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Old 20-06-2005, 21:01   #1
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Outboard charging system

We purchased a new 9,9 eleclrtic start Merc for our 26 ft boat. Boat just went in last week after winter long refit. For the initial move of boat form launch marine to dockage marine I did not have battery cables connceted to motor. Motor was pull started. Upon reading thru manual later that evening, I see a caution note, Do not run motor without battery attached due to potential damage to altenator, Oh grude! The motor was run for maybe 20-30 min without battery connected. Will this actually damage charging system? How do I determine if it is ruined? I did subsquently hook up battery, battery charge was boderline, I was able to start motor once or twice. I did not have time to run motor to see if it would charge bat. The bat is now home and will charge it before going back to boat this weekend. Any thoughts?
Thanks Ted Beyer
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Old 20-06-2005, 22:14   #2
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quik alt' test

Test and record battery voltage (at batt. terminals) with engine /alternator “off” (resting) - expect to see <12.7V.
Start engine & retest battery voltage. Battery voltage should rise significantly (to >12.7V or even to 14V) with a good alternator output. This elevated voltage will decline slightly after engine runs a few moments, but still higher than the resting voltage.
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Old 21-06-2005, 17:33   #3
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outboard charge/lighting coil

You don't actually have a "real" alternator. What you have is a rotating maget passing by a coil. One end of the coil is "ground" and the other goes to a single diode.

The concern is that unloaded, it might be possible for the coil to develop a large enough voltage spike to exceed the diode reverse voltage rating. If you get any charge current at all (probably is only rated for around 6A max anyway) then you are good.

For long engine run times it is possible to gas a battery with such an arrangement if the battery is full. If you need to regulate the output you must use a shunt regulator, which regulates by passing current directly to "ground" without hurting the charge coil. Other types of regulators are series regulators in that they drop excess voltage acrosss the active switch inside and, therefore, are not suitable for loading the coil. A good shunt regulator will allow you to run without a battery and not worry. Of course a 24V lightbulb will also work.

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