“18650” is a common format for rechargeable lithium-ion batteries. It is named 18650 because it is 18mm in diameter and 65mm in length. The “PK Brand” reference likely refers to a specific brand or manufacturer of 18650 batteries, but I don’t have information on a brand with that specific name in my knowledge as of September 2021. There are various reputable manufacturers of 18650 batteries, including Panasonic, LG, Samsung, Sony, and others. When purchasing lithium-ion batteries, it’s important to choose batteries from well-known and trusted brands to ensure safety and performance, as low-quality or counterfeit batteries can pose risks.