Close
it better to use bmp/tiff/psd for raws. png is terribly slow not only saving but also loading -_-
I only use png format for making files to upload.

btw, Xnview is a good tool for batch convertion.
resampling, gauss blur, auto/manual levels, normalize, saturation, gamma, remove alpha channel, e.t.c.
and ofcourse png compression ;)