The charging system will supply current to meet the electrical needs of the vehicle first, like lights, ignition, heater blower, radio, etc., then whatever capacity is left over will be used for battery charging. For instance- - - -ignition 5 amps, high beam headlights, 15 amps, tail lights/turn signals 10 amps, heater high fan 15 amps, radio 10 amps max- - - -if everything is on at once, that's 55 amps. that would leave 8 amps to charge the battery. More realistically, you'll probably have 20-25 amps accessory draw, and 40 amps or so available for battery charging. Immediately after starting, charge current into the battery will probably be 40 amps, and it will quickly taper off to 10 amps or less as the starting load is replenished. If you want a little overkill, use a 75 amp alternator- - - -the price won't be that much different than a 63, and you'll probably never use the full output.
Jerry
