Relay advice for switching off 12v loads when AC loads are disabled by inverter

[whatismouse]

So my inverter has a low voltage disconnect which cuts power to the AC output when battery voltage reaches a setpoint (in my case I have it set to 48.5V).

But my DC/DC converter which I use to power my 12V loads has no such disconnect.

I want to turn off my 12V loads at the same time as my 120VAC loads are cut. So one idea was to use a relay with a 120V coil. This way when the LVD cuts AC power, the relay will switch, and turn off 12V power as well.

I purchased this one: https://www.amazon.com/gp/product/B07FCJFGL9/ref=ask_ql_qh_dp_hza

Now I'm thinking its overkill because my dc/dc converter is only 30amps rated @12V anyways. And the current to run the coil in the relay, if it is several watts, means 100+ extra watts/day. Just to run a coil that I dont anticipate tripping but once every several months or so....

I could suffice with a smaller electromechanical coil that draws less wattage, but is there another solution that doesn't require activating a coil continuously with AC power?

 

[Hedges]

Solid state relay, controlled by a wall wart? (maybe a USB charger)

https://www.amazon.com/CGELE-SSR-80DD-3-32VDC-5-240VDC-Plastic/dp/B09HRX922Z

 

[whatismouse]

interesting, although I don't conveniently have an outlet near my inverter and no free breakers to add one either.

I looked for solid state relays with an AC control and a DC output, but couldnt find any. Wonder why that is? Every other combination I could find; AC/AC, DC/AC & DC/DC , but no AC/DC ?

 

[Hedges]

Gate voltage of FET is about 8V. (although SSR, "3V to 32V" has additional circuitry.)

If you have AC available to control it, use that AC to supply an outlet (cut end of extension cord?) and plug in a cube USB charger. That's 5VDC, perfect for this SSR.

Wired straight to inverter, if that can deliver too much current for the extension cord wire (in event of a short) use in-line fuse.
But having outlet there could be handy so just mount a box with one.

 

[SparkyJJO]

interesting, although I don't conveniently have an outlet near my inverter and no free breakers to add one either.

I looked for solid state relays with an AC control and a DC output, but couldnt find any. Wonder why that is? Every other combination I could find; AC/AC, DC/AC & DC/DC , but no AC/DC ?

Why not add an outlet that ties to an existing circuit?

 

[whatismouse]

so i could use one of these and cut the plug and wire it into the relay for switching?

https://www.amazon.com/Adapter-100V-240V-Charger-Recorder-Bluetooth/dp/B08C4MVLZ1/ref=sr_1_4?crid=2MXDNVM82V1DS&keywords=wall+wart+5v&qid=1671473400&sprefix=wall+wart+5%2Caps%2C240&sr=8-4

 

[John Frum]

The orthodox solution is one of these

BatteryProtect - Victron Energy

The BatteryProtect prevents the battery from being damaged. It disconnects the battery from non-essential loads before the battery is fully discharged.

www.victronenergy.com

The smart version has a Bluetooth interface that allows a configuration granularity of ~10mv.
The ac and dc will not be disconnect at the exact same time but may be close enough for your use case.
The lack of dependency on the ac circuit may or may not be desirable.
Just giving you options.

 

[Hedges]

Yes, or get a socket for that plug.

Or a USB cube, and cut a USB cable.

https://www.amazon.com/Charger-Charging-Adapter-Compatible-Samsung/dp/B08HQBM4JX/ref=sr_1_4

The orthodox solution is one of these

An alternate relay, this time requiring 24 –64V.
I was going to say required a different wall-wart to control it from AC, but has its own programmable DC thresholds, allowing it to disconnect higher or lower than the inverter.

I focused too much on the title, "switching off 12V loads"
What OP wants to do is switch off a 48V load, the 48V to 12V DC/DC converter. Or at least, he could do that rather than switching off 12V loads and leaving the converter powered.

So this should be a clean solution.

And the current to run the coil in the relay, if it is several watts, means 100+ extra watts/day. Just to run a coil that I dont anticipate tripping but once every several months or so....

Maybe with NC contacts, draw current from battery to open the relay during occasions of low battery. Energy savings, OK so long as time to recharge isn't excessive. Trick is how to get a signal which activates for low battery.

 

[TacomaJoe]

I just noticed that my GoWISE inverter has a USB outlet on it. How about yours? You might also consider a means of bypass to cover the case where the inverter dies, it would be nice to have some DC lights still working.

 

[Rocketman]

I used Victron’s Smart Battery Protect (BP-65) on my MotorHome to kill the 12v loads. I really like it. I go from 12v to 12v. They use almost no power. (Much better than the old relays that were originally on it).

They also make a 48v 100a unit.

I like that it is easy to shut down the house power, just a app on the phone. I also have it set to shutdown a little before my inverter. (The residential fridge is on the inverter), that way the inverter stays up a little longer.

 

[whatismouse]

The victron unit looks like the best option but for two downsides:

1. cost
2. the maximum disconnect voltage is 48V. I would need to be able to set it a bit higher...especially since my inverter wont accept my generator to charge batteries if they are below 48V

I also dont need all the features and capabilities of it, so I think $30 for an outlet/box/wallwart/SSR might just do the trick

 

[whatismouse]

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I just noticed that my GoWISE inverter has a USB outlet on it. How about yours? You might also consider a means of bypass to cover the case where the inverter dies, it would be nice to have some DC lights still working.

Mine has a usb (ethernet looking style) which I already use for the wifi module. Perhaps I could find a splitter and just use this...

I suppose I could put a simple switch which bypasses the relay for backup 12V.

 

[whatismouse]

oh lookey here: not only do I have a spare usb but it appears I have a relay already... (for auto-genstart perhaps?)

 

[whatismouse]

I'm a bit over my head now, but I'm not sure that I can use the dry contact signal to power a relay for my use-case. The way it is programmed, the NC will open in the case of low battery, but then it closes again when the inverter is completely off. I would need it to open for low battery, and open for off as well...

Also it still doesn't solve the power consumption constraints, because I never found a SSR with an AC coil/DC output.