Can a car run without its battery?

Dear Car Talk

Dear Car Talk | Oct 01, 2005

Dear Tom and Ray:

Could you please settle an argument? Some of my ham radio friends and I have a disagreement about the ability of a vehicle to operate without a battery. I say that a vehicle will stop immediately if you disconnect the battery when the engine is running. They say it will continue to operate with the electricity coming from the alternator. I guess the actual question is: "If you have a dead battery and jump-start the vehicle, will it continue to operate with the alternator being the only voltage?" -- Larry

TOM: Well, it's time to get on the old Wireless and concede defeat, Larry. The answer is yes.

RAY: In fact, in the old days -- before the days of solid-state voltage regulators -- we used to use that as a crude test of a car's charging system. If you disconnected the battery and the engine died, it meant that the car's charging system was kaput.

TOM: Cars are designed to run off the alternator, Larry. The battery's primary purpose is to start the engine. Once the engine is running, a belt off the engine drives the alternator, which provides the spark for continued combustion.

RAY: Aside from powering the ignition system, the alternator produces enough electricity to run the headlights, the radio and the heated massaging butt-scratcher. Plus, it makes enough excess electricity to recharge the battery so it'll be ready to start the engine again next time.

TOM: Now, is it still 100 percent true that every car today will keep running if you disconnect the battery? We don't know, because we never try this experiment anymore. With all the electronics and computers and risks of current surges in cars these days, we would never take that kind of risk on a customer's car. There's just too much expensive equipment that can get fried.

RAY: But if you and your ham radio buddies are willing to put up a couple grand to cover the cost of a burned-out alternator and a couple of fried computers, Larry, we'll be happy to test this theory on the late-model car of your choice. Let us know!


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