I've owned my 2012 Volt for just over 2 years with about 76K now on the clock. In the summer I was getting a 40 mile predicted range which I could exceed with careful driving. In the winter I was getting 30 which again I could exceed. However, this winter it's dropped to 26 and no matter how carefully I drive, I only get two-thirds of that. Is this a sign that my batteries are dying?