I use a fujitsu b-2175 which has single and double capacity stock batterys. Phantom and the usbpre will get 2 hrs with the double and 1 hr with the single. I have an elctrovaya powerpad that I use as my main battery, yeilding 5hrs of 24bit recording. If the outboard battery gets low, I can swap in the internal and vise versa until all the batteries are dead. I have 2 each of the stock batteris and the powerpad (5 + 2 + 2 + 1 + 1), so in the most extreme case, I could go 11 - 12 hours of 24bit, with cdwav.
Edit to answer your question
. For a long set 2-3hrs, you would be cutting it very close.