The basic Opencart extension feed for sitemap genartion is failed. It collects all product and output into one variable with a not an optimal database query. The result of processing is always a 500 server error or a not optimal huge xml file if we are lucky. Our class helps to provide a solution. It generates a sitemap index file and many piece of zip compressed sitemap xml. Each file is not larger than 10 Mbyte so the bots can process easier than before. The script can run standalone and you can set a cron job. See: sitemap_genartor.php
Just copy the script and the folder into the webshop root. It works with 1.5.x versions.