I've installed a voltmeter in my car, and when I first got the car a little over a year ago, I would get readings of about 14.1v at night, with the headlights on and the ELD active. With the headlights off and the ELD inactive, I would get about 12.2-12.3v. I have noticed that slowly dropping, so that last night, I got 13.6v with the headlights on, and 11.9-12.0v with the headlights off, and today, both of those have dropped another tenth of a volt -- the rate of dropping definitely seems to be increasing.
So far, I have not had any starting issues, and no factory warning lights are on, but I do notice that there is a little bit of "effort" when starting the car.
So, one of the reasons to add gauges, other then the fact that they look cool, is to be able to see when certain conditions are developing and do something about it before you're stranded on the side of the road. I'm thinking I've got a weakening battery, because even with the engine off, but with the car on accessory (which means the alternator and ELD would not be in play), I'm getting about 11.9v, which is low. In my experience, a car battery should be closer to 12.5v with little or no load. Yes, there is some load on the battery when the switched to accessory, but the load should not be that much.
However, I am open to input, as I like to have as much knowledge as possible before taking the car in for service. If it is the alternator, then my warranty should cover it, but I doubt it will cover a maintenance item like a battery.