Don't mean to ,
but here are just a few documents that should warn folks to be very cautious when dealing with batteries.

http://www.aa1car.com/library/battery_safety.htm

"WARNING: NEVER disconnect a battery when the ignition is ON, or while the engine is idling or running.
Doing so can damage electrical and electronic components."


http://ca.answers.yahoo.com/question/index?qid=20070222191231AAmSB9T

"Do not disconnect the battery with the engine running, the resulting voltage spike from the alternator can fry all your electronics."

http://www.engadineautoelectrics.com.au/battery%20care%20and%20maintenance.html

"NEVER disconnect any battery terminals from a car's battery while the engine is running (to test if your alternator is charging, for instance). This could destroy your cars charging system and it's onboard electronics!"


http://mypage.direct.ca/k/kdomries/safety.html

"Batteries can explode. I didn't believe this until one blew up while I was connecting jumper cables to it. It made a very loud noise, and splattered battery acid all over the place, not to mention flinging jagged bits of plastic in all directions at very high velocity."

http://www.loudouncountytraffic.com/2008/02/helpful-hints-for-changing-your-car.html

"Don't disconnect battery cables while engine is running (your battery acts as a filter)."


And for those that don't believe "harmless" static electricity won't do any damage, put on yer Hushpuppies, shuffle across the carpet and go work on your PC.