Most engine manufacturers deliver the electrical system so that the installer just has to hook up a cranking conductor and engine block ground cable. Once that’s done, whichever battery was selected for starting becomes the one that the alternator is charging. All return current runs through the engine block and alternator case to complete the circuit — current flows through the whole circuit, and voltage is consumed. (Starter motors similarly use the case as a grounded connection.)
When we upgrade an alternator to provide greater charging capacity, we can alter the current path to best suit the increased current flow. In most cases, directly charging the house bank with all available amperage and allowing for the minor charging of the starting battery afterward is the most efficient way of replenishing batteries.
However, several fundamental system changes have to be made to the output wiring from the alternator to meet required standards. Conductor size must be increased to reduce voltage drop and ensure safety, a calculation made from maximum expected current and distance along the conductor (and back). Overcurrent protection is now required for this conductor because we are taking it off the starter motor terminal. Cranking conductors are not required to have overcurrent protection, but instead of sharing, we are providing a new conductor that goes to the house bank, so we must install a suitable fuse or circuit breaker. This protection is also required at Paul Mirto is a digital illustrator, longtime boater and former Coast Guardsman. mirtoart.com