The problem is more than likely not the battery. What oil are you running? In extreme cold if left outside the oil will get to stiff for the starter to turn it. You have a couple options. 1. get it inside where it can warm up. 2. Get a tarp/parachute and cover the vehicle and place a space heater under the tarp and give it time, while at the same time recharge your battery. 3. Drain your oil and bring it inside to warm up, the replace it in the vehicle and quickly try to start it. I have also used a small charcoal grill under the tarp in an extreme emergency, BUT, be very careful if using this method. The best is a propane weed burner, with some metal chimney sections to divert the heat under the vehicle, again be careful to not get flames under the car. Here is a video, I usually cover the vehicle to trap more heat inside.