I used a combination of DVD batteries and Energizer 2500mah rechargable AAs.
The Marantz recognizes the external battery as the wall wart and switches seamlessly to the internals when the DVD batt runs out. The internals will run down some when using external power and I usually kept the AAs in the sled in my bag and switched out as neccesary. The last show I recorded with that deck was 2 hours, ph power on, backlight on the whole time and my wally batt was still 3/4 full.
I fretted over the 13v thing for a while, thought about getting one of those expensive adjustable voltage batts but its very tolerant of the input voltage and runs just fine off of 9-12v.