If you use one of these machines, you need to review this article by Apple:
Apple today announced a voluntary recall of a limited number of older generation 15-inch MacBook Pro units which contain a battery that may overheat and pose a safety risk. The units were sold primarily between September 2015 and February 2017 and can be identified by their product serial number.
Because customer safety is a top priority, Apple is asking customers to stop using affected 15-inch MacBook Pro units. Customers should visit apple.com/support/15-inch-macbook-pro-battery-recall for details on product eligibility and how to have a battery replaced, free of charge.