You can definitely use a lawn mower battery in a car, but it may not be the best option. Lawn mower batteries are designed for use with an electrical cord and typically have a shorter life span than car batteries. Additionally, they tend to generate more heat, potentially damaging your car's engine or other parts. If you need to power your lawnmower while you're driving, then using Lithium Iron Phosphate (LiFePO4) cells may be a safer and easier option. These batteries are encased in plastic, so they won't create any sparks or flames when used together with a conventional outlet.