You may want to avoid cheap power banks. My power bank started smoking after only the third time using it to charge a smartphone.
The power bank in question:

This 10,000mAh power bank, which I only paid $11.99 +tax for, filled my bedroom with a terible smell of burning plastic. Initially, I was worried there were wires in my home’s walls burning or wires in the attic. After some investigating, I realized it was my new power bank (linked above) that was smoking. Be careful when you buy cheap power banks. Even though this power bank had nearly 400 reviews, and the overall rating was nearly 5/5 stars, this power bank failed in what could have been a disastrous way.