Voltage drop

[rick j]

2003 DF140.
My battery was several years old and left me stranded on the lake, so I replaced it. I checked some voltages and now I'm not sure it's just the battery that was the problem.
The new battery before starting is about 12.3 volts. When I start the motor, it jumps to 14.4 or so at any RPM which I think is perfect. But when I turn on the nav lights while still running, (one small bulb in front and back and the dash lights) the voltage drops to about 13.5 and stays there. Shouldn't it still charge in the 14.4 range, no matter what I turn on? Should I suspect some trouble with the rectifier/regulator? I read a post that claims if the tach works, which it does, there's likely nothing wrong with the charging coil. Thanks for any suggestions.

 

[Grandad]

The voltage at the battery and generator should remain at a certain value, but your voltmeter (I assume) is located on the dashboard at some distance from the power source. As long as nothing is turned on, there is relatively no current flowing and the voltmeter will read the battery/generator voltage. When you turn on a load (even a small one) current now flows to and from your dashboard through the same wire that the voltmeter uses to sense voltage. Whenever there is current flowing, there will be a voltage drop, however slight. I think that if you were to use an accurate portable meter to test the voltage directly at the battery/generator, you would find that the voltage there remains fairly constant, while the voltage at various points in your electrical system will vary as current flow changes. I think 0.9 V drop is what you could expect depending of course on your wire (size) resistance. - Grandad

 

[bruceb58]

Measure your voltage right at the battery and see what happens when you turn on things.

 

[rick j]

I was reading the voltage off my Humminbird sonar/gps. I will take some reading at the battery and reply soon.

 

[rick j]

Checked voltage at the battery today. Pre-start was 12.4. With the motor running and nothing turned on, 14.78. Only dropped to 14.73 when I turned on the lights, and 14.69 with everything else on. You guys were right on, and I'm good to go fishing tonight.....