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Has this been discussed somewhere? Are there inverters that automatically start up when connected to 12V?

Rick Unplugged

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May 6, 2022
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Hi all, new member here, fairly solid in the basics around solar due to an electricity knowledgeable background, but new to actually building solar systems, so don't know all that much about the ins and outs of specific available hardware etc.

Hi Will, thanks so much for all you do! So much appreciated.

Question: Are there inverters that automatically start up, meaning, produce 240V, as soon as they get connected to 12V? I mean without manually pressing any buttons etc.

Reason I am after this:

I am running a chest freezer as a fridge; I simply use a relay to cut the power to the freezer when a temp sensor inside the freezer goes below the temperature I want. Boom, DIY "chest fridge". This has been working very well for a year and a half, and is consuming about 15% OF the power that a standup fridge of the same dimensions would use - thanks to the fact that due to the chest format, all the expensive cold air doesn't just fall out as soon as you open the door (as is the case with standup fridges), and thanks to the freezer having better insulation than fridges normally do. The compressor only runs for maybe 5 minutes every hour or so; up to 10 minutes on very warm days, which is rarely where I live.

Now I want to run this same setup off a 12V battery bank (powered by 3 x 160W panels) via a 240V inverter.

Most of the time, the chest-freezer-turned-fridge is the only device that will consume electricity from this solar setup, so the standby power consumption of the inverter would be much higher than I can accept. I can't have the standby consumption of the inverter 24/7 while most of that time no device actually needs 240V power.

So I want the inverter to start up automatically only when the fridge needs power, without me manually pushing a "restart" button or similar; and to shut down when the fridge doesn't need power.

So my questions are:

1. Are there any 240V inverters that are half affordable, that provide around at least 1kw or better 2kw constant power output, AND that power on automatically every time they get connected to 12V? Or do most of them do this? If a self restarting inverter like that exists, I can simply move the relay that currently cuts 240V to my chest freezer to between the battery bank and the inverter, and obviously replace it with a hefty high current relay.

2. If there aren't auto starting inverters - how can this best be achieved?

I can't have the inverter eating huge percentages of my solar power every day, while the freezer compressor only runs ten minutes per hour at most.

I would prefer not to convert the freezer to 12V - it is expensive, and also I like to keep things as much "off the shelf" as possible so they are easily replaced if / when they break.

Will, I couldn't find an answer to this question on your channel or web page so far - if I have overlooked that, please accept my apologies, I would appreciate a quick pointer to the right video.

Keep doing your great work! Keep being awesome everyone! Be well!

Rick
 
Yes. All inverters I have seen will begin producing power when connected at the battery cables if the power switch is already on.

What is your definition of half affordable?

What is the inverter you currently have? You can expect 5-20w idle from high frequency inverters depending on size.
 
Yes. All inverters I have seen will begin producing power when connected at the battery cables if the power switch is already on.

What is your definition of half affordable?

What is the inverter you currently have? You can expect 5-20w idle from high frequency inverters depending on size.
Hi, wow that was a quick reply, thanks heaps!

I don't currently have an inverter, I am in the process of selecting components.

By "half affordable" ideally I basically mean "the cheapest option that is unlikely to torch my shed through malfunctioning, and that is likely to run well year round for long enough that the next more expensive option wouldn't have been cheaper in the long run". Does that help?

I am hoping that I might be able to find a 2kW/4kWpeak-ish inverter that meets those criteria around the 300 USD mark at most, but don't know if that is naive to hope for.

For some more background of the project:

In a nutshell I think I'll basically build a system very similar or near identical to Will's 400W solar package, this one:


Except that I run 3 x 160W panels, and that I do need a solution to automatically switch power to the inverter on and off based on when the fridge needs AC power.

I will be running various small devices off the battery bank as well on occasion, however none of that will go via the inverter, it will all be 12V based. Think laptop- and phone chargers and a little bit of lighting on occasion.

I may eventually be running two (instead of just one) "chest fridges" as described above. They might occasionally switch on simultaneously by coincidence, which is one reason why I need a fairly hefty inverter, to make sure it can handle the startup current of both fridges at the same time, it can be rather high with some fridges. I am estimating that a 2000/4000W inverter should do that trick.

The other reason I need a 2000/4000W inverter is that I will on rare occasions need to run devices off the inverter that are rated around 1500W startup power draw / 1000W permanent power draw, for relatively short durations of time.

I don't want an all-in-one system, I want to put it together from separate 12V components, because I want to be able to replace individual components if / when they die, and because I want to maximize my chances to find such replacement components locally and in the long term.

As to the inverter on/off issue: If most inverters just start producing power when connected to 12V then I guess all I have to do is find a suitable high current relay, put it between the battery bank and the inverter, and trigger that relay based on the temperature inside the fridge.

Any suggestions for a suitable relay, or some device that may already exist that comes complete with temperature sensor?

Am I overlooking anything? Any suggestions in general? Very grateful for any insights and wisdom. I will pay it forward, I always do.

Cheers,

Rick
 
I'm planning on doing something similar with the lights in my shed and switching the power to the inverter rather than switching between the inverter and the light. Unfortunately at my place the 200w of panel and 20a MPPT controller can't keep up with the 120Wh of standby my inverter takes in a day.
 
The other reason I need a 2000/4000W inverter is that I will on rare occasions need to run devices off the inverter that are rated around 1500W startup power draw / 1000W permanent power draw, for relatively short durations of time.
How big is that freezer?!?! That's a HUGE running load for a residential freezer. I'm curious how many bodies you have stacked in there... :oops:

A snag you're looking to run head long into is the amperage draw from a 12v system. The math on that says 2000w/12v = 167a * .85% efficiency = 192a of draw and a 230a fuse. Most LFP batteries only have a 100a BMS and even lead acids are gonna struggle to provide that much current. Add in that according to the favorite amperage chart you're looking at 70mm2 wire which is pretty darned hefty.

Have you considered a 24v system? That gives you upwards of 3KW before you start pushing $Realmoney wires and fuses and lets you add a lot more panels later. Plus more of the 200Ah LFP batteries are available with a 200a BMS installed.
 
Most inverters have a switch built in for remote power up...
Just have the thermostat trigger that...

The 12V high current relay is a bad idea.
Every time the relay energized there would be a large spark, charging the capacitors, and causing damage. Eventually the relay would fail... usually failing in the on position.
 
Most inverters worth using will have a connection for remote switch on/off. No power use in the off state.
Some inverters have a standby mode that have very low idle consumption, these monitor the load and only switch on when there is a load present.

Mike
 
Put the remote start for the Inverter on a 12 volt DC timer and have it set to turn on the Inverter just 2 or 3 times per day.
 
If this was my project, switching the DC source on/off would be pretty far down on my list of things to consider. I would want to find a way to turn the inverter on/off. I think that could be done with the Victron, maybe not with off-the-shelf gear, but it could be done. I can turn the inverter off on my Multiplus while leaving the charger still on. However, Victron is not inexpensive. The standby draw I'm seeing on mine isn't bad.
 
It sounds like the best option would be to find something with a really low Sleep Mode draw that doesn't actually kick the inverter full on until there is a heavy load. I've seen ones that advertise 1-2w in Sleep Mode/Standby/Etc (marketing terms) and don't fire up until 50+watts of draw.
 
Wow, lots of answers and input, thank you everyone!

Replying as I'm reading

I would love to find a better solution than switching power to the inverter by relay. Looking into everything you all said.

First a quick thought: If I do need to switch power to the inverter via a relay, then I would probably need to place a hefty capacitor between the battery and the relay, otherwise every time the relay switches on the inverter, the inverter's capacitors charging up would put a big surge load on the battery.
 
Timselectric: Thank you, that's the kind of relay I was thinking of. Being built for 500A currents I am sure it will handle my at most 200A current to the inverter just fine.
 
Rednecktek:

It's a normal large chest freezer which should draw around 250 watts when running, and probably well over four times as much surge on startup of the compressor. The startup current is the main reason I need a big inverter; that, and the fact that on odd occasions I will need to run about 1500W devices off the inverter for short periods of time. Disclaimer: No bodies in any freezers or other places here!!

I am aware I will need about 200A when I run those high loads on occasion. I will dimension the battery bank accordingly. Currently looking at a 200Ah LiFePO4 battery that can deliver this kind of current. Yes, very hefty copper cables will be needed.

I have considered 24V, but have decided against it. I want to run a good handful of different devices directly off 12V. But more importantly, I want to be able to easily find replacement components for the system locally and into the distant future, such as inverters, charge controllers, etc; and those are more readily available and more common in 12V, especially locally. I will not need to expand this system, it is standalone for this specific purpose. If I decide to want more solare in future for other devices and purposes, I will build a second separate system.

You have a good point with availability of LiFePO4 batteries with 200A capable BMS. I can buy such a battery now, but if I need to replace it in future the availability might be an issue. Is there any reason I can't use e.g. two 100Ah batteries parallel, as long as I buy them at the same time and they are identical (=balanced against each other)? 100Ah batteries that can push 100A should always be easy enough to find long term.
 
Supervstech:

"Most inverters have a switch built in for remote power up... Just have the thermostat trigger that"

As long as that's an actual simple switch without any electronics integrated in the switch panel part, that could work. Do you know if these actually tend to be just a switch?

"The 12V high current relay is a bad idea. Every time the relay energized there would be a large spark, charging the capacitors, and causing damage. Eventually the relay would fail... usually failing in the on position."

Thank you for the warning, appreciate it. I am not dismissing what you are saying at all, but would like to understand this more. I am aware that every time I switch on power to the inverter the inverter's capacitors will charge up. However if a relay is built for say 500A at 12V, why would it suffer early failure when switching 200A?
 
Everyone:

Any thoughts on this?:

If I do end up putting a high current relay between the battery and the inverter that is a convenient way to trigger the inverter coming on in more than one way - for example, I can trigger the relay by temperature in one fridge, temperature in a second fridge, and a manual override switch in a convenient location when I need the inverter to come on for any particular purpose.
 
Mikefitz:

Thank you, see above - if it's an actual switch I could trigger it by temperature, but I can't manually push a switch, this must be automate based on fridge temperature.
 
Mikefitz, Rednecktek:

"Some inverters have a standby mode that have very low idle consumption, these monitor the load and only switch on when there is a load present."

"Find something with a really low Sleep Mode draw that doesn't actually kick the inverter full on until there is a heavy load. I've seen ones that advertise 1-2w in Sleep Mode/Standby"

That sounds like a great solution. Would you be able to point me in the direction of such inverters around 2000W/3000Wpeak, 12V, 220/240V? If the inverter actually meets its specs, 1500W/2000Wpeak would probably be sufficient for my specific case.
 
WMgeorge:

Thank you for your suggestion, but a timer based solution won't work in my case, the fridges need to have power whenever their temperature rises above a certain level so they can run their compressors to bring temperature back down again, for food safety reasons.
 
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