Wiring backup pump power

Ok so here's my idea if one were to run a backup pump power on a growing system, 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 so that if the 12V DC converter is not putting out any power [I.E. home power goes out or unit fails] 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

Just in case you're wondering...well why go through all the trouble of designing this since the pump can fail too?

Water loop:

Reservoir -----> Filter ----->Tee1----->Tee1 side1 Pump1 ----->Flow Switch -----> Backflow preventer ----->Tee2 Side1 -----> Main line to plants

Tee1 side2 will go to a second pump that will be turned on in the event the flow switch receives no flow. The line will then go to another backflow preventer and then back into Tee2 Side 2.

The backflow preventers will maintain pressure so that one of the pumps doesn't try to force water backwards through the other pump.

Another Tee can be added into the main line and two ball valves. One ball valve will turn off the line going to the plants and another ball valve will open a line going to the drain. You can use your pump to empty the reservoir for you =) If you really wanted to get fancy you could control it with timers and solenoids too and have your water change itself every week.

Thanks in advance for your input and if something other than a SPDT relay is needed let me know.

Why not invest in a UPS?.

A UPS would run for a few minutes, a 4amp 12v pump could run off a 120 ah battery for 30 hours if need be.

12v is also safer to work with and shurflo makes better spec 12v pumps for your application imo

Some larger UPS models have plugs to hookup an external deep cycle battery. Even if it's not setup, it's not too difficult to open case and re-route to external battery.

You would need to use a 12v solenoid with 2 normally open contacts and 2 normally closed contacts. After the 120 to 12 v converter, install the solenoid. Power it from the converter circuit. Wire the converter to pump circuit through the normally open contacts(+and-) and put the battery back up through the normally closed contacts(+and-). Tie the two separate circuits together after the solenoid and add any controls you want downstream from this point.

This will keep your circuits separate and in the event of a power or converter failure, will switch to 12v backup without missing a beat. No need to add check valves, tees or anything else unless you intend to add a second back up pump. That didn't seem to be the case the way I read your question.

*edit* Came back and read the question again, a little less buzzed this time. The part about the second pump settled in so I'll add this...Instead of connecting the two circuits together after the solenoid, just connect the battery circuit to the second pump, add whatever controls you want to this circuit and you're good to go. Also, because you will be using two separate pumps, you can use a solenoid with just one normally open and one normally closed contact, only passing the positive legs through the contacts.

Droopy the Double Pole Double Throw relay is what I was trying to think of.

To my knowledge no one makes a decent 12V UPS. I think one can be "made" though if the right components are chosen. This actually seems more simple than the relay.

Tripp Lite power supplies of 15A or more capacity can be used to charge batteries. They say that you should use a fuse with the name plate rating between the two so electricity doesn't back flow and fry the power supply. Be careful when choosing the power supply since the 100% duty cycle is not the same as the name plate rating. E.G., the PR60 is rated at 60 amps but I believe 42 amps is the 100% duty cycle.

I think a diode could be used between the power supply and battery in lieu of a fuse. So the circuit would go:

AC -----> power supply -----> diode -----> battery -----> pumps

Just keep it simple and don't make things more complicated than they are. I have had the same pumps going for years now with no backup and never had one go bad. And I use ecoplus, the cheapest available. I just got back from 10 days away and really never thought about my grow.

SS I believe that you're using low pressure pumps correct? Aren't you also using some kind of media too? The pumps I was designing this for are 60-70psi and will not be sitting in the reservoir.

What's more if someone were to use this design then they wouldn't have to worry about a day long power outage even if they weren't using any media. I've mentioned in other posts that my degree is in engineering and so while I do have a tendency to over engineer I think that spending an extra couple hundred on a system where there is a back up is worth it when someone could be losing a couple months electricity use/time/entire crop over a few hour power loss or a bum pump.

I do use low pressure but I do not use any media. I understand your thinking, and I was similar in ideas when I started growing. Then I learned to relax.