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Author Topic:   Charging System Problems
dave123 posted 02-12-2007 12:42 PM ET (US)   Profile for dave123   Send Email to dave123  
I just purchased a poontoon boat with a 1996 Mercury 60-HP Bigfoot motor. The voltmeter on the dash doesn't work. I don't think the charging system is working right. I tested the voltage across the battery with the motor running and without the motor running, and both times it was 12-volts DC. Should the voltage be higher with the motor running to charge the battery? What [are] some of the electrical [components] that I can [check]? I don't think the flywheel was ever pulled. Thanks--Dave
davej14 posted 02-12-2007 05:55 PM ET (US)     Profile for davej14  Send Email to davej14     
Sounds like a problem. You should be reading higher than 12V if the battery is fully charged. What are you measuring your voltage with?

Suggest that you remove the battery and have it tested

jimh posted 02-12-2007 08:22 PM ET (US)     Profile for jimh  Send Email to jimh     
When there is current flow into a battery and the battery is being charged, the terminal voltage measured at the battery will appear to rise above the terminal voltage measured when the charging current is not being applied.

The charging system of a typical outboard motor consists of:

--source of alternating current
--rectifier to convert alternating current to pulsating direct current
--regulator to control the output voltage to levels suitable for use with a flooded cell lead-acid battery

A source of alternating current is explained in detail and in an extraordinarily clear and concise manner in an article in the REFERENCE section:

and applies to outboards which have the usual under-flywheel alternator arrangement. If you have an automotive style alternator, you can find very detailed explanations of how they work on websites oriented for automobiles.

The rectifier is almost always a solid-state rectifier. Often these are built into custom modules to suit particular mounting and heat sink configurations for particular engines.

The regulator can be either separate or combined with the rectifier. Again, these are often built into custom modules to suit particular mounting and heat sink configurations for particular engines.

My advice on diagnosis of battery operated systems:

--check the battery first
--check the charging system (see above for the components)

If you have the OEM service manual it will explain the procedures for check the components using resistance and measurement tests, however these procedures often assume use of particular measurement equipment which can influence the results.

bms1939 posted 02-12-2007 10:08 PM ET (US)     Profile for bms1939  Send Email to bms1939     
A good battery will have about 12.5-12.7 volts, depending on the motors charging system you should be getting a 13-14 volt reading with the motor running. I would have the battery load tested and check all the connections to eliminate the obvious. If this doesnt work I would bring it to a mechanic to diagnos your problem. It might cost you a few bucks to do this but trial and error can be expensive too. Any electrical parts are non-returnable if you get it wrong you own it.
dave123 posted 02-13-2007 09:37 AM ET (US)     Profile for dave123  Send Email to dave123     
Thanks for the replies
I'm planning to get the boat out of storage as soon as the snow melts.
I'm positive the battery was in good condition. I purchased it new last fall.
I'll make an appointment with the local mercury dealer to look at the motor
Thanks again guys
davej14 posted 02-13-2007 02:53 PM ET (US)     Profile for davej14  Send Email to davej14     

You didn't specifically say that you stored your boat with the battery in it, I hope that is not the case. If you go through the winter months without charging, unless it is an AGM battery, you will permanently lose some capacity. With cold temperature storage if the battery discharges significantly the electrolyte can freeze and cause mechanical damage to the cells, maybe also rupture the case.

A new battery can also be defective, as others have suggested, testing the battery is a good first step.

dave123 posted 02-13-2007 05:11 PM ET (US)     Profile for dave123  Send Email to dave123     
Davej14, I did all my testing with a Fluke 77 multimeter. I don't store the boat or my camper with the batteries in them. I keep all my wet cell batteries in my cellar up off the concrete on wood and rotate float charging them.
I tested the motor running and it was 12 volts dc. While the motor was running I disconnected the battery to see if it was feeding off the battery, the motor kept running. This tells me that the output is enough to supply the motor but not high enough to charge the battery.
This battery is a Wal-Mart Maxx deep cycle.
I'm going to put on deep cycle charger and then test it.
I want to get this problem resolved before the spring rush.
Thanks for the feed back
davej14 posted 02-13-2007 05:41 PM ET (US)     Profile for davej14  Send Email to davej14     
Your Fluke meter is indicating a deficient battery unless it doesn't display tenths of volts. You are only reading the surface charge of the battery. A valid test will require applying a load. Any of the major automotive supply stores should be able to test it for you at no charge. I would still start with the battery.

BTW, I would not run the motor without the battery in the circuit because this could damage the charging system.

Let us know how you make out.

Chuck Tribolet posted 02-13-2007 07:38 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The Fluke 77 is the Boston Whaler Outrage of voltmeters.
Mine displayed two decimal digits (12.61) when I put it across
the battery in my truck just now (engine not running).

If you are getting 12ish volts with the motor running, you
have a charging system problem.


jimh posted 02-14-2007 11:30 PM ET (US)     Profile for jimh  Send Email to jimh     
You need to measure the battery voltage to greater accuracy. See the REFERENCE article on battery state of charge for a conversion between battery terminal voltage and battery level of discharge for a flooded cell lead-acid battery:

HAPPYJIM posted 02-14-2007 11:43 PM ET (US)     Profile for HAPPYJIM  Send Email to HAPPYJIM     
Wouldn't hurt to check it with another meter either.
dave123 posted 02-19-2007 08:24 AM ET (US)     Profile for dave123  Send Email to dave123     
I tested my battery and it checked out 12.75 volts dc

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