Unless you're running a LOT of high current draw accessories, like a mega-watt sound system, or hydraulic pumps to run a lowrider, a 75 amp alternator should be able to power all the accessories and have plenty of amperage left over to keep the battery charged. The original 12 volt generator only had around a 35-40 amp capacity, so the alternator is capable of supplying around twice the current the original electrical system needed. Even with high output halogen headlights, electronic ignition, and air conditioning, it's doubtful that you'd have a continuous current demand of more than 75 amps. Any charging system has two functions- - - -provide current to power the lights, ignition, and accessories, and use its leftover current capacity to keep the battery charged. Once the surge current drawn from the battery by the starter is replenished, charging current usually drops to less than 10 amps, assuming the battery doesn't have a shorted cell. Any competent auto electrical shop will have test equipment to monitor the complete electrical system and give you an exact measurement of the current demand. Like the Sun Electric Company slogan they put on all their test equipment- - - -"We test, not guess!"
Jerry
