From the gear report post below. Weather was -20's
6. Backpack Cooler for batteries: I bought a $25 backpack cooler that fits 2 drill batteries, 2 headlamps, a battery back (for lights, see #7), 2 marcum panners, marcum recon, & aqua view micro revolution. Put one of those handwarmer packs in it each day and everything stayed decently warm...and somewhat organized.
$25-30 for igloo backpack cooler from wal mart...may to choose from, that's just the one I chose.