In the Battery Energy Storage System (BESS) industry, there is no sound more expensive than the hiss of a battery cell venting gas. As grid-scale lithium-ion deployments reach record densities, the margin for error in thermal management has evaporated.

We often talk about "thermal runaway" as a chemical inevitability, but the reality is more manageable: most catastrophic BESS failures are preceded by a failure in the Thermal Management System (TMS). Whether it is a slow glycol leak, a seized manifold valve, or a drifted temperature sensor, the system meant to prevent the fire is often the first thing to break. This guide provides the protocol for inspecting liquid cooling loops to stop thermal runaway before the first cell vents.

What are the leading causes of BESS thermal runaway?

The Direct Answer

While thermal runaway can be triggered by internal shorts or mechanical damage, it is frequently allowed to escalate due to the failure of the Active Cooling System. If a liquid cooling loop fails to maintain the battery modules within their optimal operating window (typically 15°C to 35°C), the "Heat of Reaction" during high-discharge cycles can exceed the system’s dissipation capacity. This leads to a self-sustaining exothermic chain reaction known as thermal runaway.

The BESS Liquid Cooling "Checklist of Truth"

To ensure operational resilience, a physical walk-down must verify what the remote dashboard cannot. Technicians should follow this four-point inspection protocol during every scheduled maintenance event.

1. Coolant Integrity and Chemistry

It isn't enough to check if the reservoir is full.

• The Action: Use a refractometer to verify the glycol-to-water ratio.
• The Goal: Incorrect mixtures lead to internal scaling or "sludging," which coats the cold plates inside the battery modules and reduces heat transfer efficiency by as much as 30%.

2. Pump Differential Pressure (ΔP)

A pump can be "spinning" but not "moving" fluid.

• The Action: Record the pressure at both the inlet and outlet of the main pump.
• The Goal: A narrowing ΔP indicates internal wear or an impeller failure. A widening ΔP signals a blockage in the manifold. If you wait for the "Low Flow" alarm, you are already in the danger zone.

3. The Manifold Valve Audit

Remote SCADA systems often "think" a valve is open because the actuator moved, but the physical gate could be seized.

• The Action: Manually cycle the bypass and isolation valves.
• The Goal: Ensure that during a high-heat event, the coolant can actually reach the modules in the center of the container, which are traditionally the hardest to cool.

4. Sensor Correlation and Calibration

• The Action: Use a calibrated thermal imager (FLIR) to cross-reference the internal module temperature readings with the external manifold temperature.
• The Goal: "Sensor Drift" is a silent killer. If your BMS thinks a module is at 25°C but it’s actually at 45°C, the cooling logic will never trigger the chiller to high-power mode.

What are the NFPA 855 inspection requirements for BESS cooling?

The Direct Answer

NFPA 855 mandates that all Energy Storage Systems have documented, periodic maintenance of their safety-critical systems, including Thermal Management. Compliance requires a Defensible Audit Trail consisting of timestamped logs of coolant flow rates, pressure tests, and fan health. A "paper log" is no longer considered sufficient for high-risk assets; regulators require a digital System of Record that proves the inspection occurred at the asset's physical location.

Conclusion: Safety is a Physical Execution

BESS uptime is more than a revenue metric – it is a public safety mandate. In a world of "AI-driven" predictive maintenance, the ultimate firewall against disaster remains the technician with a checklist and a multimeter. By prioritizing the health of your liquid cooling loops today, you ensure that your "Growth Engine" doesn't become a liability tomorrow.

Don't leave thermal safety to a dashboard guess. Want to see how FieldEx helps hard-guard your energy assets? Book a free demo today, or just drop us a message. We’re here to help.