Encrypting the backups does not stop ransomware from encrypting the backups again locking you out of your own backups. By changing the extension of archives (for example, .zip > .ppp), this can partially protect backups from ransomware viruses.
[@Michael (author of Exiland Backup)] renaming a .zip backup archive to some other non-registered file extension is not protection against encrypting ransomware, they CAN still be encrypted and rendered unusable. Backup files can be spotted easily especially renamed backup .zip archives by size and by having a PKZIP header structure at the very start of the file.
Renaming a .zip backup with a different file extension can only protect the file if the ransomware encrypts files with common extensions such as zip, 7z, docx, xlsx, jpg, ppt, pst, etc.