DC->AC Inverter question

[force9BOAT]

Hello,

I'm looking for a DC->AC inverter that can also accept house-hold mainline power.  The idea is that the inverter output would come from the inverted DC until the DC battery voltage dropped too low and then automatically switch over to the mainline. then automatically switch back to DC after the battery has been recharged (by solar or wind). I'm sure there must be an easy way to do that without spending tons of money.  Can anyone point me in the right direction?

Thank you,

Rob

[MountainMan]

force9Boat,

I came to RE from the direction of similar systems used in RV's.  Most of the Xantrex and Outback inverters seem to be set up out-of-the-box to do somewhat the opposite.  In an RV, the idea is to always use power from the Mains if it is there (including charging from it), and if it's not there, then use battery power.  If all else fails, auto-start the generator.

I wouldn't be surprised if some of the more expensive Outback and Xantrex models can be setup to work as you described.  Really don't know, but that's one place you can start looking.

best,

jp

[Clifford]

Ohhh...

a basic UPS does just the opposite too.  

It feeds power to the batteries when there is AC available, and then uses the power when the AC is not available.

A couple of things to consider...

• Find out whether your utility company will give you a symmetric electricity buy-back.  In that case, you can just run a grid-connected inverter and use the utility company as a "battery".  You could potentially augment your system with a UPS if you need to be guaranteed 100% up-time, or setup a method to manually switch to a separate battery backup system if you loose power.
  
• If you are "grid connected", you should verify that your system can't bleed back onto the grid if the grid goes down.  Thus you may need to have a big-name inverter such as the Xantrex inverters.
  
• Most inverters have both low-power alarms and low-power shutoffs.  Perhaps you could utilize these.  For example...
  
  

Batteries --> Inverter --> Selenoid

The Selenoid would switch the house to inverter power when the inverter is powering it.

The Selenoid would switch the house to line power when the inverter was not powering it.

Ummmmm...

I'm not sure how to isolate the Selenoid so that it would only be activated by the inverter and not the line power.  Perhaps some strategically placed diodes/rectifiers would do the trick.

In theory, this would also isolate your power so that you would not leak onto the "grid" if the grid went down.  However, if your batteries were dead, you would have NO POWER during an outage.

The other thing to keep in mind....

Inverters HOG POWER, and the more circuitry that you add in the middle, the more power you loose.

If you have a small system, you might find benefits of doing things like running a separate DC circuit for lights, and then potentially have a method to recharge your batteries if they got low.

[force9BOAT]

Thank you Clifford and all for your help.  Sounds like this is a bit of a tough problem I'll have to keep researching.  My local power company does support "net metering".  Out of about 250,000 customers they say all of 18 are signed up for net metering, most using solar power.  But they also say none of them produce more power than they consume.  So none of them have any extra power to sell back to the grid.  The solar power only supplements grid power.  I guess what I'm driving at is how does one add in the supplement power (solar or wind)in order to reduce the amount of power drawn from the grid.  That is, without have to purchase some kind of $10,000 device.

I have a saltwater fish tank that requires bright lights 12 hours a day, a 100W heater and some small water pumps.  I was thinking it would be great if I could power the fish tank from my wind turbine I plan to setup but the turbine probably will not be able to supply 100% of the power needed so I need to be able to easily switch back to the mainline, preferably automatically.  I'll keep looking how to do this.

Rob

[force9BOAT]

Thank you MountainMan,

I justed started looking at the Xantrex and Outback web sites.  I'll keep looking.

Rob

[Clifford]

I haven't tried grid tied metering...

In theory you would use the grid as your battery....

The meter would spin forward at night (low wind) and backward during the day (high wind)....  

Depending on the size of your system, at the end of the month that you would probably still owe a bit of dough, as well as all ordinary connect fees (or, if you have annual averaging, you could average net surplus/loss over the entire year).

One would hope that the buyback/exchange would be 100% up to the point where there would be a net surplus.

But, with the grid tie, you wouldn't have to maintain a battery bank (mixed blessing as you might not have the ability to change over to backup power in case of a power outage and NO LIGHT and NO WIND).

My problem is that my panels don't generate enough power to run my inverter 24x7 so I try to keep it turned off as much as possible....

Another option....  not quite sure of the circuitry, but perhaps it could be designed relatively easily....

120V --> Battery Bank when batts < 11.5V

Solar/Wind --> Battery Bank up to 12.6 V or high resistance, then dump to your dump load (for wind).

Pull straight 12V off of your batteries to run the lights, heater, and pumps (without using an inverter)

Again, this would be designed so that you would be isolated from the grid (assuming you didn't have large amounts of surplus electricity), and thus you wouldn't have to worry about expensive grid-tie inverters and inverter loss and etc.

I think some of the UPS Systems work off a similar principle of feeding power into a battery bank while running an inverter and drawing power off of the battery bank, and thus have a 0ms switch-over time.

[nothing to lose]

Well I look at things backwards at times and often it works out very well, at least for me. So here's another idea from reverse.

How about you set up your system as all off grid for the stuff you want powered from the wind genny. Then to make sure the batteries never go dead and the system does not shut down, use a battery charger powered from the grid. Set up the charger system so it only comes on at say 11V and goes back off around 11.5V. Now if you have plenty of wind and keeping the batteries near full charge the grid powered charger should never come on. If you aren't making enough power with the wind and batteries get low around 11V everything is still working on the inverter and the charger kicks in to make sure the batteries don't go totally dead untill the winds pick back up again.

Would be far cheaper than a gridtie inverter probably. I think I would check the load on the batteries and see how many amps are used when everything is on. Do alittle math and figure the amps needed from a charger to maintain the battery bank at 11V when under full load. That is about the size (or a bit larger) charger I would use.

Ideally the charger should seldom or never come on, so it's not a power hog or waste, but there if needed for saftey of never letting the system shut down. Consider it as only backup power from the grid. Instead of having backup power when the grid goes down, it works the other way, backup power only if the batteries go low.

 Also of course if the grid goes down, your tank is still working, at least till the batteries go dead. But no matter what, if the grid is down and batteries go dead you would have to do something else anyway.

 After a bit of monitoring to find a good average for the power produced from the wind genny I would move loads around to ballance out with near full charged batteries all the time. So if the wind gennie powers everything except the 100watt heater fine, then I would move the heater to grid power.

Well that's how I would do it here myself, I know alot of people would not agree though. But the idea is I would be using all the power produced from the wind genny to power the loads, the charger only rarely used for backup to make sure the loads never shut down. A UPS in reverse?

As for any wasted power when the charger is running at rare times. First the power used by the loads you would have to buy from the grid anyway, so that does not count. Extra power used by the charger that is wasted, well how many years would the charger have to run to make the difference between a $200 inverter and a $1,000 gridtie inverter with just the cost of wasted power figured? The charger should rarely run, so the differnce would never be made back up. That difference in price pays for a second wind gennie to make more power later. Installing a second (or larger) wind genny later will make up any wasted power and produce even more.

[Dzisko]

Yes, such cheap way exists - an usual Back-Upses all their life easy switch to the grid and back to battery feeding with an usual relay.

We can extract it from some Ups circuits (you may find APCs Back-Ups schematics in files of Caballo) or design our own solution - I was solvingsomething like that a few years ago - if people are interested, I will try to find or redesign it.

That will look like an usual voltage sensor, power stage, and an usual relay - the only problem can be the speed of the whole process - depending of the kind of solution, it can be fast enough.

And we can ever sinchrinize the phase of the process, but that will make solution more complicated and in the most real cases is not necessary.

[henjulfox]

The Xantrex (Trace) SW series will do what your looking for - I run mine to pull from batteries if they have enough voltage but switch to grid when the batteries run low.

By my definition they are, however, very expensive.

-Henry

[jmk]

 Yes, the SW Xantrex will automaticly switch. I paid $1700 for mine. It will auto start a generator too when grid and batterys are down, plus it keeps your batterys from becomeing drawn down to far by chargeing them if your RE isn't keeping up. You can program it to sell,or just switch back and forth not feeding power to the grid. You can use it as a stand alone inverter also. If you don't want to dump your extra power in the grid you will need a sunt regulator like the Xantrex C-40.

[Nando]

ROB:

What you need is an UPS, Un-interruptable Power Supply with Zero time power transfer without any sine wave glitch.

Check APC and talk with the technical support and ask them what models have Zero time transfer switching from AC to DC battery inverter.

Quite a few of the APC UPS have Ferro-resonant outputs that store enough energy for the circuitry to detect AC drop for the internal circuitry to start the solid state circuitry to continue generating the AC. -- simple circuitry but somewhat a bit heavy --

There are other companies that do have zero transfer switching or very close to less than 1/2 sine wave if your loads can accept such switching.

Regards

Nando

[force9BOAT]

I like this idea.  The only question remaing is what kind of charger can be made to switch on at 11V and switch off at 11.5V ?

Rob

[Dzisko]

What a sense talk with them?

They respect nobody. People who not give diagram circuits, not leave a choise to their customers, only use their monopolist "repair centres", imagine what they will tell you.

They will tell you - then buy our full solution (including batteries that yes, will give you 1500 Watt during 15 minutes, otherwise that will not the full solution and they "will not responsible" and, of course, you have not right replace these with your big batteries without a risk or losing warranties)!

And this "full solution" even with your big batteries will not "legendary reliable" - heat sinks are too small - just to work sometime for a short time (no more than 15 minutes as a rule) after which "smart", and now - and "back" UPSes will shut down your computer - they designed just for that purpose.

So if you need really, not "legendary" reliable system, you have build it yourselves, and by the way - Upses have such logic - they use grid and switch to batteries only when problems with a grid appears, but our task another - we have always use batteries and switch to grid only when getting problems with batteries.

So as I said above - we can only extract detecting-n-switching part of their "secret" circuit, and redesign it to our purposes, and make better than they - that is not so hard to make better than they :-).

[Tom in NH]

I use a Xantrex 3000+ modified sine wave inverter with an array of PV panels (about 500-700 watts) and 1000 amp hours of deep cycle batteries. I had the problem of my batteries running dangerously low after multiple cloudy days with no sun. I created a grid charging circuit that does almost exactly what you wanted. I use a PIC microcontroller to monitor battery voltage. When it drops below a certain preset voltage, the microcontroller turns ON a solid state relay which turns on an IOTA 60 amp battery charger that runs off the grid. The battery charges for a set amount of time and then shuts off. It will also shut off if the battery voltage exceeds a certain preset (but that would probably never happen because of the superb quality of the IOTA charger). At the same time, the batteries are connected to my MPPT solar charge controller. The two charging systems run independently and do not interfere with each other.

The upshot is that when the sun shines my PV array charges the batteries. When the batteries drop down, the grid charger kicks in for a long enough time to bring the batteries up to a safer level. This will happen over again many times if necessary until the sun starts shining again and the PV array is able to keep the batteries fully charged. Ive been running this setup for over a year and it has been trouble free. It allows me to run a pretty heavy load on my batteries and I don't have to worry about them going dead. I get a free ride whenever the sun shines. I run my refrigerator, the ignition and blowers on my two propane heaters, the ignition and blower on my demand hot water heater, and the lights in my living room. I like to think of it as being a grid tie in reverse.

If you want more particulars on the circuitry and the PIC software, or if you'd like me to help you set up a similar system, feel free to drop me an email. woodsnh at isp.com.

--tom

[nothing to lose]

Do they make one that runs on batteries and only switches to grid if the batteries are low? That is kinda opposite of the way they normally work isn't it? I think this is what he wants to do, run on batteries first not grid.

If there are models that do that, batteries first, I might want to find one cheap used myself. Everything I have looked at so far though runs on the grid first and batteries only when the grid fails.

Although for a manual system, some UPS systems will run on external battery bank fine, so he could wire one up to run off batteries and only plug it into the grid when the batteries are getting low and unplug it when batteries are full.

Maybe put in a switch to turn on grid power to the UPS line at 11V and turn off power at 12V?

Did sort of that with a 600watt UPS manually watching batteries and when volts dropped plugged in the UPS so it switched to grid power and just pulling the plug out of the wall when batteries were charged and it switched back to battery power. Kinda a pain to have to watch it and manually switch it like that, but it did work well.

[nothing to lose]

I think allot of us would be interested in your set-up and charging controll circuit if you post it here.

 When I do this myself I do it manually. It would be nice to have a controll to turn the charger on and off as needed automatically. Also it might be good for another 120Vac to DC charger system I worry about using. Once voltage rises to full charge the simple charger does not shut off and voltage will just build till it fries the batteries. It has to be watched. Just simple caps to limit amps and rectifiers to convert to DC, no voltage limits though.

Sounds like your controll would be perfect for this type charger, on at 11V or below off at 12.5 or what ever. Would make keeping my portable battery bank charged up alot easier.

Thing to remember on any of the on off charging systems, most so called "Smart Chargers" won't work! They have to be manually turned back on after any power glitch! I know this because I used a few and anytime the power is off long enough for a UPS to beep in the house I have to go outside and turn the "SMART Charger" back on again. This is a real pain in the rear especailly durring rain or other bad weather. The charger is nice and dry in my truck, I am soaking wet in the rain turning it back on

[Dzisko]

Try this ->

http://www.otherpower.com/images/scimages/4882/v_det_n_sw.gif

[Tom in NH]

I need to draw up a schematic of the PIC and its support circuitry, and a diagram of how it works with the IOTA and the solar equipment. I'll try to make it a priority and let you know when it's complete. --tom

[Dreadstar]

A friend of mine has a remote Ham repeater site. What we did for it was install 2 110ah batteries and a single solar panel. problem is the panel will provide just enough power to run the unit all day assuming it gets decent sun.

So i tested a theory. A small trickle charger that has enough amps output to run the repeater in the even no sun and low battery power. It ramps up the draw from ac according to the panel output. Lower the panel output is the more it draws from the 110 vac. At minimum it is only drawing 6 watts or so. That is when the panel is charging the batteries.

Add in a small ups that can power the smart charger when the ac goes off. (which it does out there pretty often.) and this may be a solution for you aswell.

Will power your fishtank nicely.

one suggestion however would be to switch over to 12vdc pumps and lights. And build a current limiter ciruit for them. i am sure your windmill won't put out a constant 12vdc even using a large battery bank as a clamp.

If you can get your entire fish tank system on dc then you don't need an inverter. Much less loss that way.

[Waterfront]

That's quite a nice strategy!! Are you using this yourself?