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Ok so here's my idea to run backup 12V DC motor power, but I'm stuck on a part of the wiring.

120V AC Plug---> 120V AC to 12V DC Converter---> SPDT Relay (This is where I'm stuck. How would you wire a relay in the event that the 12V DC converter is not putting out any power so that power can then be drawn from a backup battery..I think you need a solenoid in there too but I'm not sure how)--->12DC Pump

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    \$\begingroup\$ Connect the relay coil across the output of the AC-DC converter. When the AC quits, the relay drops out and connects the battery. \$\endgroup\$
    – Dave Tweed
    Aug 27 '14 at 21:05
  • \$\begingroup\$ If the battery is suitable you can just connect the motor to the battery and the ACDC converter to the battery, through a diode. \$\endgroup\$
    – Vladimir Cravero
    Aug 27 '14 at 21:16
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You want something that runs off mains power when available, but when the mains power fails, it keeps running off battery power -- right?

Maybe it would be simpler and better to use a battery charger and a diode, rather than a relay:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The battery is a 125AH Gel Deep Cycle amazon.com/dp/B00ACNO2AO/…‌​&colid=DAVNBPJ9N92J&coliid=I2WWVMLKXTO6AN. The power supply is a Tripp Lite PR40 amazon.com/Tripp-Lite-PR40-40-Amp-13-8VDC/dp/B00006HS9D I read the Tripp lite(15A or higher model) can be used to charge batteries if you place a fuse at the name plate rating i.e. 40 amp between the two so current can't backflow and fry the unit. Wouldn't a diode serve the same purpose? Where could I find a 12V 40A diode and how would I wire it? \$\endgroup\$
    – AlligatorAndrew
    Aug 28 '14 at 4:48
  • \$\begingroup\$ I'd actually like to run two pumps off of the same power source. One pump says to fuse at 5A and the other pump says 7.5A max draw. The Tripp lite PR40 has 100% duty at something like 32A 13.5V if I remember so it provides plenty of power. I would need some sort of diode between the battery and the power supply but would I need one going to either separate motor? \$\endgroup\$
    – AlligatorAndrew
    Aug 28 '14 at 4:49
  • \$\begingroup\$ You can find dozens of diodes that can handle 14 V 40 A or more at several electronic parts suppliers. For example the MBR40L60CTG. (60 V; 40 A continuous). Good suppliers usually give a link to the datasheet that explains which pin does what. \$\endgroup\$
    – davidcary
    Aug 29 '14 at 13:10
  • \$\begingroup\$ Diodes do not serve the same purpose as fuses. You are more likely to get a quick answer to a question on this site if you hit the "Ask Question" button and post the question as a top-level "question", rather than in a "comment" attached to an "answer". \$\endgroup\$
    – davidcary
    Aug 29 '14 at 13:24
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When I was a kid My dad would turn the power off to the entire house which shut down my computer and my radio. So I devised a system where I used car batteries, a car battery charger, an inverter and a relay! You could use a simple system as well where you use a 110v relay When the 110v power drops the power comes directly from the battery. You might use a automobile relay(continues duty) based on the amps required to drive the motor. I used a separate 12v wall power supply to keep the rely on. When house dropped power my relay would switch.

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