I've looked for solutions and cannot find anything that's clear to me that accomplishes what we need.


  • 12V furnace 8A load (fan)
  • 120V to 12V adjustable power supply
  • 12V backup battery bank

The goal is to switch the furnace from the power supply 12V output to the battery bank when the power goes out (that is, no power from the power supply.)

I've set up relays to handle this switch over but they're not rated for continuous duty so they are getting hotter than I am comfortable with (just 15A BOSCH SPDT relays.)

All circuits are 12V, maximum 10A (furnace fan takes ~8A). I also have a low voltage disconnect between the battery bank and 'switching circuit' to prevent complete drainage of the batteries.

If I could find a continuous duty DPDT 12V relay or a DPDT SSR I'd have it sorted, but no luck (at least not at reasonable prices.)

What are your recommendations? Should I enlist a microcontroller (Arduino?) running off the battery bank that powers an SSR when the power supply has power feeding that power to the furnace, and when the power goes out it triggers a second SSR connecting the battery bank to the furnace?

Something like this would work, but I cannot find the right relays.

simple DPDT setup

Even an SSR-based setup. I wish I could find NC SSRs for SSR1.

SSR Based

  • \$\begingroup\$ Is there any reason why you cannot just use a big diode? There are also specific maximizer chips - why are you limiting your choices to relays? \$\endgroup\$ Feb 17, 2022 at 7:21

2 Answers 2


So 1 relay, but have the moving contact for the output and it switches between the two batteries.

There are relays used on cars rated at 10A, 20A or more.

  • \$\begingroup\$ Just realized you actually made this suggestion first, but the one above from @JRE has an image so shows up sooner. Very nice solution, thank you! \$\endgroup\$
    – revive
    Feb 17, 2022 at 17:08

All you need is this:


simulate this circuit – Schematic created using CircuitLab

That's just a single pole double throw relay. Cheaper than the DPDT. No Arduinos or SSRs required.

The battery is connected to the normally closed terminal of the relay. The power supply is connected to the normally open contact of the relay. The furnace is on the common terminal of the relay.

Look into automotive relays (for use in cars and trucks.) They have the needed DC current ratings for the contacts and the coils. They are made for 12V coil voltage.

  • \$\begingroup\$ Wow, that, is an elegant solution. One of those #facepalm, "Why didn't I think of flipping the relay contacts to the other side" moments. Very nice. I've found a continuous duty relay rated at 12VDC 250A for $20 !! So, that will surely do the job. \$\endgroup\$
    – revive
    Feb 17, 2022 at 17:07
  • \$\begingroup\$ Is there any need to isolate the ground on the power supply when it's powered down? \$\endgroup\$
    – revive
    Feb 17, 2022 at 19:11
  • \$\begingroup\$ @revive: Nope. No need. \$\endgroup\$
    – JRE
    Feb 17, 2022 at 19:17

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