Originally Posted by pizzaguy

NO. Do not do that. While we did do that in one shop I worked at, it seemed to cause more issues than it resolved; the policy was dropped in maybe a year. First of all, you are messing with an electrical system while it is energized, when you do this, that alone is a risk. Let the battery positive cable touch ANYTHING that is at 'ground' and you just caused yourself some potential issues - the least of which is that the 12 volt supply you are using just blew a fuse, and your presets are lost anyway.

One thing we finally DID realize, was that we had fewer issues by using a neat trick to stop the inductive kick that happens when the battery is disconnected and the new one connected: Adding low resistance across an inductor decreases inductance. So, how do you do this? Easy, in order to more safely change the battery, do this (the steps in red are the "inductive kick" suppressing steps, the rest is standard safe practice):

1. Shut engine off.
2. Turn high beam headlights on, if you have fog/driving lights, turn them on as well.
3. Disconnect battery ground cable.
4. Disconnect battery positive cable.
5. Cover battery positive post (just a caution, if not just be careful to not lay tools across posts of battery - the spark can ruin your day).
6. Yank old battery, dispose of properly (I burn all my old batteries, the smoke is really pretty.)
7. Put new battery in place, fasten down the clamp(s) and restraints that hold it.
8. Clean, with a wire brush, the inner contact surfaces of the battery clamps.
9. Pull off the protective cover from the positive post (that should be there on any new battery).
10. Place anti-corrosion wafers around the posts on the battery (or do step 14).
11. Connect positive cable.
12. Connect negative cable - some minor sparks will fly from post to clamp, do not be concerned.
13. Turn lights off.
14. Spray anti-corrosion spray on the posts and terminals (if you did step 10, this step is optional but never a bad idea.)

You have now down a "Throttle reset" as that information is lost without the battery in there. Some radio presets will also be lost unless you have a JVC radio.