Recently there have been cases of powerbanks going up in flames and causing bodily harm or even death here are the 3 major factors that may result in power bank catching fire:

Poor Battery Quality
Batteries are the most expensive part of a power bank. Power banks most commonly use Lithium-ion and Lithium-Polymer batteries.
In order to reduce cost, many Chinese factories use counterfeit or recycled batteries in their power banks. While this enables them to supply power banks at a very low price, these power banks carry severe safety risks. It is the battery itself that self combusts after short circuiting when a power bank explodes or melts down.
Inefficient Circuit Design
The circuit design plays a vital role to ensure that a power banks function properly. High quality circuit design and high quality electronic components allow for multiple circuit protection systems. These include high temperature detection and protection, overvoltage protection, over discharge protection and short circuit protection.
Mishandling or Improper Usage
Power banks are electronic items and are susceptible to damage by people mishandling them. Similar to any product containing a battery and electronic circuits they should not be:
Crushed
Thrown into fires
Expose to water or high humidity
Dropped
Purposely short circuited

Having proper circuit design, safety measures, testing and tightly controlled quality standards can greatly reduce the risk of battery failure.
Most importantly it's advisable to use powerbanks with plastic casings than metal enclosed powerbanks... Cause the latter is prone to short circuits and excessive heat conduction due to the metal enclosure

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so this is it

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