Pete's Solar Energy Experiemnt

December 3, 2015, quick update:
Harbor Freight still sells this kit. When it is on sale, it can be bought for as little as $139.95

Contents of this page:

Current Situation

Here's my setup (as of March 16, 2008). The only things missing from these picture is the car battery, which is in the basement, and a box holding some big rheostats that I now use to test the "MPPT" idea.
I didn't say it was going to be pretty!

Solar Panels: I can change the tilt by adding or subtracting 2 X 4's and 4 X 4's from under the back edge of the support. The panels are tied down to the concrete blocks with a piece of baler twine so they won't blow over.

Data Acquisition Unit: This DAU has 4 analog channels and 2 digital channels. This model hooks up to a serial interface, but now they have a USB model, which I'd recommend, instead. It is connected to the controller interface with a piece of 4 conductor phone wire.

Controller, Controller Interface and Load Bank: Again, I didn't say it was going to look pretty! This is about the fourth iteration. Maybe the twentieth?
This controller is the third one I have had. I don't think they can stand the stress of removing the collector input while the sun is out. This controller is a later model than the one in the picture at the top of the page. This is a pretty nice controller, but it is NOT a "MPPT" (Maximum Power Point Tracking) controller. The most notable difference between this model and the original is that this one has a digital volt meter on the front. It is connected directly to the battery terminals.
Internally, the positive side of everything is common, not the negative side. That's why I had to build my interface with the voltages going negative and the current going positive.
The controller interface board is contained within the little black box in front of the controller. On the top you can see a switch that disconnects the the battery from the DAU, and, if you look closely, just behind the switch, you can see the 0.1 Ohm 1% power resistor that is in series with the collector panel to measure current flow.
In front of the controller on the little table sits the Load Bank. It consists of 4 16 Ohm 75 Watt wirewound resistors that can be connected to the little black box in front of them in almost any configuration your little heart desires, from 4 Ohms to 64 Ohms. The switch on that black box allows me to turn the load on and off without having to pull plugs out of the box. I almost always use 8 Ohms, for now. This gives the setup a power dissipation of about 21 Watts at "normal" panel output voltage when current is flowing.
Finally, just to bug serious-minded folks, are the electrical schematic and the little interface PCB layout, almost readable.

Maximum Power Point Tracking Tests:
I don't intend to add an MPPT controller to this little system, but I do want to know how much more power I could be getting if I had one. So, I will change my setup so I can switch my charge controller in and out and replace it with some power rheostats. Then I can sit in front of the computer as it monitors panel voltage and current, changing the load resistance to see where the maximum power point is for any particular solar drive voltage compared to a similar power (wattage)output using the HF controller.
As of March 15, 2008, I have stuck 4 rheostats in a box and added them to the overall system. They are:
25 Ohms,
100 Ohms and
250 Ohms, all on a common frame, and
35 Ohms all by itself.
The reason for this combination is that
1.)I know from developing the "load bank" mentioned above what the approximate range should be, and,
2.)Those were some rheostats that were available at a surplus electronics place on the web.
This box is hooked in parallel with the panel input and controlled with an SPST switch.
To use it, I simply turn off the switch on the front panel of the HF charge controller and turn on the switch to the rheostat box.
This next image shows the overall wiring of the system at present. The image after that shows the voltage divider PCboard that goes between the charge controller and the DAU (because I couldn't get my CAD program, Turbocad Pro, V7,to save with enough resolution).

Schematic of Interface PC Board Between Controller and DAU

What's Happening Now:
The Harbor Frieght panels are hooked up and operating. I'm on the third charge controller now and it has a slight problem in that the controller locks the panel to the battery whenever the panel voltage DROPS to the battery voltage. The only problem for me is that I don't get to see the panel voltage drop as the sun goes down unless I turn the controller off, allow the panels to go to open ckt voltage, and then turn the controller on again.

I record 3 channels of information each day with the DAU (Data Acquistion Unit) hooked between my PC and the solar PV (PhotoVoltaic) system. The software that came with my $25 Dataq DAU allows me to graph the data that I store, albeit in a limited manner. I can port the data to Excel where I can rework it but, I just haven't gotten "aroundtoit" yet.
The DAU requires signals between zero volts and 10 volts. The inputs are not isolated from eachother. So, I had to design an interface between the solar system and the DAU. It is currently a passive voltage division and series current sensing system. To keep "wasted" current to a minimum, the current sensing channel is running on a voltage so low that the DAU is non linear in that region, but never worse than within 15% or so. That design is good enough for now.
Lastly, in defense of DATAQ, I bought this DAU as an entry level device in 2003 and never even took it out of the box. For the $25 it cost me, it is a great tool. They still have it, in the "serial" verson for about $25 and in the USB version for $50. REAL ones start at about $200, and they include better software.

Graphs

Here are 6 graph sets, if you are interested. Don't feel bad if they don't make full sense to you. The DAU I am using is somewhat simple-minded as is it's software. (IT WAS cheap!) I had to do some work-arounds to get it to do some of the things I wanted.

Again, here's how the graphs work: At the left of the charts you see the channel numbers, (1=1, 2=2 and 3=3). Just to the right of each channel number is the chart's title. Above and below that are the limits for that channel graph.

"1=1" is the Collector Voltage graph. The limits on the chart, .000 and -2.000, mean that the voltage is zero at the top of the chart and -20 volts at the bottom of the 1=1 trace area. The "X10C" simply means that you multiply the reading by 10 to get the actual Collector voltage. (The number .013 just above "X10C" is the reading that was at the black vertical bar that's about 4 squares in from the left.)

"2=2" is the Collector Current graph. The limits on the chart, .000 and .500 mean that no current is flowing from the collector panel at .000 and, at .500 you'd multiply by about 8.5 to get the actual current in amperes. Sorry about this, but that's the non-linearity of the DAU that I mentioned earlier. The "X10A" implies "multiply by 10" and that's good enough for you to get a general idea of what's actually going on. I use an amperage correction table that keeps my "actuals" to about 2%.

"3=3" is the Battery Voltage graph. Same everything as "1=1".

January 15th, 2008:
A totally sunny day. The voltage comes up as soon as the sun does, and about an hour later current starts to flow. The current gets up to about 1 1/2 amps, (middle chart) then slowly starts to decay about an hour and a half before the sun is gone. I say "sun is gone" because the voltage doesn't get back to zero until almost ALL the light in the sky is gone.
What you can't see here is that, during the day, I am usually (not always) adding a load to the battery to keep the controller from going into "overcharge" mode. If I let it do that, I'd not see how much current I could have generated. That, of course, is the whole reason for this experiment!
You see a few vertical bars on the upper voltage chart going all the way to the bottom of that graph image. That's the voltage going to "open circuit" when the controller shuts off because it thinks the battery is "full".
The lower chart shows that how the battery (a new car battery,not a deep cycle battery, which is what I should have) responds to the changes in solar input.