Monday, August 10, 2009

Improvised Inverter

Many people are struggling with the cost of buying material things to help them prepare.

One of the most costly (depending on the size) is an Inverter.

An Inverter is a device which takes a DC voltage, (typically 12 or 24 volts) and converts it to 120 volts AC. There are 220 volt units out there, but we will confine this discussion to 120 V units because it is typically what most of us need.

Over at ABS Alaskan is a table of common household appliances and their average power consumption. The listings of power consumption (Watts) is general, and you should look at your own individual appliances to determine their actual power consumption.

Many auto parts stores sell inverters that plug into your cigarette lighter (or power port as they are often called today) and they tend to be fairly reasonable in price. I have seen them for anywhere form $15.00 to $75.00 depending on the power rating.

There are also many companies selling the same items on the web, and through catalogs.

Did you know there is an alternative to buying sometimes costly inverters? Many times you can get the items free. You may even have a couple laying around your house right now.

How many of you have seen, or use a computer UPS, or Uninterruptible Power Supply?

The UPS is basically a 12 volt DC battery with an Inverter attached. If you're interested in how a UPS works, How Stuff Works has a concise explanation.

Typically in my experience, many computer UPS units are discarded because the sealed lead / acid or gel cell (VRLA Battery) has worn out. They're no different from the battery found in your car, truck, or SUV. The Inverter is perfectly fine, but because people often don't understand what a UPS is, or what to do with a failed lead / acid battery, or where to get a new replacement battery, the entire unit is discarded.

If you have a few UPS units that need batteries, BatteriesPlus is often the easiest solution. Be aware that sometimes it can actually cost you more to buy a battery than to replace the UPS, so take that into account when making a decision. Another reason UPS units are often discarded.

UPS units come in sizes ranging from a few hundred Watts to thousands of Watts.

Frequently, the UPS is rated in VA, or Volt /Amps. It is easy to convert Volt /Amps to Watts in DC circuits. The definition of a Watt is voltage multiplied by current, or Volt/Amps. Hence if you find a UPS that has 500 VA listed on it, it is a 500 Watt UPS, if it was DC.

We aren't dealing with a DC system. Calculating the wattage in an AC circuit (resistive load) is much more difficult. Often in AC circuits, the peak of the current draw does not always follow the peak of the voltage in the sine wave (remember AC - alternating current is a sine wave). One must multiply the RMS (root mean square) voltage by the RMS current over each moment in time and take the average.

I hear some of you out there, "Great, what does that mean to me?"

I'm glad you asked!

It means 35.

Yes, 35. I know this will really tick off many math purists and by the book power engineers out there, but one of my professors taught me that if you don't have time to figure the crap out accurately, ball park it by multiplying by .35 or 35% and subtracting that number from the VA to get Watts in a resistive load system with a light to moderate inductive load. He said if you suspect there's a healthy inductive load (some motors and other devices draw more when they first start) add another 5%, so you would be multiplying by 40% and subtracting that number from the Volt / Amp to get your Watts. He also cautioned that in larger systems, this would lead to a larger errors, but the errors would be adding to the safety margin.

Here's an example:

You have a 500 VA UPS, and you want to know how much power you can get out of the the thing with a fully charged battery.

If you multiply 500 by 0.35, you get 175. 500-175 = 325 Watts.

If you have a motor that draws more power when it first starts (also called "inrush current") you would multiply by .40. You would get 200, and 500-200=300Watts, roughly.

So, if you go to the table at ABS Alaska, you see that an electric coffee maker draws roughly 200 Watts. You could run your coffee maker safely on the 500 VA UPS.

With a UPS this small, you could round up a 5 watt solar panel, say from the top of a highway call box, and a charge controller, (from inside the yellow box) and easily keep the battery charged. It would be ideal for powering a laptop or a radio. Oh, the thought about the call box? Its for after the world falls apart. Don't do it now. The cops kind of frown on that.

Great, I have a UPS, now what?

Let's take a good look at it:

This is an older 500VA unit made by APC.

This the rear of the same unit. You can click on the image to get a larger view and allow you to read the back. The cord on the left is the AC cord that plugs into the wall. The cord to the right is a surge protected outlet. It doesn't have power if the AC power is lost and the unit is running on battery power.

Notice the battery backup outlets. They're rated for 300 Watts! Right in line with what my professor was saying. To the left of the outlets, see the the button that says "Overload Protection"? This unit doesn't have a fuse. It has a circuit breaker kind of like your house. If you try and draw too much power, the breaker trips. Some less expensive UPS units do not use a circuit breaker and have internal fuses. Keep this in mind if you happen to get one and it doesn't work. Check for a failed fuse.

Yes, it is dirty. This one sits on my workbench in my garage.

This is the bottom of the unit where you access the battery.

Here's the lead / acid battery.

The battery removed. Notice the red and black wires that connect to positive and negative? These can easily be extended and connected to a larger 12v DC power source, say a car battery?

Keep your eyes peeled for those discarded UPS units, and you can score yourself a nice inverter!

Want to look for something bigger? Say 3000 VA? They're out there. Be aware that they are heavy. Some of these things are as big as your refrigerator. Check FreeGeek, Craigslist, and other places for stuff being given away!

A nice set of solar panels and some deep cycle batteries, and a few of these.....3000VA / 2100 Watt UPS. Replace the small gel cells with the large deep cycle batteries. Notice the difference between the VA and Watt ratings is only 30%. Not as much as a margin of safety....keep that in mind when wiring up an inductive load to that nice box you lucked out and got for free.....

Why only 30%? Computers have become much more efficient, especially their power supplies and hard drives. I suspect that the new ones don't have as big of a inductive load when they initially power up compared to older computers.

Oh, one thing I forgot to mention. In the small UPS units is an alarm. When the unit loses AC power and is running on battery it will beep at regular intervals. Open the UPS and locate the piezo buzzer and disable or disconnect it. On some units there is a switch to turn it off.

On the large units, there is a silence button to stop the beeping.


ErinAndBrad said...

Jeez Cat - I wish I understood this stuff, but I don't. I will have Hubby read this sometime cause it seems fairly straight forward. oh, wish I had some geek closer to us LOL!

Maitreya said...

Wow, great advice! Never really thought about it, but now it is obvious.
I'll definitely be keepin' my eyes peeled. Thanks for the tip.

Ken said...

...damn it Brother,how do you do it???

...guess what fellas,i got another,can't believe i didn't think of that...
Thanx Catman

Bitmap said...

Good idea.

When I replaced the gel cell in my home alarm system I shopped online and found the exact same battery from the same manufacturer (if you believe the pictures shown on the websites) for anywhere from $10 up to over $45. Different brands made for more variation. That is quite a spread.

Rhino said...

great post catman,I never thought about it because the load on a small one is not enough to run my chest freezer but they would be great for a lamp,laptop,small fan,etc..I will definitely be on the lookout for these at work, maybe even recommend that a couple be replaced...know what I

Anonymous said...

Duh!! Like everyone else, I missed thinking about this. GREAT post. Just what I need to run my coffee pot.
Thanks a bunch Catman.

HermitJim said...

That's why we keep you young whipper-snappers around...because of all the things you can teach us non tech types!

Great post, Catman!

Anonymous said...

It was certainly interesting for me to read the post. Thank you for it. I like such topics and everything that is connected to them. I definitely want to read a bit more soon.
GPS jammers

Nepalien said...

Hey nice blog...
I recently added 100 Ah battery to my 600VA UPS and that powers my computer for hours during power outage.