marine 12V supply and wiring

This group has been most helpful on our re-wire job, thanks.
When sitting at the dock, the only 12 volt usage is the live well
pump. It would be real nice to have a little 12 volt marine quality supply and have its presence cut lose the live well pump from the batteries.
Doing this in a shop is easy, under a boat deck needs way better stuff. Suggestions?
Karl
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On 12/18/2010 10:49 AM, Karl Townsend wrote:

Two ways come to mind: 1) Put in a double pole, double throw relay with a 12V coil with the coil and normally closed contacts supplied by the 12V power supply. Connect the battery to the normally open contacts. Connect the pump to the relay common terminals. When the shore power has the power supply on, it will pull in the relay and power the pump from the power supply, no load on the battery. When the shore power disconnects, the relay drops out and the battery feeds the pump.
2) Use a power supply with about 15V of output. Connect the + on the power supply to the anode of a diode that will handle the pump current load. Connect the + from the battery to the anode of another diode (similar size to the first one). Connect the two cathodes of the diodes together and connect to the + side of the pump. Connect the negative of the power supply, battery and pump together. When the power supply is on, it will have a higher voltage than the battery (around 13V - 14V) and the diode between the power supply and the pump will conduct. The higher voltage on the cathode of the diode to the battery will switch it off, isolating the battery. When the shore power supply is off, the voltage on two cathodes of the diodes will drop until they are lower than the battery + voltage and the diode between the battery and the pump will conduct. The diode to the power supply will be off.
Method 1 is obvious how it works and is not dependent on the voltage of the power supply being above (at least 1.2V) the max battery voltage (with the engine off).
Method 2 has no moving parts. Take yer pick.
BobH
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Karl Townsend wrote:

Trickle charge the battery. The less stuff you put between the battery and the pump, the more reliable it will be.
--
Paul Hovnanian snipped-for-privacy@hovnanian.com
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Karl, I think your proposal is redundant. You need a marine intelligent battery charger that you always plug in when at the dock. Steve

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Karl Townsend wrote:

I wouldn't want the live well pump to be able to automatically run the battery down...
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Richard Lamb
email me: snipped-for-privacy@earthlink.net
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wrote:

Put a switch on the darn thing - if you don't want it to run the battery down, shut it off??
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