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Old 08-29-2017, 12:08 PM   #19
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Join Date: Oct 2007
Posts: 19,901
"a charging voltage from the engine's alternator or battery charger and connects both batteries together so they both receive charging current. This is what the "Both" position of a typical battery switch does."

The problems come up when the BOTH switch is not rotated to seperiate the batts after engine shut down,

OR when rotated to OFF with the alt operating , and the switch was installed with out hooking up the usually built in field cut off .

"I don't understand how the solenoid protects the batteries, is there a diode built into these? And are they manual or automatic?"

No diode is used as the solenoid is open when the engine key is off.

The signal that operates the unit to close is power from the ACC position on the engine key switch.

A secured engine will usually have a low oil pressure alarm , which is enough to trigger the boat operator to turn off the key.

"The "solenoids" used by most builders (Cole Hersee etc.) are quite often not up to that duty and I replace a quite a lot of them that have failed."

The better boat assemblers will read the stamping on the units.

Most are rated 80A cont. duty , so they spring a second $20 cost and install 2 in parallel, if a 135 A truck alt or other big amp unit is installed.

A DN 50, 24V up to 300A is not usually a small boat item, and $20 units are not suitable.
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