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Solved problems · Charging · Crew answer

Why isn't my boat's alternator charging the batteries?

If battery voltage doesn't rise with the engine running, check the belt, fuses and connections first, then test whether the regulator is sending field voltage to the alternator.

Symptom

With the engine running, battery voltage stays near its resting value instead of climbing to charging voltage, and the battery monitor shows little or no charge current. A charge light may stay on, or the regulator display may be blank.

Likely cause

Most charging failures are in the wiring rather than the alternator: a loose or slipping belt, a blown fuse or breaker in the output or regulator supply, or corroded connections. Next most likely is a regulator that isn't sending field current, or an alternator with worn brushes or failed diodes. Opening a battery switch, or a lithium BMS disconnecting, while the alternator is charging can also damage it.

Stop now if

The alternator, a cable or a connection is hot or smells burnt, or voltage climbs above about 15 V on a 12 V system. Shut the engine down and find the cause before charging again; overcharging can damage batteries and electronics.

Before you touch the wiring

Before tightening or disconnecting alternator or regulator wiring, stop the engine, turn the battery switch off, and disconnect the battery negative for the output cable. The output terminal is live from the battery even with the engine off. When measuring with the engine running, keep hands, tools, meter leads and clothing clear of the belt and pulleys, and don't run with the belt guard off.

The fix, in short

  1. Check the belt, fuses and connections first. Engine off and battery isolated as above: check belt tension and wear and look for black belt dust. Check every fuse and breaker in the charging circuit, including the regulator supply fuse, then clean and tighten the output, ground and battery connections.

  2. Measure battery voltage with the engine running. Compare resting voltage with voltage at a fast idle. Normal charging lifts it to roughly 13.5 to 14.6 V on a 12 V system, depending on battery type and charge stage. No rise means no charging.

  3. Check the regulator's power, sense and field wires. With the engine running, an external regulator should show battery voltage on its power and sense wires and a few volts or more on the field wire. Battery voltage present but no field output usually points to the regulator or its ignition/enable wire; follow your regulator's troubleshooting chart.

  4. Field-test the alternator only with care. Regulator manuals describe a brief full-field test that bypasses the regulator to show whether the alternator itself can charge. Voltage is unregulated during this test: switch off electronics, watch a voltmeter, keep it to seconds, and do not do it on a lithium bank. If the alternator still won't charge, have it bench-tested by an alternator shop.

  5. Protect the alternator from sudden disconnects. Never turn the battery switch off with the engine running. On lithium systems, make sure the BMS can stop the alternator safely, or that alternator protection is fitted, before the battery can disconnect.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Charging: Engine alternator with internal or external regulator
  • Batteries: 12 V lead-acid or lithium house bank

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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