A bad idea on older cars too. On my way home from work one day I noticed my ammeter was reading way high. '66 Ford F-100 with 352. Stopped at a gas station and told the mechanic to check it out. He disconnected a battery cable while it was running - don't know which one. The heater fan took off like a jet engine, and a bunch of fuses blew, and a couple tail light bulbs. And I shut it down almost instantly with the ignition key. Drove it home (about 1/2 mile) and fixed everything. Think the alt had failed with an open diode. Unusual fail mode.