How to spot a fake power bank

A 20,000mAh cell has to weigh something: around 350 to 400 grams. If it feels light in the hand, the capacity on the box is not the capacity inside it.

Check the charge time too. A real 20,000mAh pack does not fill in an hour on an ordinary charger, whatever the listing says.

And look for the output rating printed on the body, not just on the packaging. Missing markings mean nobody certified it.