You want to do bulk file converting? For free? Your issue is going to be finding which raw formats are supported. Not all raw formats are the same.
Try these:
https://www.lifewire.com/free-image-converter-software-programs-2622865
http://www.nchsoftware.com/imageconverter/index.html
You may need to use Adobe DNG converter first:
https://helpx.adobe.com/photoshop/using/adobe-dng-converter.html