I thought my Planet Bike Superflash was defective, because it shut off in the cold despite installing a new pair of batteries. I was just about to ship it off for service/replacement, but I checked the charge on the batteries anyway. Much to my surprise one had a full charge and the other was dead. Note: these are supposedly fresh batteries from the same blister pack.
Since replacing the dead battery, it's been working now for several weeks without a hitch, and I've been riding in cold temps. Before you blame the light, check your batteries.