Grid Powered Battery Charger - The Plans
Tom Woods from http://www.altenergyweb.com/gridcharger.htm
You get a really good feeling when you plug in some lights and appliances and power them with your own electricity from the sun or wind. You get a really bad feeling when you run those items and the clouds come for days or weeks or months on end. The batteries get lower and lower. Capitulation comes when you have to unplug those appliances and hook them back up to the public utility power. This circuit lets you keep the appliances plugged into your inverter. It won't make the wind blow or the clouds go away, but it will keep your batteries safely charged even if you have a generation shortfall. I call it the Grid Charger Voltage Switch.
The circuit utilizes a PIC microcontroller to monitor battery voltage and control a solid state relay, turning it ON for a time period when battery voltage falls below an adjustable low voltage set point and OFF if it exceeds an adjustable high voltage set point.
The circuit is extremely useful in alternative energy applications such as wind or solar, which experience long periods of calm or clouds when no power is generated to feed the battery array. The switch works in tandem with your charge controller. You charge your batteries using your wind or solar generation capacity just like you always do, but if the batteries get low because your wind or solar stops, this device will automatically turn on a grid powered battery charger to protect the batteries from chronic undercharging. It will permit you to run continual loads even when the sun doesn't shine or the wind doesn't blow.
I like to think of this switch as creating a sort of reverse grid-tie arrangement. The alternative energy system is connected to the grid, but instead of sending excess power out onto the grid, this arrangement uses grid power when it is needed to meet electric demand and keep the batteries charged when conditions are not good for energy production. Unlike a true grid-tie, this system will not make your electric meter run backwards, but you don't need the expensive equipment and your electric power will not stop when the public utility grid goes down.
Construction (Please Note: Revised March 4, 2006)
Build the circuit on a breadboard, PIC prototype board, or printed circuit board. You will also need to program the PIC 16F676 chip with the program downloaded from the link on the next page. If you purchase a circuit board or kit on the last page, you will find more detailed assembly instructions in the assembly guide link at left. The parts list gives all necessary components. You will need to size the battery charger according to the size of your battery bank. Likewise, the solid state relay must be able to handle the current draw of the charger you select. Minimum control voltage of the relay is 3vdc. Connect the relay in series with the hot leg of a 120vac line such that when the relay is turned ON, a connection is made between a 120 volt wall outlet and your battery charger. Connect wires between the battery inputs on the board and the battery bank's positive and negative and negative terminals.