Dear Automotive Professional,
Please do not, under any circumstance or for any reason, disconnect the
battery while the car is running. Doing so can burn out my alternator,
and can cause high voltage transients to burn out my car's computer or
other electronic parts. Such damage can cost thousands of dollars to
fix, and can create a diagnostic nightmare.
Please do not be insulted. I'm sure you already know not to disconnect
a battery while the engine is running, but there are a few automotive
professionals who do not know. There are even some who argue that they
do it all the time, and it's never caused harm. I'm sure you're
familiar with such people and understand that so far they've just been
If you're a professional who never disconnects a battery with the
engine running, there's no need to read further.
For more information on the risks of disconnecting the battery with the
engine running, see the Car and Deep Cycle Battery FAQ, section 9.2.12.
If you are one of those people who disconnects batteries with the
engine running, do not do it with my car. If you need to test the
output of the alternator, use a clamp-around ammeter, or measure the
battery voltage with the engine off, then running, and note the voltage
increase (or lack thereof) supplied by the alternator.
Thanks for your time, and once again, if you're one of the vast
majority who knows better than to disconnect a car battery with the
engine running, my appology for making you read this.
Very truly yours