Systems+Engines

Ask Sail: Voltage Drop

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David McMillin of The Woodlands, Texas asks:

When I turned on my recently installed chartplotter, I got a “low voltage” message. Using a digital voltmeter, I measured 9.7 volts at the circuit breaker on the load side. Voltage on the battery side was 12.5 volts. I removed the breaker and measured the resistance across it. Each time I turned it on, I got different readings, ranging from 4 to 11 ohms. This breaker supplies power to several other electronic systems (wind, speed, backup chartplotter, etc.), and as more equipment is turned on the voltage gets lower. A check of the seven other breakers on the panel also showed various resistances. Any idea what could cause this?

Nigel Calder replies:

You are apparently dropping 2.75 volts from one side of the breaker to the other (12.5 to 9.7). Let’s say the chartplotter draws 0.5 amps. (They typically have a low power drain.) Ohms law gives us a resistance of 5.5 ohms, which is consistent with what you are seeing. If this is correct, there is high resistance in the breaker, probably due to corrosion or damage to the contact surfaces. The breakers are almost certainly not designed to be taken apart, in which case there is not much you can do about this.
You could try turning the breaker on and off repeatedly to see if this will clean up the points, but I don’t hold out much hope. The resistance when closed should be well under an ohm.

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