Many boats are equipped with 12-volt DC-powered refrigeration. While it’s convenient to leave the refrigeration operating when the boat is at a shore-power-equipped dock, if shore power fails or is disconnected, the refrigeration system will continue to operate, drawing power from the battery bank.
Some 12-volt DC-powered refrigeration systems are equipped with low-voltage cut-out circuits; however, the sensing circuit is present to protect the refrigerator’s DC/AC inverter and compressor from damage that could result from operation with insufficient supply voltage. In addition, the voltage at which the power flow to the compressor is interrupted may be low enough to drain the batteries, in some cases to a degree that can compromise battery life. Depending on how the boat’s battery bank is connected, it may be impossible to start the engine until the batteries have been recharged.
Fortunately, you can wire your circuit to disconnect automatically, so you need never return to a boat with flattened batteries due to failed shore power.
The necessary components are: a 120-volt, AC coil relay whose contacts can pass about 10 amps, a couple of fuse holders with fuses, and some insulated, stranded boat wire. As shown in the diagram, the relay coil is connected to the boat’s AC shore-power system through a 1-amp fuse and a single-pole, single-throw toggle switch. The normally open contacts of the relay are connected in series with a 10-amp fuse between the boat’s 12-volt system and the feed wire to the refrigeration system. When leaving the boat, the switch that controls power to the refrigeration system is turned off. At the same time, the switch that provides power to the relay is turned on. Applying power to the relay closes its contacts, connecting the 12-volt system to the refrigeration system by bypassing the normal refrigeration control switch. The refrigeration will operate as long as shore power is available.
Some boats are equipped with an inverter that will energize automatically when shore power is disconnected. On boats so equipped, it is necessary to turn off the inverter before leaving the boat. Should you decide to install this battery-protection circuit, be sure the AC circuit is not energized. Remove the shore-power cable from the inlet at the boat; you don’t want some well-intentioned person to reinsert the shore end of the power cable while you are doing the wiring. If the boat has a 12-volt/120-volt inverter, be sure to disconnect the positive battery cable that feeds the inverter to avoid being surprised by an unexpectedly energized 120-volt circuit.