Don't Disconnect the Battery
with the Engine Running
Copyright (C) 2005 by Steve Litt
day a well meaning mechanic or naive jump start expert 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.
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 yield 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
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 even possible
for the alternator to put out 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 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.
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(C)2005 by Steve Litt. -- Legal