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post #5 of Old 12-28-2007
Join Date: Jan 2007
Location: Seattle Washington area
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hlkaulfman Lee

It sounds like some “tech” is talking out of both sides of their mouth to you. Everything is not working properly; a 10 volt DC output off an alternator will not replenish a 12 volt battery. Others have pointed this out. Your intuition was correct. Your charging source (alternator) must have a higher voltage output than the battery it is trying to charge up.
Secondly if “everything is working right” then why are you being told by this “tech” to drop down to a smaller prop to increase the revolutions in an attempt to boost voltage of the alternator’s output if the alternator output is fine in the first place?
When the Nissan outboard is in neutral you are reading 14.4 volts. When you are in gear, and under power, is your engine’s revolutions faster than they were when the motor was in neutral? If so your alternator output should be at least 14.4 volts. The alternator voltage output should remain stable within a fine range. It should not drop down. As the outboard slows down and eventually stops during the shutdown process the alternator output will fall off –that’s normal. But the alternator’s voltage output should remain stable throughout the normal operating range of your engine: idle, reverse, forward and full throttle.
To figure your electrical load you need to calculate your total amperage draw. Your VHF, depth gauge instrument and tiller pilot should have a wattage rating stamped on them or on listed in the paperwork. Divide the wattage rating of each electrical consumer by system voltage (your case a 12 volt system). This will yield your hourly amperage draw for each electrical consumer; then simply add them all up to come up with your total electrical demand in amps.
For example if your tiller pilot draws 40 watts of power then 40 (watts) divided by 12 (volts) yields 3.3 amps an hour. In that case your tiller would draw 3.3 amps of power each hour at 12 volts. If you had a fully charged 100 amp hour battery (meaning it will deliver 100 amps for one hour or one amp for 100 hours) that battery would last approximately 30 hours and 18 minutes at that draw down rate (100 divided by 3.3 answer is in time).
You may want to calculate your total electrical demands and compare it to the output capability of your alternator to see if you have any capacity left. I suspect a 6 amp alternator is very small, perhaps too small to run your electrical system and charge your battery at the same time. You may end up investing in a battery charger and the necessary power cords to hook up to marina power to keep your battery charged up.



Last edited by Northbeach; 12-28-2007 at 01:43 AM. Reason: math errors-getting late
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