/diy/ - Do It Yourself

Question time:
I'm building a solar glider designed to fly infinitely in the stratopause, but that shit's cold.
The problem is that above and below this slim range, shit's even colder.
I doubt the solar cells would mind traversing this range all that much, but trying to find a suitable battery technology is proving difficult.
I am thinking lithium cells submerged in Isopropyl with nichrome wires for heating, all in an insulated tube, but I'd really rather only have to heat this all with an outside temperature of 0c, not -60c (-80f+/-).

All I need is a technology suitable as a backup battery, to do nothing other than power the flight controller and servos 'till the plane can thermally soar back to a suitable height.
I'd happily burn through the lithium bank's charge to do nothing more than keep themselves warm, I'd be flying well above the clouds so energy from the plane's panels will always be more than enough, but overnight, should the cells get too low, the batteries depleted, I'd need something to keep it going till morning because, if it dipped below the cloud line, it'd be game over for the plane.