The failure of the No. 1 generator field excitation return wire terminal lug due to improper installation of the field excitation return wire by maintenance personnel; the subsequent failure of the No. 2 generator due to overvoltage, caused by the momentary closure of the No. 1 generator contactor, as a result of the flight crew's attempt to reset the No. 1 generator, while an overvoltage condition existed at its POR, due to the shorted failure of the No. 1 GCU diode CR17; and the rapid failure of the aircraft's main battery, accelerated by its existing depleted condition, following an extended period of inadequate charging, due to a damaged electrical system.