Several years ago I began building my own wind turbines and
solar panels to provide power on my remote, off-grid property. A charge controller is an
essential part of any wind or solar system to ensure the batteries aren't over or under charged. The charge controller monitors the battery
voltage and switches the batteries off charge when they are fully charged,
and switches them back on charge when they reach a pre-set level of discharge.
This is a new and improved charge controller design based on the 555 chip.
So I set myself the goal of greatly simplifying my solar/wind charge controller circuit. I wanted to get it down to only one IC if possible, and reduce the number of other components as much as possible. I also wanted to make sure it only contained easy to find components that should be obtainable pretty much anywhere in the world. That way maybe more people would be able to build it without running into problems.
One of my friends suggested I switch to using one of the popular microcontroller chips and replace all the analog circuitry with one chip. That would certainly get the parts count way down. However, I was worried that the microcontrollers would be too expensive or difficult to obtain in some parts of the world, and too difficult for non-technical people to program. I decided to stick with analog circuitry for now, though the microcontroller option is a possibility for the future.
Here is the schematic of my original charge controller circuit.
The heart of the charge controller circuit consists of a voltage divider, two comparators, and an S-R flip flop. My original
idea was to redesign it using the LM339 Quad Comparator IC. I'd need two of the comparators for this circuit, and could make an S-R
flip-flop using the other two left over comparators on the chip. I played around with this idea for a while, and
even bread-boarded a few test circuits. I was having some trouble getting it to work right though. So I shelved the project for a
while and worked on other things.
One other project I as working on was a PWM motor speed controller for the pump I use with my recirculating sluice box
that I use for gold prospecting.
The speed controller uses a 555 timer chip. While looking at a diagram of the internal structure of the 555 chip, I was struck by how closely it
resembles my original charge controller circuit. Suddenly I realized I could redesign the charge controller circuit using the 555 chip and greatly
simplify the circuit and reduce the part count.
I set to work. In only a very short time, I had a working prototype circuit bread-boarded. It worked right the first try, which is rare for me. I almost always make
some sort of bone-head mistake wiring things up.
Here is a schematic of the new charge controller circuit. Click on it for a larger, clearer version.
The relay is a
general purpose SPDT automotive relay rated at 40 amps. It should be very easy to find. Get one from an auto parts store, or salvage one from a junked
car in a scrap yard. I have included a pinout for the relay for ease of connection. 40 Amps may seem like overkill, but it allows for expansion in the
future. You may start with only one small solar panel, then add a few more later, possibly a wind turbine, and a bigger battery bank. Eventually the
charge controller will need to switch some serious current. Why not build in the capability from day one? All other parts are specified below.
Most of the parts can be purchased at your local
Once I had the prototype working on the breadboard, I built another unit on a piece of Radio Shack Protoboard for use in the field.
It came together in only a couple of hours, and again, worked the first time (I must be living right lately). This more rugged version
will get mounted in a box and given a thorough testing in the field.
Once you have the circuit built, it is time to tune or calibrate it. I use 11.9V and 14.9V as my low and high set points for the controller. These are the points where it switches from sending power to the batteries to dumping power into a dummy load, and vice versa (a dummy load is only needed if you are using a wind turbine, if using only solar panels, the dummy load line can be left open).
Probably the best way to tune the circuit is to attach a variable DC power supply to the battery terminals. Set the power supply to 11.9V. Measure the voltage at Test Point 1. Adjust R1 until the voltage at the test point is as close to 1.667V as you can get it. Now set your variable power supply to 14.9V and measure the voltage at Test Point 2. Adjust R2 until the voltage at the test point is as close to 3.333V as you can get it.
Test the operation of the charge controller by running the input voltage up and down between about 11.7 and 15.1 Volts. You should hear the relay pull in at about 14.9 Volts and open at about 11.9 Volts. In between the two set points the controller should stay in whichever state it is in. The Charge and Dump buttons can be used to change the state of the controller when the input Voltage is between the two set points.
Before you write to me and tell me that my lower set point is too low and I am over-discharging my batteries, consider that the battery voltage isn't normally going to get that low except under load. If the load were removed, the voltage would recover over time back up to well over 12V. So the batteries aren't as deeply discharged as you might think at first glance. Also, the high voltage limit will be different for different battery types, like flooded vs AGM. Consult the spec sheet for your particular type of battery.
Once I had the circuit working, I mounted it inside a semi-weatherproof enclosure. The relay is on the left side.
I used a barrier strip to make wiring everything together easier. I used heavy gage wire for all the high-current connections. This thing
was designed to switch up to 40 Amps after all. I also included a fuse in line with the solar/wind input line.
Here is another view with the lid in place. I used this enclosure because I happened to already have it on hand,
not because it is the best one for
the job. For permanent outdoor use I would prefer to use a more rugged and weather-proof enclosure like I did for
my original charge controller design. However, I like the fact that I can see the LEDs through
the translucent lid and tell which state the charge controller is in at a glance, and I didn't have to drill any extra holes in the box for the LEDs.
This box will work for field testing purposes.
Here is a side view of the unit showing the feed-through barrier strip with all the connections to the outside. There are connections for the positive side
of the battery(s), the positive input from a solar panel or wind turbine, the positive side of an optional dummy load, and three ground connections.
Here is another side view showing the charge and dump buttons. The charge controller will automatically switch between charge and dump
when the battery voltage reaches the low and high set points. Between the set points the controller will remain in whichever state it
is in. These buttons allow me to manually toggle the charge controller between the two states.
Here is a photo of the first real field test of the new charge controller design. It seemed to be working good in my bench tests, but
I wanted to make sure it worked right under real-world conditions. So I set up one of my home-made
60 Watt solar panels outside my
workshop and used it to charge up a deep-cycle battery using the new charge controller. It worked great. The charge controller let power
run into the battery until it was fully charged and then switched to dumping power so as not to over-charge the battery. Perfect!
Here is a closer photo of the setup. The battery is a 36 AH deep-cycle unit often used in personal mobility scooters and motorized wheel chairs. I find
that they work well in small-scale wind and solar power systems. The Volt meter
is showing 12.64 volts on the battery, which is essentially fully charged. The battery was nearly fully charged when I started this test. It took only
a short time for the solar panel to top it off and the charge controller to switch over to dumping. A highly successful test.
Multiple solar panels and/or wind turbines can be connected to this unit. All the power sources can be connected in parallel and fed into the single input
connection. Each individual solar panel or wind turbine needs to have its own blocking diode though. Here is a diagram of a typical system with a wind turbine
and two solar panels feeding the charge controller. Typically an AC inverter is included in the system to power AC loads. Click on the image for a larger version.
I may develop a printed circuit board for this project, if time permits and there is sufficient interest. I'll post further updates on this project as it progresses.
Here is another view of the setup. As you can see, I have also included a cigarette lighter style plug for powering 12V DC loads. It is a
complete solar power system in one small (but heavy) package. All I have to do is connect a solar panel or two to it. I can't wait to try it
out on my next camping trip. I'll have plenty of power in the wilderness.
I have finally decided to ditch my old battery bank, which I had been lugging around on my wilderness outings for years. It was a bank of 14 smaller 12V
AGM batteries. I got the batteries essentially for free, so I wired them in parallel, put them in a plastic bin, and used them with my portable
solar/wind power system. The setup was very heavy and unwieldy. I kept telling myself I'd get rid of it and go with one big battery once
the little batteries started dying. They hung on for years and years. I must have been treating them right. Finally they started loosing capacity
and dying off one by one. So I went out and bought one big battery to replace them all. It is about the same size and weight as a car battery,
but it is a deep-cycle design, perfect for solar/wind systems. It has about the same AH capacity as the old bank of 14, but
is much smaller and quite a bit lighter. It only set me back about $200. If it lasts as many years as the old bank, I'll be very happy. My back will
also be happy that I don't have to lift the old bank of 14 batteries anymore.
Just to clear up one detail mentioned in the video, this project was not actually created to be a contest entry. I had already had my "Eureka
Moment" about using the 555 to replace a whole bunch of parts in the original design, and was building the prototype before I even heard about
the 555 Contest. It just turned out to be great accidental timing. Hearing about the contest did spur me to quickly finish the prototype and get this web
page up in a timely manner though.