I have a car still with a 6v system. And I'd like to get a portable jump starter in case I break down and need a jump start (again). Last time I had to be towed since they can't jump start a 6v with a 12v unit. I been looking for a 6v/12v jump starter (cause I also have an antique 12V Model A) and evidently they don't make a 6v. Anyone know who does? If not as an alternate I thought i could just get a spare battery, keep it fully charged and take it whenever I take to the road. Got any thoughts? Any downside to that idea?