Power banks store far more energy than smartphones, with some models reaching 25,000mAh, compared to less than a quarter of that in most phones, increasing heat buildup and potential risk.

Most power banks use lithium-ion or lithium-polymer batteries, which can become unstable if charged beyond full capacity without safeguards. This may trigger "thermal runaway," a chain reaction in which rising temperatures accelerate further heat buildup and can lead to fire.

Even without immediate failure, prolonged charging can degrade battery cells over time, reducing capacity. As the cells deteriorate, they may begin to swell, raising the risk of damage or malfunction, according to tech site Slash Gear.

Other risk factors include physical damage, extended exposure to heat, and incompatible cables. Charging a device on soft surfaces such as beds or under pillows can trap heat and prevent ventilation, increasing the danger.

A smartphone charges using a portable power bank.

Manufacturers offer differing guidance. Brands such as UGREEN and Ambrane say their products are designed for safe overnight charging, while Anker advises users to avoid it.

Tech site BGR also recommend not charging a power bank while using it unless it supports passthrough charging, which allows simultaneous charging and discharging without excessive heat buildup.

Singapore’s Consumer Product Safety Office advises consumers to buy from reputable retailers and choose products with safety certifications, warranties, and protections such as overcharge, short-circuit, and temperature control, along with automatic shutoff when fully charged.

Users should stop using a power bank immediately if they notice excessive heat, swelling, unusual odors, leakage, discoloration, or visible physical damage.