So I got this single Li-ion power bank. It's a 18650, and I wanted to add another 18650 in parallel. I want to use the same circuit which produces a constant 5v. Is it safe to do it that way? in parallel. If no, what is the cheapest & easiest solution?
Not a good idea. The answer is controversial and can be argued both ways based on safety concerns. If you connect two 18650s in parallel, they should be identical and used together from day one. You should not add a second battery after the first one has been used.