If you are planning to buy a power bank from online or a particular store then make sure the company that you are buying from has some good will such as Intex digiteck etc. You can buy around 10000 or 12000 mah. They will cost you around 600 to 700 in Indian Rupees when you buy first make sure to charge it fully because then it won’t work properly if you don’t charge it I have and Intex power bank which is around 750 rupees of 12500maH which charge my phone twice as my battery is around 4000 Mah it is a very good Power Bank so if you buy it online you can go for Intex power bank they are very very reliable and Long battery life with fast charging.