First don't even think of trying irfanview for this. It doesn't handle transparency at all. Because I got fed up of that little feature I changed over to
XnView, another graphics viewing program (IMO the best free 1 I've seen) that handles transparency well in program. I make no guarantees how it does in the batch conversion stakes though as I haven't had a need to use it yet. But it does have comprehensive batch processing built in.
As an alternative that I haven't tried at all, there is
Easy Graphic Converter Free! by Etrusoft. Like I said, I haven't tried it so don't know if it's any good.
TR