[ndutton]

First, you don't know yet what your alternator output voltage is so let's not get ahead of ourselves. You might not even need a ballast resistor.

Second, voltage drop can be significant due to insufficient wire gauge and circuit length. Typically boat wiring, especially old boat wiring, is a hot mess.

Third, think of a battery as a bucket of water filled and drained through a single hole at the bottom. It takes more pressure to push water into the bucket than the static pressure at the hole. If the pressure was the same the bucket would not fill, if it's less the bucket would drain. In this analogy, pressure = volts.