[MReid]

The battery acts as a large capacitor to the alternator. When it is fully charged, the voltage regulator either inside of or external to the alternator senses the lack of voltage differential and turns off the alternator output. As the battery begins to discharge, the voltage differential grows and the regulator resumes alternator output. When the battery is abruptly removed from the circuit, the regulator senses a huge voltage differential momentarily and the output of the alternator output spikes. This spike is strong enough to burn out the rectifier circuit and damage the regulator. Hooking the cutoff up the way I posted above, prevents the differential spike and damage. It is in effect, the same as turning off the key except all power is removed from other circuits.