The LED can't keep the battery warm when it's subzero. On Tuesday I had to turn my MJ880 off in favour of a knog to make sure the rear Magicshine stayed on til I got home.
on my smart R1 the light goes dim in the sub zero temps atm but brightens up wheni take it inside . i'm thinking of using a bit of foam around the batteries to keep them warmer.
It might be the cheap rechargable nimh cells that i'm using though.