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Author Topic:   Diagnosis of Cause of Three Battery Failures
Don Russell posted 06-15-2013 12:34 AM ET (US)   Profile for Don Russell   Send Email to Don Russell  
I have just purchased a Mercury 60-HP FourStroke outboard and have a 650-Ampere cranking battery to start and run my Humminbird 998C HD SI Combo. Once I get to my fishing destination and start fishing I can only get about three hours of fishing in before my depth finder gives me an indication of low voltage and then my engine will not start. I have put three new batteries in and the same thing continues to happen. HELP


L H G posted 06-15-2013 12:49 AM ET (US)     Profile for L H G    
It sounds like the engine is not charging the battery. Take it in for a charging system check-up.
swist posted 06-15-2013 10:33 AM ET (US)     Profile for swist  Send Email to swist     
Although I would check what lhg said first, the other possibility is that there is some accidental high load on the battery from somewhere. If you don't have the instruments to test for this, you can disconnect one of the battery connectors and brush it against the terminal (that you just disconnected it from). With everything shut off, you should not get any kind of spark. Test is obviously better if not done in bright light.

3 hrs is not much time. If a load/leak is draining the battery it is not a small one. But then again, it's not so large that the alternator can't keep up with it when the engine is running, so you never notice a problem then.

Tom W Clark posted 06-15-2013 10:43 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
If the three hours spent fishing does not involve operating the outboard motor, it's difficult to blame the dead battery on the charging capability of the motor.
L H G posted 06-15-2013 11:57 AM ET (US)     Profile for L H G    
My thought was that is if there is no battery charging, the engine will draw down the battery firing the plugs, then the depthfinder draw further reduces the battery voltage so that there is not enough to fire the EFI system or trun the starter. A failed rectifier or voltage regulator could also be the problem.

With this happening to the THREE NEW BATTERIES, what else could it be with a 60HP engine? Is the engine new? If so, warranty will take care of it.

This winter I had this same thing happen to me on an L-6 Merc tower. The stator, which generates the AC current under the flywheel, had failed, and I had no current generation or charging at all. The engine was running off the battery reserve, which eventually would run out. The bad stator also killed the rectifier.

Doesn't a Merc 60 4-stroke have an automotive type alternator?

jimh posted 06-17-2013 01:47 PM ET (US)     Profile for jimh  Send Email to jimh     
A battery rated as having 650-Amperes usually means it can deliver that amount of current for a very short time if the battery is fully charged. A battery of that type is known as a starting, lighting, and ignition (SLI) battery and is designed to provide a lot of current for engine starting, but as soon as the engine is started the engine is supposed to immediately generate charging current and re-charge the battery.

Many outboard engines won't start unless the battery voltage during cranking is above a minimum threshold. Mercury engines are somewhat famous for this. Mercury usually specifies very large batteries for starting in order to give a generous voltage during cranking.

It would be unusual for a little GPS--Chartplotter--SONAR combination device to consume more than a few Amperes of current. Such a device ought to run for ten hours on a battery without really killing the charge.

I recommend you perform this procedure:

--disconnect the battery from the boat power distribution;

--charge the battery with a good 120-VAC-operated charger until it is at full charge. This may take more than 24-hours.

--take the battery off the charger, let it rest for 8-hours, then measure the terminal voltage. If the battery is at full charge the terminal voltage will be 12.9-Volts. You have to measure with an accurate meter, as 0.1-Volts is a big swing in charge.

--reconnect the battery to the boat.

--start the outboard engine

--go for boat ride of an hour or two

--shut off the engine and measure the battery terminal voltage; it should be right back at the initial reading you made above.

You can infer the state of charge from the battery terminal voltage. See

for details.

If the battery is not being maintained at full charge by the Mercury outboard, you have a problem in the charging system.

You are welcome to "take it in for a charging system checkup" but if you have any sort of familiarity with electrical device and own a decent voltmeter, you can assess the charging system yourself in about ten seconds. Connect the voltmeter to the battery terminals and start the engine. Run the throttle to about 1,500-RPM. You should see the terminal voltage on the battery rise to above 12.9-Volts. Typically you will see about 14.5-Volts. If you do not see the battery terminal voltage change when the engine is running, there is a problem in the charging circuit.

It is also possible to cook a 12-Volt lead-acid storage battery to death with too much voltage. If the outboard charging circuit is not properly operating, it can put out very high voltage, more than 16-Volts. This will quickly boil off the electrolyte in a battery. If you have a sealed battery, like a maintenance-free SLI battery or an AGM battery, you can kill them with too much voltage.

ASIDE to LHG: I don't think a bad stator could cause the rectifier to fail. More likely a bad rectifier caused the stator to fail. Or a bad regulator.

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