Why do lithium batteries explode?

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Samsung's decision to halt sales of the new Galaxy Note 7 because of reports of battery explosions is an extraordinary step for a tech giant to take.

The firm said it had identified a battery issue but did not elaborate.

But if a lithium-ion battery cell charges too quickly or a tiny manufacturing error slips through the net it can result in a short circuit - which can lead to fire.

One expert urged the industry to find safer alternatives to lithium.

"I think one should be concerned and push towards safer battery tech," said energy storage expert Professor Clare Grey from Cambridge University.

"That should be an important focus on research and industry development.

"While most manufacturing flaws will be picked up during initial testing, it's not an infallible process."

However Prof Grey also said that people should not panic.

"I'm standing at an airport - every single person would have to stop what they are doing if we took their batteries away from them," she said.

"We all take risks in our lives - we drive cars sitting on top of flammable organic liquids. Other tech is coming along that is safer."

Common causes

There have only been 35 cases of the Galaxy Note 7 catching fire reported worldwide following 2.5 million sales, Samsung says.

The lithium ion batteries used by Samsung are common across the tech industry - so what makes them hazardous?

Image source, Ariel Gonzalez
Image caption,
A Galaxy Note 7 reportedly caught fire shortly after its charger was unplugged

It's important to understand a little about how they work. Simply they contain a cathode, an anode and lithium.

The cathode and anode are separated by an organic liquid called an electrolyte and a porous material called the separator.

The lithium travels through the separator, within the liquid, between the two.

Quick charge

If the battery charges too fast, generating heat, lithium plates form around the anode which can create a short circuit.

"Normally you would have a battery management system that controls the rate at which you charge," said Prof Grey.

"Batteries are optimised so that you don't charge too fast - if you do that you will plate the lithium."

This is also why battery charging can be a frustratingly slow experience, she added.

Other faults that can cause a short circuit include contamination by tiny fragments of metal during the production process or minute holes in the sealing, which might not become apparent until the battery has been charged a few times as the materials expand and contract.

"The manufacturing has got a lot more standardised than it was 10 to 15 years ago," said Prof Grey.

However battery packs - combining battery cells to generate more power - can be problematic and this is increasingly common. Batteries containing 12 cells, for example, are readily available for laptops.

"The more you put together, the higher the likelihood that some will fail," she added.

"There are still flaws emerging but it's getting better. It is a challenge - with so many being produced, you just need one error."

Signs to watch out for

There can be symptoms indicating that a battery is about to fail, said support and repairs provider Geek Squad.

"Sometimes, a battery will start to swell and bulge before it fails completely, as the internal cells rupture and break," it says on its website.

"But the bulge doesn't always happen. If not, you might notice that your device is a little warmer than usual - but let's be honest, our phones get fairly warm during standard usage anyway."

The firm suggests disposing of any batteries displaying these signs.