Johnson Controls’ start-stop batteries are designed to improve mileage by about 5 percent and help carmakers meet new federally-mandated efficiency standards of 54.5 mpg by 2025. When a vehicle with start-stop batteries comes to a halt, so does its engine, saving fuel and boosting miles per gallon. The car’s advanced battery keeps the lights on and the stereo playing until the pedal is pressed and the engine restarts. Stop-start systems include not only the battery but brake sensors and other custom car parts and should add about $500 to the purchase price of a new car.