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Old 08-12-2017, 08:30   #16
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Join Date: Jan 2007
Location: Maine
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Re: Electrical question.....

Just a data point many don't consider.

A small Westerbeke 30B Three starter motor, mounted to my bench, will settle out at about 100A with no load on it at all, just spinning in free air.

To get that same starter spinning, mounted to the same bench with zero load on it, requires approx 370A, this is called the in-rush. When you connect this to a motor the cranking current will not be smooth and it would not be 100A. The in-rush, depending upon battery health, size and SOC, can easily peak over 400A and cranking current will pulse between about 250A and 150A for this particular starter. Of course the average engine will also go from loaded starter to unloaded in about 0.75 - 1.25 seconds.

You may be interested in this article.

Voltage Transients in
Marine Electrical Systems


Marine How To Articles
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Old 09-12-2017, 11:15   #17
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Re: Electrical question.....

Thanks Maine Sail.

I just spent some time reading the article that you linked and posted. Very helpful and goes right to the original question about transient loads/spikes.




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Old 09-12-2017, 12:14   #18

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Re: Electrical question.....

While the Marine How To is very interesting, I would note that the author says he's rarely seen transients above 15V or 20. Back when LED tail light assemblies were new in cars and LED nav lights were unheard-of on boats, I did a little research. And one of the largest suppliers of LEDS to the automotive industry stated explicitly that they did not offer any warranty on their products, at all, unless the circuits included proper transient protection.

This because they had documented normal "car" starter motors throwing 600VDC spikes and larger (bus and truck) starters, still in 12v systems, throwing 2000VDC transients.

So you can and should except 600VDC spikes in normal boat systems as well. Very short term (microseconds) spikes, but that's still a very long time, as seen by the solid-state components in radios, navigation electronics, and LEDs. (And the folks who make all that stuff often go on record as saying spikes are the #1 reason their equipment fails.)

While you're wiring...the circuits for navigation and other electronics could use spike protection on them, as well. FWIW.

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