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 the point of connection with the battery (or batteries), as it is live at both ends. 

If the new cable has only its normal insulation, over-current protection has to be a maximum of 7 inches along the conductor length. If we provide extra protection in the form of sheathing, conduit or something similar, we can extend the distance to 40 inches. 

We may also have to install a separate grounded (negative) cable to the alternator to provide a more reliable and direct path for return current, rather than using the engine block and alternator case as a current-carrying conductor.

Roger Hellyar-Brook runs a marine consulting business, repairing and upgrading boats of all types. He has spent more than 40 years in the marine industry and is the former manager of the systems program at The Landing School in Arundel, Maine. 

Paul Mirto is a digital illustrator, longtime boater and former Coast Guardsman. mirtoart.com  

This article originally appeared in the November 2017 issue.