Ed,
check out this thread, just a little further down the page:
http://taperssection.com/index.php?topic=63681.0as I said in there, I'm using an external 12v NiMH battery, 4.2 AH. although I've never run the battery to completely empty, I know that it was still going fine at 6 hours...
in that same thread, Jon states:
i get 5.5 hours on 2500 mha AA
we're both using the S/PDIF input on the HD-P2, and, some quick calcs yields this:
2.5 AH = 5.5 hours
so my 4.2 AH battery should = 9.25 hours of run time.
based on this, it looks like either internal rechargables, or an external pack will work, and will provide enough run time. Personally, I like the idea of having a lot of extra time, which is why I went with the external pack.