Interstate makes some 6-volt gell cells that you could use in series. I have them in my motor home. They run the hole house heat and all, all night without starting the car. They dont leak since there gel. I just dont like the spiral cells in really cold weather unless you use the car every day. A regular wet cell might freeze a little and slush up but theres plenty of room inside for expansion. The spiral cells freeze and must break or something and cause a short inside the battery.