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Author Topic:   Diagnosis of Electrical Power Distribution Problems
dwfrank7 posted 12-06-2010 04:15 PM ET (US)   Profile for dwfrank7   Send Email to dwfrank7  
I was trying to test the voltmeter on my 2004 [Boston Whaler MONTAUK boat]. I put the tester on the violet ignition wire and then lost all power to guages and to the motor. Anyone out there have a clue[?]
jimh posted 12-06-2010 04:50 PM ET (US)     Profile for jimh  Send Email to jimh     
Tell me what setting was used on the "tester," and I will tell you my theory of the crime.
dwfrank7 posted 12-06-2010 06:24 PM ET (US)     Profile for dwfrank7  Send Email to dwfrank7     
Found it, it was a fuse at the motor. I was having problems with my old battery losing water. So I put a closed cell Optimax battery in, and now the volt meter is only at around 11, even when the boat is running high rpms. Is this normal?
Jefecinco posted 12-06-2010 07:14 PM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
I assume you mean an Optima battery?

The short answer is that it is not normal.

There is no reason I know of for an Optima AGM to cause the charging volt meter reading to go below the normal output. What is the volt meter reading with the ignition on but the engine not running? That should be battery voltage. When the engine is running the volt meter reading should be alternator/regulator output.

I recommend you check all electrical connections between your new battery and your engine.


dwfrank7 posted 12-07-2010 12:13 AM ET (US)     Profile for dwfrank7  Send Email to dwfrank7     
Thanx butch! The volt reading is the same,which tells me bad conection or alternator, regulator. I'll check it out. Any tips on how to properly check the voltage coming from the engine? Thanx
davej14 posted 12-08-2010 01:08 PM ET (US)     Profile for davej14  Send Email to davej14     
11 Volts sounds like a bad battery cell. Pull the battery and charge it outside the boat. Recheck the battery voltage several hours after charging. If it is still low then check your battery warranty for a replacement.
Jefecinco posted 12-09-2010 10:56 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     

I concluded, perhaps wrongly, that the OP had recently installed an Optima AGM battery in his boat. If he has a newish AGM it is quite unlikely it has developed a defective cell but it is certainly possible and your advice to check the battery is valid. Because the boat's volt meter reads the same 11 volts when the engine is not running and when it is running I suspect the problem is either caused by a defect in the charging system (alternator or regulator) or by a high resistance connection between the battery and the charging system.


Assuming the wiring has been checked and is good I believe your boat's volt meter reading when the engine is running will provide the engine's charging output voltage. However, the boat volt meter has a coarse and perhaps slightly inaccurate scale. For accurate readings a good hand held digital meter would provide better information. I'm not sure of the best method of use for getting the reading you want from a hand held meter. Hopefully someone with knowledge of your engine can provide a more useful option.


davej14 posted 12-09-2010 04:15 PM ET (US)     Profile for davej14  Send Email to davej14     
You can also get a pretty accurate voltage displayed on most fish finders. You may have to go to the options menu to select "display voltage".
jimh posted 12-12-2010 01:45 PM ET (US)     Profile for jimh  Send Email to jimh     
The voltage measured by the voltmeter contained internal to marine electronic devices like chart plotters or depth sounders is typically the voltage supplied to the device. It is the voltage on the branch circuit to which the device is wired for its power. This voltage is not necessarily the same as the battery voltage, and in actual practice is likely different than the battery voltage.

The voltage measured at the load in a branch circuit of a secondary power distribution will be lower than the battery terminal voltage due to voltage drops in the distribution circuit. The amount of voltage drop or difference from the battery terminal voltage will depend on the size of the wiring used in the distribution, the current flowing through those wires, and any voltage drops which occur at connections, fuse, circuit breakers, and switches used in the circuit. It would not be unusual for there to be a difference in voltage of 10-percent.

In establishing the state of charge of a lead-acid storage battery, a variation in the voltage measurement of 10-percent introduces a rather wide range of uncertainty. For example, if we assume the battery terminal voltage is 12.9-volts, a drop of 10-percent in the distribution would mean the voltage in a branch circuit would be 11.6-volts. If one tries to assess the battery's state of charge from a measurement of 11.6-volts in the branch circuit, the conclusion would be the battery was almost 90-percent discharged. In fact, a battery with 12.9-volts at its terminals has 0-percent discharge--it is at full charge.

Even with only a 5-percent drop in the distribution, a battery with a terminal voltage of 12.9-volts will be read as 12.25-volts. This indicates about 50-percent discharge. Again, a rather serious error in the measurement of the actual condition of the battery.

My initial question about the setting used on "the tester" which produced the unexpected failure of the power distribution has not been answered. I am still curious to know what setting was used on "the tester" when the power was lost.

jimh posted 12-14-2010 08:11 AM ET (US)     Profile for jimh  Send Email to jimh     
"Any tips on how to properly check the voltage coming from the engine?"

To check the charging system of an outboard motor, follow this procedure:

--obtain an accurate voltmeter, usually a digital multi-meter, with an accuracy of at least three-percent;

--connect the leads of the voltmeter together, and hit the meter zero function;

--connect the leads of the voltmeter directly to the terminals of the battery connected to the engine;

--note the battery terminal voltage when the engine is not running;

--start the engine, and increase the engine speed to a fast idle, typically about 1,000-RPM to 1,500-RPM. This moves the engine alternator into a speed range where it is able to produce some charging current;

--again measure the battery terminal voltage.

--compare the two voltage measurements; see below.

When the engine is not running, the battery terminal voltage should be in the range of 11.5 to 12.9-volts, depending on the state of charge of the battery. When the engine is running, the battery terminal voltage should increase to a voltage that is above 12.9-volts and perhaps as high as 15-volts. The amount of voltage increase will depend on many factors, including the state of charge of the battery, the ability of the alternator to produce charging current at the engine speed used, the regulation of the alternator voltage output, and the engine speed used in the test. Typically an alternator will produce more charging current (and thus increase the terminal voltage of the battery) as the engine engine speed is increased. The engine speed necessary to produce maximum charging current from the alternator varies, depending on the design of the charging system and the engine. Some engines will not reach their maximum charging output voltage and current until running at speeds of 2,500-RPM or more, and it may be difficult to measure the battery terminal voltage unless the boat is underway.

For information on the battery terminal voltage as a measurement of the state of charge, see

Battery Charge

These are my "tips on how to properly check the voltage coming from the engine."

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