The key with any portable power packs is to keep them warm, in the house, by the front door if you wish. A power pack in the trunk at 10 degrees is useless. Even though cold temperatures thicken the oil (another reason to use a lower pour point synthetic), the cold really takes the punch away from any battery. Personal experience starting an Isuzu pup diesel at 10 degrees with a weak battery: my cheap 500A power pack (at 68 degrees) did a great job boosting the truck's battery.