Disconnecting either battery cable with the engine running is risky at best and could be catastrophic. With an oscilloscope waveform monitoring voltage instead of a voltmeter, I've seen voltage spikes in excess of 100 VDC as the charging system attempts to keep current going into the battery as the connection is broken. Back in the dark ages when cars ran DC generators, disconnecting the hot (+) cable (not the ground) was done as a shade tree method of checking to see if the generator was charging. Even back then, people who understood electricity didn't do it. Now, with solid state components such as diodes and integrated circuits, disconnecting a battery cable with the charging system in operation is stupid squared! Use a voltmeter!
Jerry
