My Factory World
The foremost thing about a power bank that comes to in our mind is the capacity. The capacity of a power bank is measured in Milliampere Hours. More the capacity of the power bank, more the number of times it can charge your smart phone or tablet. Depending on the capacity of the best power banks, they can be used to charge your smart phone multiple times.
A power bank with a 15,000 mAh value should be able to fully charge your phone with a battery capacity of 10,000 mAh 4 times.
Hence it is always best to ask people who have first hand experience about the electronic product. If you want to buy power bank offline & Online, My Factory World is also a good option.