Keep it above 5 degrees. Mine quits just below zero, (I didn’t have an accurate thermometer). Not sure an insulated battery would help, you would need a heat source, Unless your traveling a long ways before you drill your first hole its not a problem. Then just put the battery in your pocket. I just put the battery in the portable after I drill. I got the extra battery deal and I keep the extra in a (gasp) Ion battery bag. stocked with hand warmers, only used the warmers once when it was -30. have never needed the extra battery, even on long weekends,
but I’m not really a run and gun guy.
The trick in the owners manual about running it with no load for awhile to warm the battery didn’t work for me, just gives the multiple beeps.
I really like this auger!