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post #1 of Old 12-27-2007 Thread Starter
hlkaulfman
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Nissan alternator problem

I need the help of someone that understands batteries and DC electrical systems.

Here's the deal.

I have a 6hp Nissan long shank outboard in the motor well of my CD25. I had an alternator (read coil with rectifier) installed. When I measure the output of the motor in neutral I get something in the vicinity of 14.4 volts at something less than full throttle. When I put the motor in gear the output falls to about 10 voltsat full throttle. After extensive discussions of this situation with Nissan tech support, I am told that everything is working properly and that at 10 volts the battery is being charged.

Will a battery charge from a source from which the voltage is lower than the current charge in the battery (i.e. will 10 volts charge a battery that is registering 12.4 volts)? It was always my understanding that it wouldn't but, I am not sufficiently certain to argue the point with a professional technician.

I am also told that, due to the size of the boat, the motor is unable to achieve the rpms for which the motor is rated. They suggest switching from an 8 pitch prop to a 6. The idea is that this will let the motor operate at higher rpms and the alternator to put out more power.

The alternator is rated at 6 amps. I realize that this is not much but, on a typical day of sailing I am not using much power on a C25. I typically motor 2-3 hours in a day of sailing. Shouldn't that put at least 12 amp hours into the battery while a handheld gps, listening to a VHF, a depth gauge, and tiller pilot would not draw near that much?

I hope there is someone out there that understands all of this stuff and can explain it until a novice can understand. I definitely need ?DC Systems for Dummies?. I'll certainly appreciate any help anyone can give. I'm "bumfuzzled".


If you have any thoughts, let me know.
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