A Photovoltaic Battery - Charge Controller

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The completed circuit board should be put into a weatherproof enclosure of some kind. Radio Shack's part number 270-224 does the job nicely.

USING THE CONTROLLER

Only four connections have to be made to install the charge regulator in your photovoltaic system. Looking at Fig. 4, you can see that the negative lead is common to all the components, linking the negatives from the PV array and from the battery. The other two solder points connect to the positive legs of the system. One goes to the positive output on the generator, and the other — on the resistor side of the circuit — connects to the positive post of the battery.

After you've installed the controller, you must adjust the voltage sensor so that it will switch the relay at the proper time. An easy way to do this is to start with a somewhat discharged battery and turn VRl fully clockwise, so that the relay contacts are closed and full current is going to the lead-acid cells. As the battery recharges, monitor the voltage across its terminals with a voltmeter. When the level reaches 12.6 volts, turn VR1 counterclockwise just until the relay opens. This puts the system on trickle charge.

Or, if you don't have a voltmeter handy, you can simply watch the cells as they charge. When you notice the cells bubbling, adjust VR1 just until that relay opens. This process is a little tricky, though, because a small amount of bubbling will occur before the actual gassing point is reached. Be careful not to mistake this natural gassing for the vigorous, rolling "boil" you're really looking for.

Unfortunately, the charging voltage of a battery varies with its temperature. The colder the cells are, the higher the voltage that's required to produce the chemical changes. Ideally, a controller would adjust to this automatically, but — in order to keep this project relatively simple — our device doesn't have a temperature sensor built in. Consequently, you'll occasionally have to adjust the controller's activation point manually to compensate for temperature . . . in accord with the chart in Fig. 5.

This setting isn't very critical, though, since a certain amount of error can be tolerated by the lead-acid cells. As long as you keep the battery reasonably warm and sheltered (as you should anyway), small temperature changes won't force an adjustment.

The controller I've described here will easily handle a full five amps of power and can do the job of commercial units costing $100 or more! Amazingly enough, you can build it yourself in a few relaxing evening hours, for less than $20. And with the device in place, you'll be able to rest assured that the battery for your photovoltaic generator is receiving just the right amount of current for efficient charging.

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