Step 8: Build the charge controller
There are lots of controllers for solar and wind power systems. Anyplace that sells alternative energy stuff will have them. There are also always lots of them for sale on Ebay . I decided to try building my own though. So it was back to Googling for information on wind turbine charge controllers. I found a lot of information, including some complete schematics, which was quite nice, and made building my own unit very easy. I based my unit on the schematic of the one found on this web site:
That web site goes into a lot of detail about the controller, so I'm only going to talk about it in fairly general terms here. Again, while I followed their general recipe, I did do some things differently. Being an avid electronics tinkerer from an early age, I have a huge stock of electronic components already on hand, so I had to buy very little to complete the controller. I substituted different components for some parts and reworked the circuit a little just so I could use parts I already had on hand. That way I had to buy almost nothing to build the controller. The only part I had to buy was the relay. I built my prototype charge controller by bolting all the pieces to a piece of plywood, as seen in the first photo below. I would rebuild it in a weatherproof enclosure later.
Whether you build your own, or buy one, you will need some sort of controller for your wind turbine. The general principal behind the controller is that it monitors the voltage of the battery(s) in your system and either sends power from the turbine into the batteries to recharge them, or dumps the power from the turbine into a secondary load if the batteries are fully charged (to prevent over-charging and destroying the batteries). The schematic and write-up on the above web page does a good job of explaining it. Much more information on building the charge controller, including larger and easier to read schematics, can be found on my web site at http://www.mdpub.com/Wind_Turbine/index.html
In operation, the wind turbine is connected to the controller. Lines then run from the controller to the battery. All loads are taken directly from the battery. If the battery voltage drops below 11.9 volts, the controller switches the turbine power to charging the battery. If the battery voltage rises to 14 volts, the controller switches to dumping the turbine power into the dummy load. There are trimpots to adjust the voltage levels at which the controller toggles back and forth between the two states. I chose 11.9V for the discharge point and 14V for the fully charged point based on advice from lots of different web sites on the subject of properly charging lead acid batteries. The sites all recommended slightly different voltages. I sort of averaged them and came up with my numbers. When the battery voltage is between 11.9V and 14.8V, the system can be switched between either charging or dumping. A pair of push buttons allow me to switch between states anytime, for testing purposes. Normally the system runs automatically. When charging the battery, the yellow LED is lit. When the battery is charged and power is being dumped to the the dummy load, the green LED is lit. This gives me some minimal feedback on what is going on with the system. I also use my multimeter to measure both battery voltage, and turbine output voltage. I will probably eventually add either panel meters, or automotive-style voltage and charge/discharge meters to the system. I'll do that once I have it in some sort of enclosure.
I used my variable voltage bench power supply to simulate a battery in various states of charge and discharge to test and tune the controller. I could set the voltage of the power supply to 11.9V and set the trimpot for the low voltage trip point. Then I could crank the voltage up to 14V and set the trimpot for the high voltage trimpot. I had to get it set before I took it into the field because I'd have no way to tune it up out there.
I have found out the hard way that it is important with this controller design to connect the battery first, then connect the wind turbine and/or solar panels. If you connect the wind turbine first, the wild voltage swings coming from the turbine won't be smoothed out by the load of the battery, the controller will behave erratically, the relay will click away wildly, and voltage spikes could destroy the ICs. So always connect to the battery(s) first, then connect the wind turbine. Also, make sure you disconnect the wind turbine first when taking the system apart. Disconnect the battery(s) last.