You're at the TSA checkpoint. You've got your shoes off, your laptop is in a bin, and suddenly a frantic agent is asking if you have any loose "backup" power banks in your checked suitcase. It feels like a hassle. It feels like a weird rule from 2005 that just never went away. But honestly? There is a very scary, very real reason why why can't you take lithium batteries on a plane remains one of the most strictly enforced safety protocols in global aviation.
Lithium batteries are basically tiny, dense energy bombs. That sounds dramatic, I know. But when you pack a high-capacity lithium-ion cell into a pressurized tube flying at 35,000 feet, you aren't just carrying a way to charge your iPhone. You're carrying a chemical cocktail that, if it fails, cannot be extinguished by a standard fire extinguisher.
The chemistry of a "thermal runaway"
The big scary term you’ll hear experts like those at the National Transportation Safety Board (NTSB) use is "thermal runaway." It’s a bit of a nightmare scenario. Essentially, if a battery is crushed, overheated, or has a manufacturing defect, it can enter a self-heating cycle. The battery gets hot. That heat causes the next cell to get hot. Then the whole thing vents gas and bursts into flames.
Water won't stop it.
Most people don't realize that lithium-ion battery fires are chemical fires. They create their own oxygen as they burn. This means even if you try to smother the flame, it keeps going. On the flight deck, pilots have specific "fire containment bags" for tablets and phones that start smoking, but if that fire starts in the cargo hold? You’re in trouble. There’s nobody down there to put it out, and the Halon gas systems in the luggage compartment are designed for luggage fires, not chemical-metal fires that burn at over 1,000 degrees Fahrenheit.
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Why the cargo hold is the danger zone
You might be wondering: "If they’re so dangerous, why can I bring them in my pocket?"
It’s all about visibility.
If your vape or your laptop starts smoking in the cabin, you’re going to notice. The flight attendants are trained for this. They’ll grab the fire bag, dump the device in, and the threat is neutralized. But if your spare "mophie" battery is buried deep inside a checked suitcase under 40 pounds of cotton t-shirts and polyester sweaters? It could be fully engulfed before the smoke detectors even trigger an alert in the cockpit. By then, the structural integrity of the plane is at risk.
Remember UPS Airlines Flight 6. In 2010, a Boeing 747 crashed in Dubai because a large shipment of lithium batteries ignited in the cargo hold. The crew couldn't see through the smoke. They couldn't breathe. It was a tragedy that fundamentally changed how the FAA and ICAO (International Civil Aviation Organization) look at battery transport. That one event is largely responsible for why the gate agent asks you three different times if you have batteries in your checked bag.
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The 100-Watt-Hour Rule
There is a specific limit you need to know about: 100 watt-hours (Wh). Most consumer electronics—your phone, your MacBook Air, your Kindle—are well under this. For example, a standard 16-inch MacBook Pro has a 99.6 Wh battery. They literally designed it to be as large as possible without breaking the FAA’s "no-questions-asked" limit.
If you’re a professional photographer or a drone pilot, you might be carrying batteries that exceed 100 Wh. In those cases, you actually need airline approval. And if you’re carrying something over 160 Wh? Most of the time, that's a hard "no" for passenger planes altogether. You'd have to ship that via specialized cargo freight.
Real-world mishaps that still happen
Even with all the signs at the airport, people mess up. A few years ago, a passenger's spare battery fell into the mechanism of a reclining First Class seat. When the passenger moved the seat, it crushed the battery. The resulting fire caused an emergency landing.
It’s not just about "old" batteries either. Brand new ones can be just as sketchy if they are cheap knock-offs. We’ve all seen those $5 chargers at gas stations. Those are the ones that keep safety inspectors awake at night. They lack the sophisticated Battery Management Systems (BMS) that tell a battery to stop drawing power when it gets too hot. Without a good BMS, a battery is just a ticking clock.
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Damaged batteries are a different beast
If your phone has a "bloated" look—where the screen is popping out slightly—do not take it on a plane. Period. That bloat is caused by gas buildup inside the battery casing. It is the literal doorstep of a fire. If you drop your phone and the battery gets dented, that can also create an internal short. TSA doesn't always check for this, but as a responsible traveler, you should know that a compromised battery is a liability the moment the cabin pressure changes.
How to pack like a pro
Knowing why can't you take lithium batteries on a plane is half the battle; the other half is actually packing correctly so you don't get stopped.
- Keep them in your carry-on. This is the golden rule. If it has a battery, it stays with you.
- Tape the terminals. If you’re carrying loose "18650" cells or similar, put a small piece of electrical tape over the metal ends. This prevents them from touching a coin or a paperclip in your bag and short-circuiting.
- Use a dedicated case. Don't just throw spare batteries into a messy pocket. A plastic battery case costs two dollars and keeps them from bumping into each other.
- Don't charge in your bag. Never hook your phone up to a power bank while both are inside a backpack. Heat can't escape, and that's how "thermal runaway" starts.
The future of batteries and flight
Researchers are working on "solid-state" batteries. These don't use the flammable liquid electrolyte that current lithium-ion batteries do. When those become mainstream, the rules might finally loosen up. But for now, we are stuck with the chemistry we have.
Airlines aren't trying to be difficult. They are trying to prevent a chemical fire that is virtually impossible to put out in mid-air. It’s a small price to pay for not having the floor of the airplane melt beneath your feet.
If you're unsure about a specific device, look for the "Wh" rating on the back. If it says something like 14.4V and 5000mAh, you just multiply them: $14.4 \times 5 = 72 Wh$. You're good to go. If that number is over 100, call the airline 48 hours before you fly.
Actionable steps for your next trip
Check every single pocket of your checked luggage for forgotten "emergency" chargers or old vapes. If you find one at the gate, don't just shove it in the seatback pocket—keep it in your personal item where it can stay cool and ventilated. If your device ever feels unusually hot during a flight, notify the cabin crew immediately. They have the gear to handle it; you don't.