The cold weather puts an additional need on the battery equal to about 20% of a full charge. Also, it takes an additional 20% for the battery to perform it's usual functions in weather at or below 35 degrees. You unplug it in the morning and your short 40% before you sit down in it. I had a 1968 Ford Fairlane with a 302 that HATED winter. It was in perfect tune and relatively low mileage. I put in a new coil, rotor, dist. cap, battery,rebuilt the 4bbl. carb (remember those?) everything! It just wasn't a winter friendly car! It was a dream above 35 degrees!