Why do portable chargers
sometimes catch fire?
A power bank swollen inside a bag, a smartphone left charging overnight while its owner fell asleep — you may have seen news stories along the lines of "we never thought it would actually catch fire." These devices usually work fine for years. So why does this happen? Let's start by looking inside.
A power bank gets dropped and knocks its corner on the floor. It looks unmarked, charges normally afterward. "Seems fine, I'll keep using it" — so that night, it sits by the pillow, charging while its owner sleeps.
Hours later, there can be a small crack, then smoke pouring out within moments. It's not unusual for the gap between the drop and the fire to stretch from hours into days. "It still worked, so it must be fine" doesn't always hold.
Why does this happen? The key lies in the battery's internal structure.
Two reasons, and only two
Inside a portable charger, positive and negative electrode sheets are tightly wound together along with a flammable liquid. All that keeps them from touching directly is a single sheet as thin as paper.
An impact or improper charging can tear this barrier. The positive and negative electrodes then touch directly, and a large current flows. The heat this produces drives further reactions, which produce still more heat — a self-feeding chain begins.
Because these two facts combine, fires in portable chargers are said to be "unpredictable in timing, yet hard to stop once they start." Let's look at each in turn, using familiar comparisons.
Reason 1: the inside looks like a "Swiss roll"
Open up a portable charger and you'll find thin sheets wound round and round, much like a Swiss roll. Broadly, there are two kinds of sheet: one for the positive electrode, one for the negative. A flammable liquid fills the space between them.
If the positive and negative sheets ever touched directly, electricity would rush through all at once, creating a large current (a short circuit). To prevent this, a single sheet as thin as paper sits between the two. The fact that we can normally use these batteries safely comes down to this one thin sheet.
Reason 2: why the heating is hard to stop once it starts
Heat produced when the separator tears drives further reactions in the surrounding flammable liquid. Those reactions generate more heat, which drives the reaction further still — a state where heat keeps calling for more heat. With a campfire, cutting off the surrounding air (oxygen) puts out the flame. But the material used for a battery's positive electrode often has the property of generating its own oxygen as it reacts, so cutting off outside air doesn't necessarily stop the reaction.
And a fire doesn't necessarily start the instant the tear happens. As in the opening scene, cases have been reported where heating progresses after damage occurs, and a fire breaks out hours or days later. That's why "it's fine right now" doesn't guarantee it stays fine.
A torn separator isn't only caused by external impacts like drops or crushing. Using a cheap, non-genuine charger, or charging and storing a device in extremely hot places (like a car in direct sun), can also cause thin metal spikes to grow inside the battery and damage the separator. Even with no visible damage, caution is still needed.
- Use certified chargers and cables, such as ones bearing a PSE mark. Cheap, poor-quality products can have inadequate current and voltage control.
- Avoid charging inside bedding, near a pillow, or on top of flammable materials. Charge somewhere you'll notice right away if something goes wrong.
- Don't use a battery that has been dropped, hit hard, or has swollen. Even if it looks fine, the inside may be damaged.
- If you notice unusual heat, a strange smell, or swelling during use or charging, stop charging immediately, unplug it, and place it away from anything flammable, on a metal tray or concrete surface.
- If it is already smoking or on fire, don't try to put it out yourself — leave the area immediately and call emergency services.
- Use your smartphone or portable charger while it's charging for 10–20 minutes, then gently touch it
- Check whether it feels "just slightly warm" or "too hot to hold"
Under normal charging, devices usually stay at "just slightly warm." Knowing this feeling makes it easier to notice the "unusual heat" described above. If it's too hot to hold, stop charging and unplug it right away.
Summary
Portable chargers don't catch fire simply because "they're fragile." It's said to come down to two things combining unexpectedly: ①the inside is made of a flammable liquid and a thin separator, and ②once that separator tears, a chain reaction of ever-increasing heat begins.
Looking fine on the outside doesn't mean it's fine inside.
A damaged battery can catch fire only after a delay.
For more on how batteries store energy in the first place, see this article.
Want to go deeper? ― Terms, formulas, and textbook connectionsLabels show whether each part is middle-school level or university-level, so you know what you're reading
- MSCovered in middle-school science
- HSCovered in high-school "Basic Chemistry" / "Basic Physics"
- HS+High-school "Chemistry," or advanced/sidebar content in textbooks
- Univ.Not taught in high school — university-level specialist content (electrochemistry)
- ResearchNot yet settled even at university level — an active research question
MSTerminology: this phenomenon has a name
- Lithium-ion battery: the formal name for the battery type with the structure described above. Widely used in smartphones, portable chargers, electric vehicles, and more.
- Cathode / anode: the formal terms for the "positive side / negative side" used in the main text.
- Separator: the "thin barrier" from the main text. A thin resin film that keeps the cathode and anode from touching directly.
- Electrolyte: the "flammable liquid" from the main text. A liquid that carries ions, usually a flammable organic solvent.
- Short circuit: a state where the cathode and anode connect directly, and a large current flows all at once.
- Thermal runaway: the formal name for the "runaway heating" from the main text. A state in which heat keeps generating more heat, with temperature rising rapidly and continuously.
MSHSChecking with a formula: how much energy is packed into one battery
Let's estimate how much energy is stored inside a typical smartphone power bank. The units here are mAh (milliamp-hours), V (volts), Wh (watt-hours), and J (joules).
| In symbols | W = Q × V (temperature rise: ΔT = W ÷ ( m × c )) |
| In words | Battery energy = stored electric charge (capacity) × voltage |
| Where it comes from | Power = voltage × current; energy = power × time (middle-school science). Current × time is capacity, so capacity × voltage gives energy |
| W | Energy stored in the battery (in watt-hours, or joules) |
| Q | Capacity = charge that can be drawn out (in amp-hours) |
| V | Battery voltage (in volts) |
| m, c | Mass of water being heated (kg) and specific heat (about 4200 joules per kg per degree) |
| Battery capacity | 3000 mAh (= 3.0 Ah, typical for a common device) |
| Voltage | about 3.7 V (typical for one lithium-ion cell) |
| Heat needed to warm 1 L of water by 1°C | about 4200 J (approximate specific heat of water) |
| Energy (Wh) | 3.0 × 3.7 = 11.1 (Wh) |
| Converted to joules | 11.1 × 3600 = 39960 (J) |
| How much can it warm 1 L of water? | 39960 ÷ 4200 ≒ 9.5 (°C worth) |
One smartphone's worth of battery works out to roughly enough energy to warm 1 L of room-temperature water by nearly 10°C. This is only an ideal-case estimate, but the fact that such energy is packed into a palm-sized component is thought to contribute to how fiercely these fires can burn.
HS+Univ.Reason 2, precisely: the chemistry of thermal runaway
HS+When the separator tears and a short circuit occurs, a large current concentrates at that spot, raising the local temperature. This temperature rise can trigger a reaction that decomposes the cathode material.
Univ.Most lithium-ion battery cathodes use metal oxides. Once temperature crosses a certain threshold, this cathode material decomposes in a reaction that releases oxygen while generating heat. The released oxygen further accelerates the decomposition and combustion of the surrounding electrolyte (a flammable organic solvent). Even if outside oxygen supply is cut off, oxygen keeps being generated inside the battery, so the usual firefighting idea of "cut off oxygen to extinguish" doesn't straightforwardly apply. This is one reason lithium-ion battery fires are said to be "hard to put out."
📖 For more on battery mechanisms and thermal runaway: Wikipedia, "Lithium-ion battery"
Univ.Why does the fire start hours or days later, rather than right away?
With every charge and discharge cycle, lithium ions travel back and forth between the cathode and anode. Repeated fast charging at low temperatures, or repeated overcharging, can cause needle-like structures of metallic lithium to grow on the anode's surface — these are called dendrites. Dendrites are thought to grow gradually, and one day pierce through the separator, causing a short circuit. The reason a fire can start hours or days after damage occurs, rather than immediately, is thought to be that damage and dendrite growth progress slowly before eventually reaching a short circuit.
ResearchWhat isn't fully understood yet
Lithium-ion batteries are a widely used technology, but some aspects of their safety are still being researched.
- There's no established way to predict in advance which battery will catch fire, or when. Algorithms that try to detect early warning signs from tiny quirks in charge/discharge voltage or small changes in internal resistance are being researched, but how far these can be built into real products remains a work in progress.
- Development of harder-to-burn electrolytes is still ongoing. Research continues into liquids that resist burning, and into "all-solid-state batteries" that use no liquid electrolyte at all, but balancing performance and cost remains a challenge (research into the underlying mechanism of energy storage itself is also covered in a separate article).
- Even among batteries of the same model, why only some catch fire isn't fully explained. Tiny manufacturing impurities or variation in the assembly process are thought to play a role, but exactly how minor a factor can act as the trigger is still under investigation.
In other words, this article too reflects "what is understood so far." The precautions introduced above are reasonable measures that follow logically from the mechanism, but they are not a guarantee that every accident can be prevented.
Connections to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| MS | Science: basics of current and batteries | The basic idea of positive/negative electrodes, Figure 1 |
| HS | Basic Chemistry: batteries and ions | Terms like cathode, anode, electrolyte; the energy calculation |
| HS+ | Chemistry: oxidation-reduction reactions (often treated as advanced content) | The starting point for cathode-material decomposition and exothermic reaction |
| Univ. | Electrochemistry / materials science | The chemistry of thermal runaway, dendrite growth and short circuits |
| Research | Battery engineering / safety engineering (unresolved) | Early detection of anomalies, harder-to-burn electrolytes and all-solid-state batteries, causes of individual variation |
| ― | Disaster prevention / safety education | Using certified chargers, storage/charging location choices, response when a problem is noticed |
- Fire and Disaster Management Agency (総務省消防庁), "Public advisory on fires involving lithium-ion batteries" (accident cases during charging, guidance on initial response).
- National Institute of Technology and Evaluation (NITE) (独立行政法人製品評価技術基盤機構), product accident information related to portable chargers and lithium-ion batteries.
- Electrochemistry and battery engineering textbooks, covering the structure of lithium-ion batteries and the mechanism of thermal runaway.
- Ministry of Economy, Trade and Industry (経済産業省), general public advisory materials on the safe use of portable chargers (including the PSE mark certification system).
※This article is a general-audience science explainer. Follow the guidance of fire services, local authorities, and product manufacturers for actual emergencies. The figures given are approximations meant to aid understanding, not precise values.