I wonder, whether O.R. team has considered using lithium manganese dioxide batteries within current design. One of important features these batteries have is ability to withstand "capacity leak" even in cold water, which greatly impact widely available Li-Ion battery packs.
For instance here is some technical info on battery pack from Duracell, claiming pretty good shell life as well:
http://www.duracell.com/oem/Pdf/others/LithBull.pdf