12 volt jump starter

A jump start, also called a boost, is a method of starting an automobile or other vehicle
powered by an internal combustion engine when the vehicle's battery has been discharged.
A second battery (often in another vehicle) is temporarily connected to provide starting
power to the disabled vehicle. Once the disabled vehicle's engine is running, its
alternator or generator should recharge the dead battery, so the second battery can be
disconnected after the engine starts.

Most passenger vehicles use a 12-volt car battery that provides power to a starting electric
motor for the engine. When the engine is running, electrical power from its alternator
restores the charge to the battery in preparation for the next start. When a battery is
discharged, as for example by headlights left on while the engine is off, the car's engine
will not "turn over" when the ignition key is turned and the vehicle will not start.