Electrical system in cars

A friend of mine asked me how this worked, and I couldn’t tell him b/c I thought that this wouldn’t work, but it does for some reason.

After starting a car (Camaro) the negative on the battery was disconnected and the car continued to run along with everything else inside (power windows, radio, etc).

It was supposed to demonstrate how to test to make sure the alternator is working properly. I wouldn’t have believed it if i hadn’t seen it.

Once the car has started, all the electrical power the vehicle needs comes from the alternator. However disconnecting the battery for any length of time with the engine running is a Bad Thing, and can damage your alternator or other electrical system components.

Q.E.D. is correct… all of the car’s electrical energy comes from the alternator[sup]1[/sup], even at startup. (Cranking power comes from the alternator during the previous driving session.)

Don’t disconnect the battery after startup. IIRC, you could get away with this on old cars. But the regulation systems on new cars is dependent on the battery being in the circuit.

[sup]1[/sup][sub]Technically, there is one exception. The cranking power during the first startup after you install a new battery truly comes from the battery…[/sub]

Yes, it’s a (stupid, unprofessional) way to show that the alternator is charging–though it doesn’t show how well it’s charging, as a properly done test would. It’s also a way to sometimes fry the alternator and possibly some electronic control units (“computers”).