Here's why Samsung Note 7 phones are catching fire

The Samsung Galaxy Note 7 could explode. Read this to satisfy your burning curiosity.

Sean Hollister Senior Editor / Reviews
When his parents denied him a Super NES, he got mad. When they traded a prize Sega Genesis for a 2400 baud modem, he got even. Years of Internet shareware, eBay'd possessions and video game testing jobs after that, he joined Engadget. He helped found The Verge, and later served as Gizmodo's reviews editor. When he's not madly testing laptops, apps, virtual reality experiences, and whatever new gadget will supposedly change the world, he likes to kick back with some games, a good Nerf blaster, and a bottle of Tejava.
Sean Hollister
7 min read
Watch this: Explaining the Note 7's battery flaws (with cake)

Editor's note, January 2017: Samsung has finally revealed what went wrong with the exploding Galaxy Note 7. Here's everything you need to know as of today.

Original story follows:

You plug your smartphone into the bedside charger and place it on your nightstand with care.

You wake to find your nightstand in flames, smoke billowing everywhere.

Or maybe your Jeep. Your hotel room. Your entire home.

How could this have happened? Simple: your phone is a Samsung Galaxy Note 7 -- and it's one of over a hundred that have spontaneously burst into flames.

After 35 reported incidents of overheating smartphones worldwide, Samsung made the unprecedented decision to recall every single one of the Galaxy Note 7 smartphones sold. That's said to be 1 million of the 2.5 million that were manufactured. (Since the recall was first announced, the number of explosive Note 7s has nearly quadrupled.)

The company stopped all sales and shipments of the Note 7, worked with government agencies and cellular carriers around the world to provide refunds and exchanges for the phone, and apparently it still wasn't enough: as of October 10, 2016, as many as five of the supposedly safe replacement Note 7 phones caught fire as well, and Samsung asked all users to shut down their phones. On October 13, Samsung officially recalled every single Note 7, including replacement units.


Don't leave a Galaxy Note 7 on the charger. Return it for a refund now.

Josh Miller/CNET

Once again, every US carrier has halted sales of the Samsung Galaxy Note 7, and a second recall is underway.

But why did these phones even catch fire to begin with? On January 22nd, 2017, Samsung finally explained: it's the batteries.

Why would a battery cause so much damage?

Here's what we found out back in September 2016, when this article was originally published.

The basics

The science behind phone battery fires is actually pretty simple, and fairly well understood. Much like the infamous exploding hoverboards, phones use lithium ion battery packs for their power, and it just so happens that the liquid swimming around inside most lithium ion batteries is highly flammable.

If the battery short-circuits -- say, by puncturing the incredibly thin sheet of plastic separating the positive and negative sides of the battery -- the puncture point becomes the path of least resistance for electricity to flow.

It heats up the (flammable!) liquid electrolyte at that spot. And if the liquid heats up quickly enough, the battery can explode.

Watch this: We finally killed the Droid Turbo 2

Above: what happens when you puncture a phone's battery.

The Galaxy Note 7 certainly isn't the first phone to catch on fire, or even the first giant recall. By 2004, a spike in cell phone battery explosions prompted this CNET article. In 2009, Nokia recalled 46 million phone batteries that were at risk of short-circuiting. Exploding phones have even allegedly killed people.

No brand or model is necessarily safe: for instance, unlucky iPhone owners allegedly suffered nasty burns from exploding devices in 2015 and 2016. And though the Galaxy Note 7 is making headlines right now, other Samsung phones have also burst into flames, like the Galaxy Core that allegedly burned a 6-year-old child earlier this week.

We've known for years that lithium ion batteries pose a risk, but the electronics industry continues to use the flammable formula because the batteries are so much smaller and lighter than less-destructive chemistries. Lithium ion batteries pack a punch, for better or for worse.

Statistically small

Just because a simple phone could turn into a destructive inferno doesn't mean that it will -- even if it's a new Galaxy Note.

Enlarge Image

The FAA is strongly warning passengers not to use or charge a Note 7 on a plane, and many airlines are explicitly banning their use.

Donald Sadoway

According to an unnamed Samsung official who spoke to Yonhap News, the Note 7's manufacturing defect affects less than 0.01 percent of all Note 7 handsets sold. Some quick back-of-the-envelope math, and you're potentially looking at fewer than 1,000 defective phones. "It is a very rare manufacturing process error," a Samsung rep told CNET.

But it's the damage those phones can cause, and the frequency with which they're causing damage, that makes the Note 7 dangerous.

While CNET tends to hear about just a few exploding devices each year, Samsung's Galaxy Note 7 has caught fire as many as 112 times after only one month on sale.

(That's based on official tallies of 92 incidents in the US, plus at least 17 in Korea, 1 in Taiwan and 2 in Australia.)

Update, September 15, 2016 at 2:00p.m. PT: Updated tally with official US incident count from the Consumer Product Safety Commission.

Why Note 7?

What makes the Note 7 different: Samsung may have accidentally squeezed its batteries harder than it should.

According to a unpublished preliminary report sent to Korea's Agency for Technology and Standards (obtained by Bloomberg), Samsung had a manufacturing error that "placed pressure on plates contained within battery cells," which "brought negative and positive poles into contact."

"The defect was revealed when several contributing factors happened simultaneously, which included sub-optimized assembly process that created variations of tension and exposed electrodes due to insufficient insulation tape," a Samsung representative tells CNET.

Watch this: Samsung explains what went wrong with exploding Note 7 battery

Or, in plain English: the thin plastic layer that separates the positive and negative sides of the battery got punctured, became the shortest route for electricity to zap across the battery (that's why they call it a "short-circuit"), and became a huge fire risk.

What does pressure have to do with it? MIT materials chemistry Professor Don Sadoway explains that today's cell phone batteries are made by literally pressing together a stack of battery components -- and that battery companies are under pressure (no pun intended) to cram in as much battery capacity as possible.

Enlarge Image

Samsung's Note 7 battery holds an impressive 3,500mAh, despite its slim profile.


"Imagine if you had a toilet paper roll and it wasn't packed tightly," says Sadoway. With the same size roll, you'd run out a lot quicker.

At first, Sadoway has two theories: perhaps Samsung simply pressed so hard that the positive and negative terminals poked right through the separator and managed to touch.

Or perhaps it's the sponge-like separator itself that got squished. Normally, says Sadoway, the separator allows the liquid electrolyte to pass through pores connecting the negative and positive sides of the battery, even as it keeps the two terminals separate. "If they press really hard, they constrict the pores, the resistance goes up and you generate more heat," says the professor.

But there's another, more interesting theory: perhaps Samsung's batteries are skewering themselves on their own tiny spears.

Why didn't the phones catch fire immediately?

When Sadoway explains these theories, one thing doesn't seem to add up. Today's cell phone batteries generally charge faster (and get hotter) when they're first plugged into the wall, not at the end when they're trickle-charging the last few percent to reach their maximum capacity.

But these Note 7 phones didn't explode right away. In practically every reported instance of a Note 7 catching fire or exploding, it happened after the phone was plugged in and left charging, sometimes overnight.

Then, there's the little matter of how Samsung plans to make these phones safer -- by issuing a firmware update that keeps the Galaxy Note 7 from charging to more than 60 percent of its full capacity. How could that possibly help, if things heat up the moment a phone is plugged into the wall?

Sadoway has a theory -- albeit one without proof. What if only part of the battery was squished improperly, so that the phone couldn't tell when it was 100 percent charged, and kept on charging the cell?

When lithium ion batteries are continually trickle charged, the lithium ions can start to cover the surface of the negative contact in a coating of lithium metal through a process called "plating." And in extreme conditions, that lithium metal can form tiny spikes (called "dendrites") that can poke right through the separator, creating -- you guessed it -- a short circuit.

That would seem to line up with the "variations in tension" Samsung says it found inside the defective battery cells.

"My guess is by backing off to 60 percent charge, they'll be well below the threshold where these things happen," says Sadoway. "Imagine we're trying to fill our gas tank, we don't have a really good regulator, and we don't want to spill the gas all over our shoes. We want to make sure we're cutting off the flow well before this thing gets to overflow conditions."

Samsung didn't respond when asked for comment on the theory.

What happens next

These are just a few theories based on one battery expert's remote analysis of Samsung's initial findings. We don't have the whole truth yet, and the truth is what Samsung and government agencies around the world are looking for as we speak. Just one mystery: why the replacement batteries might also be exploding.

We'd confirmed that the replacement Notes had a battery from a different supplier -- the manufacturing issues were found in batteries built by Samsung SDI -- but maybe that wasn't enough.

Anyhow, organizations like the US Consumer Product Safety Commission have officially stepped in to recall the Galaxy Note 7 and figure out what happened, and they've since recalled the replacement units too.

For you, what happens now is simple: if you're a Note 7 owner, you should strongly consider returning your phone, whether it's the "safe" model or no.

You could wait to find out what Samsung and the CPSC uncover, since battery fires are so rare even in risky phones -- but Samsung and the CPSC are already advising that you should shut your Note 7 down.

You can still exchange your Note 7 for any other smartphone at any major US carrier. Or, get a refund and use it to buy one of these excellent alternatives.

Update, September 15, 2016 at 2:00p.m. PT: The CPSC has officially approved the Galaxy Note 7 recall and exchange program in the United States, and Samsung says new Note 7 smartphones with safe batteries will be available at "most retail locations no later than September 21, 2016." We've updated this story to reflect that.

Update, October 10, 2016 at 1:31p.m. PT: As many as five of Samsung's replacement Note 7 smartphones have also allegedly caught fire. All US carriers have stopped selling the phone. Samsung and the US Consumer Product Safety Commission are investigating.

Update, October 10, 2016 at 4:21p.m. PT: Samsung and the CPSC have officially advised that all Galaxy Note 7 phones -- new and replacement -- should be shut down.

Update, January 23, 2017 at 6:11p.m. PT: Samsung has officially explained what went wrong with its Note 7 batteries, and identified them as the cause of the burning phones.