What Is Thermal Runaway? (And Why Your EV Won't Randomly Explode)

You've seen the videos. Black smoke, orange flames, firefighters looking confused. Here's what's actually happening — in plain language, no chemistry degree required.

Table of Contents

• The Campfire That Feeds Itself
• What's Inside Your EV Battery
• When Things Go Wrong
• The Three Ways It Can Start
• Why EV Fires Look So Dramatic
• The Safety Systems Working For You
• LFP vs NMC — The Safer Option
• Real Numbers — How Rare Is This
• What To Do If It Happens
• Key Takeaways

The Campfire That Feeds Itself

Imagine a normal campfire. You light it, it burns, and when the wood runs out, it dies. Simple. Now imagine a campfire that, as it burns hotter, produces more wood — which makes it burn hotter still — which produces even more wood. You can't starve it. You can't just pour water on the top. The fire is now feeding itself from the inside.

That is thermal runaway.

Thermal runaway is not a fire that happens to a battery. It is a fire that is the battery — a chain reaction where the heat the battery generates causes it to generate more heat, faster and faster, until it can't stop.

The word "runaway" is exactly right. Once it starts beyond a certain point, nothing can stop it from the outside. It has to burn itself out.

What's Inside Your EV Battery

Before we can talk about what goes wrong, you need a quick picture of what's in there.

Your EV doesn't have one big battery — it has thousands of small cells, each about the size of a AA battery (cylindrical type) or a flat pouch. These cells are bundled into modules, and modules are assembled into a pack that sits under your car's floor.

Each cell has three layers inside — a positive electrode, a negative electrode, and a liquid between them that allows electricity to flow (called electrolyte). Under normal conditions, this liquid is stable. Under too much heat, pressure, or damage — it is not.

Think of each cell like a small, pressurised tube holding a flammable liquid. Normally perfectly fine. But give it enough heat or a physical shock, and things can go sideways.

When Things Go Wrong

Here's the simplest version of what happens during thermal runaway:

The Three Ways It Can Start

There are really only three root causes:

Too hot from the outside. Your car sits in direct sun at 50°C for hours. Or there's a fire nearby. Or the cooling system in the battery pack fails. The battery gets cooked from the outside in.

Electrical abuse. The battery gets charged too much (overcharged) or has too much current pushed through it too fast. This generates heat internally from the electrical resistance — basically the cells are being overworked.

Physical damage. A crash, a road hazard, or even a tiny manufacturing defect that went unnoticed for months. Physical damage can cause a short circuit inside the cell — a direct connection between the positive and negative electrodes — which dumps all the stored energy as heat in seconds.

Why EV Fires Look So Dramatic

If you've seen EV fire footage and thought "that looks way worse than a petrol fire" — you're not wrong, visually. Here's why:

Petrol fire: Burns the fuel. Once the tank is empty, it goes out. Firefighters know exactly what they're dealing with.

EV battery fire: The battery is simultaneously the fuel and the source of oxygen for the fire. It contains chemicals that don't need external air to burn. Pouring water on the outside cools the surface, but the inside keeps generating heat.

The re-ignition risk is the part that surprises most people. A battery that appears to be out can restart hours later because the cells are still hot inside and still slowly reacting. This is why fire departments soak EVs in water for extended periods — sometimes even submerging the entire vehicle.

The Safety Systems Working For You

The reason thermal runaway is still relatively rare despite millions of EVs on the road is because modern packs have multiple layers of protection:

The BMS — Battery Management System. This is the brain of your battery. It monitors the temperature, voltage, and current of every cell (or group of cells) constantly. If anything looks abnormal, it reduces power, slows charging, or shuts down completely.

Thermal management. Most EVs have active cooling — either liquid cooling pipes running through the battery pack or air cooling. This keeps cells at their ideal operating temperature and prevents any cell from getting dangerously hot.

Physical barriers. Modern pack designs include fire-resistant barriers between cell groups. If one cell or module does enter thermal runaway, these barriers slow or stop the spread to neighbouring cells. This buys occupants time to evacuate.

Venting systems. Cells have pressure relief vents designed to release gas in a controlled direction — away from neighbouring cells — rather than rupturing explosively.

LFP vs NMC — The Safer Option

Not all EV batteries are equally flammable. There are two main types you'll encounter in Indian EVs:

LFP (Lithium Iron Phosphate): Used in Tata Nexon EV (newer packs), BYD vehicles, most e-buses and 3-wheelers. This chemistry is inherently more stable. It needs to get much hotter before anything goes wrong, and when it does fail, it releases far less energy.

NMC (Nickel Manganese Cobalt): Used in many passenger EVs and higher-range vehicles. More energy-dense (which is why range is better) but less thermally stable. It fails at lower temperatures and releases more energy when it does.

Real Numbers — How Rare Is This

What To Do If It Happens

Quick Knowledge Check

Reader Poll

Key Takeaways

• Thermal runaway is a chain reaction where heat generates more heat — a self-feeding fire inside the battery cells.
• It can be triggered by physical damage, overcharging, or extreme external heat.
• EV fires are rarer per vehicle than ICE fires, but harder to extinguish and can re-ignite.
• Modern EVs have multiple protection layers — BMS, cooling systems, physical barriers — that make thermal runaway rare.
• LFP chemistry is significantly more resistant to thermal runaway than NMC.
• Never ignore a battery temperature warning. It is not a minor alert.

Resources and References

Official Safety Guides

• AIS-156 Phase 2 (2023) — India's national EV battery safety standard. Published by Ministry of Road Transport and Highways. Summary available at: https://morth.nic.in
• BIS EV Safety Guidelines — Bureau of Indian Standards overview of EV safety requirements. https://www.bis.gov.in
• NFPA 502 (2023) — National Fire Protection Association: Standard for Road Tunnels, Bridges, and Other Limited Access Highways — includes EV fire response protocols. https://www.nfpa.org

News and Explainers

• Merrill, J. (2023). "Why Electric Vehicle Fires Are So Hard to Put Out." The New York Times. https://www.nytimes.com/2023/04/22/climate/electric-vehicle-battery-fire.html
• Squatriglia, C. (2022). "What Really Causes EV Battery Fires?" Wired. https://www.wired.com/story/ev-battery-fires
• AutoDesk Research (2023). "EV Fire Statistics: Per-Vehicle Incident Rates vs ICE Vehicles." https://www.autodesksustainability.com

For the Curious — Next Steps

• How Lithium-Ion Batteries Work — US Department of Energy explainer. https://www.energy.gov/science/how-lithium-ion-batteries-work
• Battery University — BU-803a: Evaluating Unsafe Battery Conditions — plain-language safety overview. https://batteryuniversity.com/article/bu-803a-safety-concerns-with-li-ion

Further Reading — EVPulse Series

• → Intermediate: Thermal Runaway — The 5 Stages Your BMS Is Racing Against
• → Expert: Thermal Runaway — What Actually Happens Inside a Cell Before It Catches Fire
• → Master: Thermal Runaway — ARC Testing, Propagation Modelling, and Pack Certification

This is the Beginner level of the EVPulse Thermal Runaway series.

→ Next: Thermal Runaway — The 5 Stages Your BMS Is Racing Against (Intermediate)

Published on EVPulse — India's most technically rigorous source for battery technology and EV engineering coverage.