The authorities will be reviewing the product safety requirements of personal mobility devices (PMDs) such as e-scooters, which now do not need to be tested and certified compliant before they are sold in Singapore.

This was announced by Spring Singapore and the Land Transport Authority (LTA) in a joint statement yesterday, in response to queries from The Straits Times.

It comes after more fires involving e-scooters and electric bicycles were recorded this year. Between January and September, the number rose to 31, up from 19 in the same period last year. The two most recent fires took place last week.

Experts have said cheap batteries, which are usually of lower quality and tend to overheat, could be behind the recent fires.

"No-brand" batteries can be bought for as little as $60, compared with batteries from well-known brands that cost between $200 and $300.

PMDs use mostly lithium-ion batteries but as these batteries are subjected to high humidity, dust and vibration when used in PMDs, they have a higher chance of failure.

Faulty or lower quality batteries that are unable to stop charging when full also pose a fire risk.

Safety guidelines for the use of PMDs now cover only their usage in terms of weight, width and speed. They do not cover batteries.

In the statement, Spring Singapore and LTA did not indicate when the review will start or how long it will take.

Experts said the review could look into and address the issue of fire safety, as well as the current general guidelines.

Dr Soh Chew Beng, deputy director of the electrical power engineering programme at the Singapore Institute of Technology, said battery packs could be made to undergo electrical testing.

For example, a special test can tell whether a battery pack will rupture or explode if there is overcharging. A temperature test will find out if the component cell batteries are able to maintain their operating currents, voltage and temperature limits during charging.

Professor Rachid Yazami, from the Energy Research Institute at Nanyang Technological University, said while it is difficult to pinpoint factors that contribute to fire risks in PMDs, one way to reduce the risk is to use batteries that can stop charging when the battery temperature goes above 60 deg C.

"It becomes riskier when the temperature goes beyond that," he said.

Under new rules, only owners of power-assisted bicycles are required to register with LTA, by Jan 31 next year. Those who fail to do so could be jailed and fined.

Mountbatten MP Lim Biow Chuan, who sits on the Government Parliamentary Committee for Transport, said he hopes the review will lead to requirements for PMDs to be certified compliant and to be registered with the authorities in future.

He said: "It'll then be easier to track down devices that have been involved in accidents, or pose a risk to others."