Thermal Runaway, HV Emergency Response, and First-Aid Procedures

Why Thermal Runaway Matters

Lithium-ion (Li-ion) battery packs power modern hybrid and battery-electric vehicles. They store enormous amounts of energy in a compact space, which is exactly what makes them useful — and exactly what makes a failure dangerous. When a Li-ion cell loses control of its internal temperature, a chain reaction called thermal runaway can propagate from one cell to the next, releasing toxic gases, intense heat, and fire that can reignite hours or even days after it appears to be out. Every automotive technician working near high-voltage (HV) systems must understand this process and know what to do if it begins.

Stages of Lithium-Ion Thermal Runaway

Thermal runaway does not happen instantly. It moves through recognizable stages, and early recognition gives you the best chance to protect people and property.

Stage 1 — Internal stress and off-gassing. A cell begins to overheat due to a short circuit, overcharge, physical damage, or manufacturing defect. As internal temperature rises, electrolyte begins to break down and the cell vents flammable and toxic gases — most commonly hydrogen, carbon monoxide, and various organic vapors. You may detect a sharp or sweet chemical odor. The battery management system (BMS) may trigger a warning lamp or fault code.

Stage 2 — Venting. Pressure builds inside the cell until the safety vent opens or the casing ruptures. Gas release becomes visible (vapor plume) or audible (hissing). This is a critical decision point: the situation is worsening and evacuation should already be underway.

Stage 3 — First ignition. Vented gases ignite, producing an open flame. Heat from this fire drives neighboring cells into the same failure sequence. Water application on a lithium-ion battery fire is debated in some circles, but fire departments and OEM emergency response guides widely support large volumes of water to cool the pack and surrounding structure. Never attempt to fight a battery fire with a standard dry-chemical extinguisher alone.

Stage 4 — Re-ignition. This is one of the most dangerous characteristics of Li-ion fires. Even after flames appear extinguished, cells deep inside the pack may still be above their thermal stability threshold. Re-ignition can occur minutes, hours, or longer after the initial event. Fire service agencies and OEM emergency response guides (ERGs) stress that vehicles involved in battery fires must be monitored for an extended period and should not be returned to an enclosed structure without confirmed pack temperature stabilization.

Warning Signs to Recognize

• HV warning lamps or BMS fault codes displayed on the dash or scan tool
• Unusual heat from the floor, battery tunnel, or undercarriage
• Hissing, popping, or crackling sounds from the battery area
• Chemical odor (often described as sharp, sweet, or solvent-like)
• Visible vapor or smoke from battery vents or undercarriage
• Swollen or deformed battery casing

If you observe any of these signs, treat the situation as a developing emergency.

First-Responder Actions in the Service Bay

HV warning lamp active (no visible smoke or fire): Stop work immediately. Do not open the battery enclosure. Follow the OEM ERG for that vehicle — most direct you to disable the service disconnect plug using insulated HV gloves (Class 0 minimum, rated for the voltage of the system) and to scan for fault codes before proceeding. Ventilate the bay. Notify your supervisor.

Smoke from the battery compartment: Evacuate all personnel from the bay immediately — do not attempt to diagnose. Activate the facility fire alarm. Call 911. Provide first responders with the vehicle's OEM ERG so they know battery location, voltage level, and recommended approach. Keep bystanders at a safe distance. Do not re-enter until the fire service clears the area.

Electrical shock victim: Do not touch the victim with bare hands if the HV source may still be energized — you will become a second victim. De-energize the source following lockout/tagout (LOTO) procedures if you can do so safely and quickly. If de-energization is not immediately possible, use non-conductive means to separate the victim from the source. Once separated and the scene is safe, call 911, then begin hands-only CPR if the victim is unresponsive and not breathing normally. Use the nearest AED as soon as it is available. Continue until emergency medical services arrive.

Hands-Only CPR and AED Essentials

For an unresponsive adult with no normal breathing:

1. Call 911 (or direct someone else to call).

2. Begin chest compressions — hard and fast, at the center of the chest, at a rate of 100–120 compressions per minute, with a depth of approximately 2–2.4 inches. Allow full chest recoil between compressions.

3. When the AED arrives, power it on and follow the voice prompts — attach pads, stand clear for analysis, deliver shock if advised, then immediately resume compressions.

4. Do not stop unless the victim shows obvious signs of life, a trained provider takes over, or you are physically unable to continue.

PPE and LOTO Reminders

Wear appropriate PPE whenever working near HV systems: Class 0 or higher insulating gloves with leather protectors, safety glasses, and arc-flash-rated face protection where required by the task. Always verify gloves are within their test date and free of damage before use. Follow your facility's LOTO procedure — confirm the HV system is de-energized and locked out before any service that requires opening the HV circuit.

Key Takeaway

The stages of thermal runaway are predictable. Early warning signs give you time to act. Your job as a first responder is not to fight the fire — it is to get people out, call trained help, and provide life-saving assistance to any victim. Know your OEM ERGs, know your facility's emergency plan, and keep your CPR skills current.