Below you will find information troubleshooting marine charging systems.   We do not claim to be experts in all fields.  We cannot promise to be correct on all issues, nor can we address the content of these pages.  (Please read our legal disclaimer page)  But, I hope you will find this page useful.  We want to thank Richard Martin, Vice President of ARCO Marine Corp., for his technical assistance with this page.  Remember if you need technical assistance, we are only an email away.  We recommend that you get assistance from a trained individual, if you are not fluent with electrical systems.

One of the most important things to remember when troubleshooting marine electrical systems is to start your testing at the battery first.  Regardless of the symptom, the battery is the heart of the system and every electrical device depends on the battery to operate properly.  Boats are stored for months without charging the battery.  All batteries self discharge when stored.  Self-discharge rates are even higher when moisture is present.  Once a battery becomes discharged sulfation occurs.  Sulfation is a chemical action that occurs when batteries are stored for a period of time in a discharged state.  Hard, bulky, crystal-like substances are formed on the lead plates inside the battery.   This causes the pores in the plates to become clogged and useless.  Don't just throw a charge on the battery and expect it to perform properly.  I hear this all the time "the battery is brand new," "I just charged it up, it's good".  You can't assume anything when it comes to troubleshooting electrical systems.

In order to test a battery properly you must apply a load equivalent to 50% of its cold cranking amp rating for 15 seconds and maintain a terminal voltage of 9.6 volts.  This is the only way you can accurately test a battery.  The recommended C.C.A. rating for a battery that is used to crank most V8 inboards is 650 amps. In order to test this battery you would need to apply a 325-amp load on it for 15 seconds.  You need a tester that is capable of applying at least a 300-amp load to accurately test most batteries used in V8 Gas or Diesel inboards.  These will usually be a variable carbon pile load type.  On smaller engines like outboards, or 4 cylinder Gas engines, you can do this with a smaller handheld 100 amp load testers.  The smaller type of tester is available from most automotive stores.

Start your testing by checking the belts for wear and belt tension.  Many alternators have been replaced due to a loose or glazed belt.  With today's high amperage alternators, belt condition and tension are critical in proper alternator performance. Connect the voltmeter leads to the battery post, not to the cable ends.  Crank the engine and increase the rpm to a fast idle.  Observe the voltage reading.  On most systems the correct voltage will read between 13.8 volts and 14.2 volts.  Some systems with gel cell batteries may have a lower voltage setting of 13.5 and 13.8 volts.  You can use an ammeter to check for alternator output, it is an easy and safe test, but this cannot tell you the specific problem, only give you an indication of what the may be the issue.  If the voltage reading is below 13.5 volts, connect the positive voltmeter lead to the output post of the alternator and the negative to the ground post of the alternator.  You must be very careful to connect only to the output post.  If the lead touches the alternator case, it can short out your alternator.  Crank the engine and increase the rpm to a fast idle.  Check the voltage reading.  If the voltage reading is within range, resistance in the charging circuit is the problem.  This is a common problem with many marine alternator systems, especially if an in-line amp-meter is being used.  The manufacturer of the boat runs many charge circuits through plug-in connections for easy installation.  These connections work fine when they are new.  After time they tend to corrode and cause severe voltage drops.  When a standard alternator is replaced with a higher output alternator, you must replace the charging circuit wiring with the proper gauge wire to handle the higher output, otherwise you can cause a fire.  In order to test the output circuits for resistance you must discharge the battery somewhat so the alternator will charge at its full output.  The best way to accomplish this is with a battery load tester.  Another way is to turn on a few accessories for a short period of time.  Turn off the accessories and connect the positive voltmeter lead to the positive output terminal of the alternator and connect the negative voltmeter lead to the positive battery post, not the battery cable end.  Crank the engine and increase the rpm to a fast idle while observing the voltmeter.  If the reading goes above 0.2 volt the circuit has excessive resistance.  Connect the positive voltmeter to the negative output terminal on the alternator and the negative voltmeter lead to the negative post of the battery and perform the same test as done with the positive output side of the charge circuit. Most alternators today require some type of ignition or switched voltage in order to start charging.  If you are not sure how to test all the circuits on an alternator, call someone who is familiar with them and ask for help.  It can save you a lot of time, trouble, and expence if you do something wrong.

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Boatfix.com specializes in marine engine parts and boating accessories; including risers, manifolds, alternators, starters, water pumps, bilge pumps, pistons, gaskets, lifejackets ( PFD ), hoses for Mercruiser, Crusader, OMC, Volvo, Sea Maxx, Evinrude, etc. (Gas and Diesel)- you name it.  If it goes on a boat, it's in our catalog!