If I lived in severely a cold weather area where batteries died all the time, I'd have two. With an A/B switch controlling which battery was active. When you're on battery A, then battery B is totally disconnected, and vica versa. We used to have something like this on our ambulances back when I ran calls. I can't remember for sure if it was a total disconnect of the "off" battery or if it still could be charged by the alternator. A total disconnect would be best, if you could remember to use A one day, then B the next so that both batteries stayed charged. You'd also probably want to have a large capacitor hooked across the batteries to keep your car's clocks, etc. powered for the few milliseconds as you switched from A to B. Finding a place to mount a second battery could be problematic in a small car. In the trunk maybe? Mounting one in a truck or full size car might not be too much of an issue.