voltage drop when accessories are turned on 2007 mercruiser 5.0 mpi

by Preston Spraul
(Englewood Ohio USA)

when engine is running, any accessory turned on causes voltage drop. (blower,lights,stereo amp,bilge pump) drop is from 14 volts down to 12.5 to 13. I have a standard 70 amp alternator which on a 100% load test produced 77.5 amps. cleaned all wire and cable connections and replaced ground strap on neg terminal on engine block. Also added another ground wire from alternator to block and replaced 465 cranking amp battery with a new 960 amp battery. Still no change.