USB power banks are selling for very high prices. Most provide 2000 to 3000 maH of charge. Some give 5000 mAH but cost around 6,000 rupees! Buying these expensive power banks is stupid. I bought a 20,000 maH power bank for Rs. 1500 but the downside of that is that it take 2 whole days to charge. There is no point in going overboard like that either.
Instead, buy USB charger cases for 18650 type battery. Don’t buy the 18650 batteries. Take them out from an old laptop battery. Laptop repair shops and IT waste recycling units may have stock. Take care when you remove the cells from the laptop battery. The solder on the cells are quite sharp and will tear your flesh easily.
Once, you have removed the cells and brushed off the excess metal and solder, they are good to use in one of these charger cases. They even have a LED that makes the power banks usable as torches!