Thread: Never Disconnect the Battery with the Engine Running

A mechanic friend told me that used to work but in modern cars, not only does it not work but WARNING it could cause a lot of damage. See below:

Never Disconnect the Battery with the Engine Running

Copyright © 2005 by Steve Litt

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The Key to Everyday Excellence

WARNING:

Some day a well meaning mechanic or naive jump start specialist will attempt to disconnect your battery with the engine running. If you value your car and your money, don't let him!

The man from the road service thinks he's being helpful. After he can't jump start your car, he puts in his own known good battery and starts your car. So far so good -- your battery was definitely either run down or bad, and he's proven it.
Now he decides to "test" your alternator by disconnecting the battery. After all, the car's ignition should be able to run on just the alternator's power alone.

Wrong!

The moment he disconnects either lead from your battery, it's entirely possible he caused thousands of dollars in damage. Here's why...
Your battery does more than just provide electricity. It also shorts AC, spikes and transients to ground. Removing the battery from the circuit allows those spikes and transients to travel around, endangering every semiconductor circuit in your car. The ECU, the speed sensitive steering, the memory seat adjustments, the cruise control, and even the car's stereo.
Even if your computers and stereo remain intact, in a great many cases removing the battery burns out the diodes in the alternator, necessitating a new alternator. If disconnecting the battery interferes with the voltage regulator's control voltage input, it's possible for the alternator voltage to go way over the top (I've heard some say hundreds of volts), frying everything.
Even the initial premise was wrong. If you disconnect the battery and the car conks out, you don't know if it conked out due to insufficient alternator current, or whether the resulting transients caused your ECU (the car's computer, which controls fuel mixture, timing, and much more) to spit out bad data, shutting down the car.
Nobody should EVER run your engine without a battery.
And yet when you tell them not to, they'll roll their eyes. "I'm a professional. I do this every day. It's fine!" They'll sound so authoritative. So commanding. So in charge. So intimidating. But they're wrong.
The problem, of course, is that disconnecting the battery doesn't always damage something. It does it only sometimes. Less experienced jump start professionals and automotive technicians figure if they got away with it a few times, it must be OK.
Don't let them do it on your car. When you call for roadside service, or take the car in for possible electrical problems, or have your battery and charging system checked at a "battery shop", give them this letter.
Steve Litt is the documentor of the Universal Troubleshooting Process, has created a course based on the UTP, and is the author of Troubleshooting Techniques of the Successful Technologist and Rapid Learning: Secret Weapon of the Successful Technologist. He can be reached at Steve Litt's email address.