Ask SAIL: Battery Power Drain

I recently bought a share of an older boat, a Cape Dory Intrepid 9-meter, and put new 12-volt AGM batteries in her. Something, however, is draining the batteries.
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Peter Letson, Greenfield, Massachusetts asks:

I recently bought a share of an older boat, a Cape Dory Intrepid 9-meter, and put new 12-volt AGM batteries in her. Something, however, is draining the batteries. At one point, after a couple of weeks absence, they were down to less than one volt. I have traced out the wiring and found an unidentified object (see photo) between the battery selector switch and the distribution panel. The two lighter gauge wires in the picture connect to the output terminal of the selector switch and the ground. Power to the distribution panel has been a “sometimes thing.” My questions are: What does this thing do? What functionality would we lose if I took it out of the circuit and connected the selector switch directly to the distribution panel?

Nigel Calder replies:

You are looking at a solenoid, an electro-magnetic switch. When you turn on your selector switch it energizes an electro-magnet, which pulls together two heavy-duty points to close a circuit between the larger cables that supply power to your distribution panel. Given that the energizing side of the electro-magnet is connected to the output side of your selector switch, the solenoid should be disabled when the selector switch is off. It therefore should not drain your batteries when the switch is off, so this is probably not the cause of your problem. However, I can think of no reason to have the solenoid in the circuit (it duplicates the function of the selector switch), so I would take it out. If this does not solve the power drain problem, you definitely need to deal with it, as it will rapidly wreck your expensive batteries.

Solenoids normally serve as remotely operated switches in high-current circuits. Typically, there is a switch to supply power to the electro-magnet through the small wires, closing the circuit to the bigger cables. This way the big cables don’t have to be brought to the switching location. The most common use is in an engine-cranking circuit. In the old days solenoids looking just like yours were mounted independent of starter motors. Now they are mounted on the starter itself.

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