Last week I was cleaning the attic and I found an old Mustek 800Pro UPS I knew (and forgot) I had. It had no battery but It worked. I also had three 12V/7.2AH SLA batteries. Searching the net on this generator subject I noticed several similar builds that shared the same idea. They were using an inverter for AC output but I decided to repurpose that UPS unit I found. This is a controversial subject but using a UPS unit has its perks. I'll get to this part later on.
So I headed to a local hardware store and came home with the needed stuff. The main thing was the project box. It had to be able to contain everything, be sturdy enough to take the weight and provide some side panels for the external connectors. I found this blue tool box that eventually gave my project a name (very original, I know). The next steps describe the 7 days process. Total cost of the project (excluding my work) - around 180 USD. Cheap, silent and very useful.
DISCLAIMER: This project is NOT for people that do not have advanced skills in electric work and electronic circuits. While building this project LETHAL voltages are available on some parts of it. I take no responsibility for any injuries that may occur while building this by other people.
ADVICE: If you feel like taking this with you on a holiday trip, avoid getting there by airplane as you'll never pass airport security with this. (Good luck trying to explain the security officer that this box full of wires and stuff, with something that looks like a timer on the lid is just a glorified UPS unit.) :-)
Step 1: Layout and Planing
Inside this box I wanted to have the UPS electronic board along with three 12V/7.2AH SLA batteries I had around. The batteries will be connected in paralel for a total amount of 12V/21.6AH battery bank. Not much but enough tor light loads. I first dismantled the UPS unit and kept only the mainboard and the transformer. Then I arranged all the components inside the box. It soon occured to me that I had to arange things so that the weight get evenly distributed. As the box had a top -centered handle I had to arange things like the last photo of this step shows. Any other layout was out of balance and made the box impossible to carry around. Then I decided I have to reinforce the box as the bottom was soft and started to bulge a bit when lifted. See the next step.
The schematic shows the general layout. I'm no technical engineer but you should understand it. It's actually pretty simple. Like a LEGO game for grown-ups. Except that, let me repeat this, it all should be made by people with proper skills. The 220V live lines are LETHAL when touched. Not depicted here, the grounding wires were all preserved and connected to the input plug ground wire. This way, whenever a power surge happens, it can stay connected to any wall socket and use the grounding of the grid. When camping however no grounding will be available. (Unless you're willing to stick a copper rod 2 meters deep into the ground and connect it to any grounding wire of the generator.)
Step 2: Getting the Box Ready
Reinforcing job done, I got my good old DREMEL and started to cut out the power connectors and switches. This was a pig of a job. The dremel is too fast for cutting plastic and it melted everything in its path. It all went like I was cutting butter with a hot knife. No second chances. Just mark your shape and start the mess. Oh, be advised, the smell of melted plastic is horrible as always. Use a vell ventilated area when doing this. Try not to inhale the fumes.
The DC panel (the fourth photo of this step) needed some special treatment. I wanted to have a fan cool down the UPS board as I always noticed that MUSTEK UPS running hot wether its charging or providing power. That will shorten your battery life. So I used a 60 mm PC fan I had around. At first I wanted to have a switch for it to turn it on or off but then I remembered I had a small circuit I built a long time ago that was a thermal switch for fans. (insert here two days looking for it ). I found it and I decided it will be great as it had a thermistor I could use on the cooling radiator to read the temperature and turn the fan on or off accordingly. I cut out the fan hole and mounted a protection grill over it. The small fuse visible on top is for the DC plug.
Step 3: Assembly Time
All the electrical connections were insulated and verified. Do not take shortcuts when doing this. It's important that you make this part properly as dangerous voltages are involved and you or your loved ones may risk electrical shock when using this unit.
The entire assembly involved several undo operations as the space got cramped in places and I had to mount some things and then the ones on top of them. The last photo also shows the external battery connectors (two 8mm bolts) that were connected the the positive and negative of the battery bank. Those got two plastic spacers (one red and one black) indicating the polarity. I'll also put a plastic cap on them to avoid shorting them. Having external connectors is very important. You could put a solar regulator on those and use a solar panel to recharge the bank. You could also start a car off them if your car battery dies. Or you could just connect aditional batteries to increase the capacity of your bank.
Step 4: Lid Connections
The link is here: http://www.thebackshed.com/windmill/articles/PanelMeter.asp
The three LEDs are taken from the UPS front panel . The yellow one turns on when it's working in "offline mode" providing power to the power sockets. The green one turns on when it's working as an UPS. The red turns on whenever AC overload occurs or battery voltage gets too low (11.8V). Then the power output is automatically stopped to protect the battery. This is one of the great features of using an UPS as an inverter as you already have charging/generating/discharge protection in one unit. Be aware that these UPS units are not designed to run for extended amounts of time. Therefore I used a Pentium II cooler on top of the existing one as well as a cooling fan. As long as you handle the heat, you're fine. Oh, one more thing. The UPS had a lound buzzer that I discarded. The status LEDs are enough.
The lid (last photo of this step) was powered (bank voltage and LED connections to the UPS board) via an Ethernet CAT5 wire. This one had eight pairs of wires. Two were used as power supply (and battery voltage) for the voltmeter and the remaining three pairs for the status leds. A messy job involving a gluegun and steady hands (the rest of the unit was already completed and the lid could only be opened about 100 degrees). I will cover all these wires with a rectangular plastic cover to protect & hide them.
Step 5: Testing
I have a 5W solar panel I want to test this unit with but it's way too small to charge the entire bank. Maybe in a month or so, if the sun keeps shining. :-) The generator powered a small TV set with no problems. That TV was a 60W CRT type. I figure as long as I keep my load on this unit under 500W things will be fine. I'll edit this instructables and post the results here as soon as I run some real load tests.