Checking a Dead Battery - SailNet Community
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Checking a Dead Battery

After sitting through the winter season, our 320 (which is only two years old) won't start on battery #1. It starts fine on battery #2 and on BOTH. Battery #1 is reading fully charged as is #2. Any hints or suggestions before we resort to a costly inspection?
Peter and Mary

Tom Wood responds:

Dear Peter and Mary,

Electrical detective work can be fun. The only thing that can be deduced from your description is that there is nothing wrong with the starter. We don't even know whether the batteries are wet, gel, or AGM.

Get a voltmeter—if you don't have one on board, you need one anyway. A digital voltmeter is nice, but a simple analog cheapie will suffice for light work. Turn on your battery charger and check the voltage at the battery terminals while they are charging—it should be between 14.0 and 14.4 volts. If not, your charger has malfunctioned on bank #1, causing your dead battery. Shut off the charger, start the engine, and test your alternator output in the same manner. If battery #1 doesn't show any charge and you have a diode (splitter) in your alternator output line, the diode may be fried, requiring replacement and some more detective work to find out what caused it to fail.

If all is well at this point, charge both batteries fully and put on your Sherlock Holmes hat. Let the batteries stand for at least eight hours after they have finished charging, then test the voltage on both again—it should be between 12.8 and 13.0 volts. If battery #1 has fallen off in voltage, it is probably damaged beyond repair—replace it.

If both batteries test well, set the battery switch on #1 and test the voltage on the common output terminal of the battery switch and again at the starter. If these tests show a voltage drop, start checking all the wiring and connections, both positive and negative, from the battery to the switch, the starter, and to ground. Clean and tighten these connections.  It is even possible that the battery switch has failed.

Batteries on boats can, and very often do, fail in a two-year span due to sulfation. Unless the boat is used very often, or has a good quality three-stage shore-power charger, or has solar or wind chargers keeping the voltage up, two years is about all you can expect from most batteries. See Don Casey's article, A Little Help from the Sun.

Tom Wood

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