems united under LONGi ONE. Alongside conventional voltage, temperature and overcurrent monitoring, iCCS uses additional indicators to identify abnormal cell behaviour at different stages of its development.

Thermal runaway has detectable stages

A deteriorating cell gives off different warning signs as the problem develops. Its electrical characteristics can change first. Later, gases can form inside the cell, pressure can rise and the cell can expand. If the problem continues, the safety valve can open and electrolyte and gas can escape. Changes in voltage and temperature can become more pronounced as the cell moves closer to thermal runaway.

These are the warning signs that LONGi's iCCS is designed to monitor. Instead of relying mainly on voltage and temperature, it adds earlier indicators such as changes in internal resistance, pressure, safety-valve status and electrolyte leakage.
Recent battery research supports the importance of monitoring these earlier physical changes. A 2025 study published in Communications Engineering found that mechanical strain can reveal abnormal cell behaviour before significant external temperature changes under several abuse conditions. The researchers point to a fundamental limitation of surface-temperature monitoring: heat generated inside a cell takes time to reach its exterior.

Research on large-capacity LFP cells reaches a similar conclusion. A 2025 Journal of Power Sources study evaluating voltage, temperature, strain, internal pressure and external force found internal pressure to be an effective parameter for tracking the development of thermal runaway. A separate experimental study on 280 Ah LFP cells found that mechanical stress changes provided early indication before safety-valve opening.

These findings are relevant to the design principle behind LONGi's iCCS: a cell can provide detectable electrical and physical signals before conventional temperature monitoring indicates a critical thermal condition.

LONGi iCCS extends the detection window

Conventional cell monitoring typically relies on voltage and temperature data. LONGi's Intelligent Cell Contact System (iCCS) adds further indicators and analysis to identify abnormal cell conditions earlier.

One is direct-current resistance, or DCR. Changes in internal resistance can provide additional information about a cell's condition before more obvious warning signs appear. iCCS combines this with temperature sensing, pressure and leakage sensors, as well as current and voltage monitoring. Together, these monitoring functions allow iCCS to track different signs of abnormal cell behaviour as they develop. According to LONGi's technical data, the iCCS system can provide thermal runaway prediction three to six months in advance.

As a cell moves closer to thermal runaway, physical changes provide further warning. Pressure can rise, the cell can expand and, at a later stage, the safety valve can open and electrolyte or gas can escape. LONGi states that iCCS can provide a thermal runaway alarm three to five minutes in advance at this stage. This creates several layers of detection rather than relying on a single alarm threshold.

The sequence is important. Changes in internal resistance can provide information about developing abnormalities, while pressure, leakage and other physical indicators add further information as the cell condition progresses. Voltage and temperature remain essential safety parameters throughout the monitoring process. The different indicators therefore provide complementary information about the condition of the cell rather than relying on any one parameter to identify a developing fault. The value of iCCS therefore lies not in replacing conventional BMS monitoring, but in adding further cell-level indicators and extending the period during which a developing  abnormality can potentially be recognised.

Cell-level detection matters because propagation starts locally

Utility-scale battery energy storage systems contain large numbers of cells organised into modules, packs and larger system architectures. A thermal event, however, can originate in an individual cell.

Once heat and hot gases from that cell affect neighbouring cells, a local failure can develop into thermal propagation. Experimental research continues to show that heat transfer, hot gases and ejected material from failing cells can contribute to propagation through battery modules.

This is why detection at cell level matters. The earlier an abnormal cell can be identified, the greater the opportunity to respond before its condition affects neighbouring cells or a larger part of the storage system.

LONGi's iCCS combines cell-level electrical monitoring with physical indicators associated with cell deterioration. That information feeds into the wider BMS and BESS safety architecture, adding an earlier detection layer before fire protection becomes relevant.

Earlier detection can reduce the safety and economic impact of a BESS failure

The benefit of predictive cell monitoring is not limited to fire safety. A developing abnormality identified early can potentially be investigated during maintenance before it becomes a thermal event. Once thermal runaway propagates beyond the originating cell, the potential consequences can extend to additional cells, modules and other system components.

For an asset owner, that difference also has an economic dimension. More extensive damage can mean more equipment requiring inspection or replacement and longer periods of reduced availability or downtime. Early detection cannot eliminate battery risk, but it can create more opportunities to address a problem while its physical consequences remain limited.

Fire detection and suppression therefore remain essential elements of BESS safety, but they address a later stage of the risk chain. Predictive monitoring moves part of the safety strategy further upstream: the objective is not simply to detect a fire faster, but to identify abnormal cell development before there is a fire to detect.

For developers, EPCs and asset owners comparing BESS technologies, this raises several questions: Which parameters are monitored at cell level? Can the system detect physical as well as electrical changes? Can it identify abnormal development before a significant temperature rise? And how early can those indicators provide actionable information?

LONGi's iCCS addresses this earlier part of the safety chain by combining DCR analysis and cell-level electrical monitoring with physical indicators including pressure, safety-valve status and electrolyte leakage. The commercial relevance follows from the engineering: earlier identification of a developing cell problem creates more opportunity to intervene before a cell-level abnormality develops into a larger system event.