askST: How do I prevent my phone from overheating?

Smoking batteries are the hot topic of the month following an unprecedented decision by world's largest handset maker Samsung to pull the plug on its flagship Galaxy Note7 barely two months after its launch.

The move follows several reports of the phone emitting smoke or catching fire even after they have been exchanged for supposedly safe replacements during a global recall programme in September. Samsung had initially put the blame on faulty batteries, but now, it seems there may be more complications.

Reader Farah Seth asked: "How do I prevent my phone battery from overheating? Is it dangerous to leave my phone to charge overnight?"

Tech editor Irene Tham finds out why batteries now seem more prone to overheating.

Professor Rachid Yazami at Nanyang Technological University's Energy Research Institute said the chances of mobile devices exploding or catching fire are five in a million based on about 20 billion lithium-based batteries in use today.

But the risk is increasing as device makers push boundaries to make batteries charge faster and last longer.

"Intense rivalry among companies may have encouraged engineers to push materials to their limits, therefore increasing risks," said Prof Yazami. He is one of three researchers credited with laying the groundwork for today's lithium-ion battery used in almost every consumer electronic device including laptop, mobile phone and power bank.

He said lithium-based batteries overheat under extreme conditions, including extreme temperatures (below zero degree celsius or above 65 degree celsius), mechanical stress and when being designed to fast charge.

An important component in a battery - to prevent what is commonly known as a short circuit - is the separator between the positive (cathode) and negative (anode) electrodes.

This separator is often made of a microporous polymer material. As long as there is no rupture in the separator, there is no risk of a short circuit to trigger overheating and a thermal runaway.

Here lies the problem: To make batteries charge faster and last longer, battery makers tend to reduce the thickness of the separator film.

During charging, the separator often comes under mechanical stress due to the swelling of electrodes. A thinner separator has a higher chance of rupturing, which then puts electrodes in direct physical contact creating a short-circuit. Intensive heat will be generated melting the separator, which allows for more short-circuiting. This is how smoking, fire and explosion come about.

Battery makers frown on thick separators as they create higher internal resistance, slowing down the charging. A thick separator also means less space for electrode materials, which store energy.

"The cost of fast-charging is, unfortunately, safety," said Prof Yazami.

Since there is no way of telling if a battery is good or faulty, consumers should take the extra precaution not to leave their devices to charge overnight. Consumers should also remove any clutter of devices plugged onto power extension sockets for prolonged charging.

Samsung's production halt for an entire model due to overheating and fire hazard is unprecedented. But lithium-based batteries are notorious for overheating.

Although there have been no reported cases of the Note7 catching fire or exploding here, there was a report earlier this year of a deadly fire caused by drone batteries left to charge overnight. The fire on June 9 last year engulfed a home in Parry Avenue and claimed two lives.

Overheating problems also led to electronics retailer Challenger recalling 12,000 units of its Valore portable power banks in April 2014. Nokia, Dell, Apple, Lenovo, Panasonic, Sharp, Toshiba, Hitachi and Fujitsu had also recalled products due to overheating batteries.

More askST stories here.