A jump start, also called a boost, is a method of starting a vehicle with a discharged starting battery.A temporary connection is made to the battery of another vehicle, or to some other external power source.hey usually have a 24 V supply using two 12 V automotive batteries in series: it is therefore possible to jump-start a vehicle with a 12 V electrical system using only one of the two batteries. Vintage cars may have 6-volt electrical systems, or may connect the positive terminal of the battery to the chassis.