As long as you do not cancel your domain name, it will be automatically renewed for another year before the date of expiration. You will receive an e-mail notification about the renewal 90, 60 and 30 days before the date of renewal of your domains and services. The e-mail address for notifications can be defined in your Customer Center.
Based on that notifications you have enough time to charge your customer account if you don’t have enough credit or cancel your unneeded domains. If you don’t have enough credit on your customer account it is not possible to execute an automatic renewal of your domain. Information about the renewal process and the consequences of a lack of credit can be found on the information page of each top level domain.