Turning the key or pressing start completes a control circuit. A relay or solenoid then connects the high-current starter circuit. The starter turns the crankshaft until combustion becomes self-sustaining. Once running, the alternator supplies electrical loads and recharges batteries through the vessel’s configured charging system.
A slow crank is a circuit problem until proved otherwise
Battery open-circuit voltage alone is not enough. Test voltage at the battery and starter while cranking, then measure voltage drop separately on the positive and negative paths. Loose lugs, corroded isolators, undersized cable, poor engine-earth connections and failing solenoid contacts can imitate a bad starter.
Alternator considerations
A standard alternator designed around lead-acid charging may overheat when connected through an unsuitable system to a large lithium bank that accepts high current for long periods. System design may require current limiting, temperature sensing, external regulation, a battery-to-battery charger or another manufacturer-approved approach. Alternator capacity, belt wrap, pulley load and engine idle output all matter.
Battery switches must be operated only as their system permits. Disconnecting an alternator from its load while running can cause damaging voltage transients unless the installation provides protection.
Work on 230 V AC generator and shore-power systems is separate from routine 12 V fault finding and may be restricted to appropriately licensed people. Australian boat electrical installations are covered by applicable legislation and standards, including the current edition of AS/NZS 3004.2 where relevant.
Related: No-crank diagnosis · Charging alarm diagnosis