The Tascam DR-100 gets pretty crappy battery life. a regular pair of batteries will get you 20 min. with phantom power. the rechargable battery gets about 90min. fully charged. maybe a little bit more.
I used 2 of the energizer lithium batteries at the Fox the other night and recorded for 2 hrs without one bar from the battery/power light missing.
They are $10 for 4 at publix. maybe you can find them cheaper.
GREAT backup's for the DR-100.