Making a Home-Made Inverter

I'd love to have the money for a large solar array, something that could gather enough electricity and be able to store it so that I could run most of my house off-grid. Maybe in a few years I'll be able to start building that dream, but for now I'll have to be content with puttering around with small-scale solar power.

Not that the small-scale stuff is useless; it's a great way to learn more about the individual parts and get some hands-on experience that may translate into good practices if/when I ever get a larger system set up. I have a 50 Watt panel hooked up to two deep-cycle, 100 Amp-hour, batteries at the cabin we built years ago, which is more than enough to keep our cell phones charged and run a few LED lamps when we spend a few days up there. The inverter my friend installed in the system will put out 120V AC for the few things we have that run on standard household power, but it wasn't cheap and it's a small one. I've been looking for a small system that I can play with at home, something to use as a test-bed for projects that I tinker with when I have the time.

While sorting through my various electronics and electrical toys, I've found a few that run on non-standard voltages and their chargers all use 120V AC:

• Handy-talkies that require an amateur radio license to use legally (I'm studying for one) have a 7.4V battery.
• The cordless tools that I buy use a 20V DC battery pack.
• My laptop computer runs on a 19V DC battery. 
• There are a few others that fall into the same category of “useful but not available in my standard voltages”*, so I've been looking for a way to keep them running. 

The ideal method would be a solar panel hooked up to a battery bank with an inverter to output 120V AC so I can use the chargers that came with each device, but those are expensive if bought new. so it's time to scrounge around and see what I can find or modify.

* Like selecting a caliber of firearm, I try to standardize my electrical toys. I've settled on 4 systems that cover most of my needs:

• 1.5V DC: AA and a few AAA batteries to feed the flashlights and small radios. Common and cheap, with good rechargeable options available.
• 5.0V DC: Standard USB power for charging cell phones and electronics. Easily converted to by using car chargers and “wall warts”. Rechargeable power packs are cheap and easy to find, we've reviewed a few over the years.
• 12V DC: Car battery power as well as the standard output for home solar arrays. Deep-cycle batteries aren't cheap, but will last for years if maintained properly. A deep-cycle marine/RV battery will work longer than a car battery due to differences in the internal construction making them more tolerant of deep discharge, but giving them less surge (starting) capacity.
• 120V AC: Standard household power, not as easy to convert to as the DC choices, but it comes in handy for running the tools and toys that you can find in most stores.

There are other options that I've chosen to avoid, like the lithium-ion rechargeable batteries for some of the higher-priced flashlights (18650- 3.6V) and the higher voltage AC (208/230V AC and 3 phase power) that I have limited need for and can survive without. I like flashlights but I don't need one that can be seen from the International Space Station, so the simple LED lights that I use are just fine. Likewise, the only things in my house that use 230V AC are the air conditioner and the furnace blower motor. I can live without air conditioning and have back-up heat if the furnace is out. I'm looking into a 120V AC blower for the furnace to be able to run it off of my small generator, but that's a project for the future right now.

The computer techs at work recently upgraded several of our networked systems and they brought in a complete change of equipment. The PCs were too outdated to be of much use, but they also replaced the Uninteruptable Power Supplies (UPS) that several of them were hooked up to. I managed to snag one out of the dumpster and took it home to play with.

A simple UPS is a battery charger feeding a battery and an inverter at the same time. As long as 120V AC power is available, the UPS will keep the battery charged and the inverter supplying 120V AC to the outlets. If the main power goes out, the battery will feed the inverter without a pause or interuption. This means we have the basis for a simple home-brew solar power system, just minus the solar panels.

The battery in the UPS I found was dead and while I could easily replace it for less than $50, it is rather small at only 6 Amp-hours. With a few crimp-on connectors and some longer wires, I can hook it up to a standard car battery or marine/RV battery and get closer to 100 Amp-hours as the UPS will “invert” the 12V DC from the battery into 120V AC for the appliance.

Even after buying a new batter, I still got a combination battery charger and inverter for free. This inverter is rated at ~250 Watts, so it's not going to run a microwave oven, but is more than enough to keep a ceiling fan going at high speed (about 100 Watts at most). Charging a laptop or portable DVD player to keep the kids entertained after the sun goes down may be close enough to their version of normal to relieve some of the stress and anxiety in a SHTF situation.

Connecting a solar panel to the battery will charge it, but how fast it charges depends on the size of the panel. There is some electrical balancing involved in adding batteries and panels to a system (stay with all the same size/brand of battery and panel and you'll be fairly safe), but in a pinch I can at least keep some of my odd-ball electronics up and running.

For a better explanation of solar arrays, we posted a thorough article a few years ago. You can also use search for posts with the "solar" tag by clicking on this link, or this use the search box in the upper left corner to find our posts covering basic electricity.

Keep your eyes open for chances to repurpose things. Businesses toss out lots of stuff that is outdated but still works, and your local online sales sites or thrift stores may have used UPS units for sale at much lower prices than buying new.