A re-enactment of an accident where a portable battery pack subject to recall ignites. (Provided by National Institute of Technology and Evaluation)

Portable battery packs for charging smartphones and tablets risk overheating or bursting into flames if they are used without taking precautions to protect them from damage.

The number of accidents involving the external batteries soared to 76 last year, from three in 2013, according to the Consumer Affairs Agency.

The agency on July 31 warned people to avoid leaving them in high-temperature spots such as inside a vehicle under the burning sun.

"Put the battery pack in a bag if you're going to a pool or beach so it won't get wet. If you sense anything unusual such as swelling, getting hot or emitting a foul odor, stop using it immediately,” the agency advised.

A total of 162 accidents have been reported in the six years since June 2013.

Of these, 78 cases involved the battery pack emitting smoke, overheating, or catching fire. In 39 cases it caused fires in buildings.

Eighteen people have sustained injuries including burns due to the faulty devices, the agency said.

In September 2018, a woman took a Shinkansen carrying her portable battery pack, which was not connected with her smartphone, in her bag.

The battery burst, burning both her legs. The bullet train's operation was suspended for about 15 minutes and police and fire department officials looked into the cause.

Even one month after the accident, the woman's burns had not completely healed and she needed to continue to see a doctor regularly.

In April 2017, a man riding a train was surprised to feel the portable battery pack in the breast pocket of his suit suddenly begin to feel hot.

Luckily, his train was stopping at a station, so he ran out and threw the battery pack on the platform. Scarcely had he done so when a pillar of flame shot out of it. The man escaped injury, with only his jacket pocket singed.

Lithium-ion batteries are used in most portable battery packs, according to the agency.

If a battery pack is dropped or squished, the lithium battery inside it can become deformed. As result, the battery may emit smoke or ignite. If the cord used to connect the battery pack to a smartphone gets wet, its end terminal can short-circuit.

External batteries are subject to regulation under the electrical appliances and materials safety law. Since February, only the products that carry the PSE mark have been allowed to be sold. PSE stands for Product Safety Electrical Appliances and Materials.

The agency recommends that consumers shopping for portable battery packs check if products have the mark before purchasing one.

People who already own external batteries should check whether the one they own is subject to a recall via the agency’s website at (https://www.recall.caa.go.jp/) only in Japanese.