In addition to the voltmeter you need to carry a jump starter for you CMax. Several times I've had to jump start my 2013 because the 12V battery died while the car was sitting in the garage and I had driven the car the previous day. The dealership told me in town only driving will not sufficiently keep the battery charged and I need to get it out on the highway now and then. That's a big unknown because how do you know if the alternator is charging the battery and at what speed. Yes, they need an altimeter and a voltmeter on the dashboard to let you know if your 12V battery is charging, is being charged or is charged!!!
