A car battery disconnect switch is a required piece of safety equipment if you’re building a car that you wish to enter into sanctioned racing events. This device is a switch that is easily accessible and can be used to immediately disconnect the battery from the car’s electrical system. This is especially important in serious accidents when a spark from the electrical system can ignite fuel in either its liquid or gaseous states. Some of these are located on the fender right above the driver side front wheel. Others will be located within easy reach of the driver. They can also be located in the engine compartment right next to the battery. If they’re hidden, they can also be used as an anti-theft device.
For more info, see How to Recharge a Car Battery