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Grid Powered Battery Charger - The Instructions
by Tom Woods from http://www.altenergyweb.com/gridcharger.htm

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Grid-Charger Voltage Switch Materials

PB1 1 reset switch (momentary, normally open)
SW1,SW2 1 ON time select switches or jumpers
C1 1 .1uf electrolytic
C2 1 10uf electrolytic capacitors
C3, C4 2 18pf ceramic capacitors
R1 1 2k 1/4 watt resistor
R2, R5, R6 3 10k 1/2 watt resistor
R3, R4 2 50k 10 turn potentiometer
R7 1 100k 1/4 watt resistor
LED1 1 LED 5mm T1
U1 1 78L05 5v 3 terminal voltage regulator
U2 1 PIC 16F676
XT1 1 32.768khz crystal
DIP14 1 14 pin DIP socket
1 Printed Circuit Board or breadboard
1 120vac Solid State Relay
1 120vac Battery Charger

Be careful not to accidentally reverse the polarity of the 12 volt dc supply, or you will destroy the 78L05 voltage regulator. When connecting the LED, the flattened side should face away from pin 9 of the PIC. Observe caution and electrical codes when working with 120vac. Protect the PIC from static discharges.

After building the circuit you need to adjust the low and high voltage setpoints. You will need a voltmeter and an adjustable power supply capable of delivering from 10 to 15 vdc. Follow these steps:

  1. To begin, turn the low voltage adjust pot, R3, fully counterclockwise.
  2. Turn the overvoltage adjust pot, R4, fully clockwise.
  3. Adjust the power supply to the maximum acceptable voltage for your battery bank. I chose 14.95 volts for my minimum battery voltage. This is just below the over voltage cut-off of my inverter.
  4. Connect the power to the board. Be very careful not to accidentally reverse the polarity of the supply. At this point, the LED should blink ON, then OFF.

    Troubleshooting If the LED does not turn ON when power is supplied, press the reset button to see if that turns it on. If it still does not blink, there is something wrong with your assembly and you will need to troubleshoot the cause. Inspect it closely with a magnifying glass for bad solder joints. Check if the PIC or LED is placed backwards or if pins are not properly seated. Test the supply voltage between pins 1 and 14 on the PIC to insure it is 5v. If you did not use the supplied printed circuit board, double-check all your connections against the schematic.

  5. Turn R3 clockwise until voltage at pin 8 of the 16F676 measures 2.5 volts.
  6. Adjust the power supply to the minimum acceptable voltage for the battery bank. I chose 11.95 volts in my system.
  7. Turn R4 counter clockwise until the LED turns ON. If you overshoot, back off 1/2 turn on R4 and press the reset button to turn off the LED. Then slowly turn R4 counter clockwise until the LED turns ON.

The switch is now set to turn ON at the low voltage setting and run for a period determined by the two jumpers, from 1 to 4 hours. If the battery voltage exceeds the high voltage setting, the unit will shut off. The reset button toggles the relay ON and OFF, and the LED will blink to indicate the number of times the unit turned on since the last reset. To adjust the length of time the switch stays ON, change the jumpers on the board:
SW1SW2 Duration
y y 1 hour
n y 2 hours
y n 3 hours
n n 4 hours

Once the adjustments are made you can connect the Grid Charger Voltage Switch to your system. Use small gauge wires to connect your battery bank to the battery terminals shown in the schematic. Use similar size wire to connect between the solid state relay control terminals and the pads labeled "relay" in the schematic. Make a power cable using #12 or #14 wire. Connect the hot (black) wire of this cord in line with the 120vac connections on the relay. Put a plug on one end of the cable and a receptacle on the other end. The relay, thus, becomes a switch allowing household current to pass or not, depending on the state of the relay. Plug the cable into a wall outlet. The other end will receive the chord for the battery charger. The photos below suggest one possible way you might choose to make the connections.

A Plug for a Really Good Battery Charger
My battery system consists of 10 12 volt batteries , 125 AmpHrs each. I chose the DLS55 charger made by Iota Engineering. These are excellent chargers, not the cheapest you'll find on the market, but they deliver precisely controlled current to your batteries. Mine has run flawlessly since I got it.

A finished assembly might look like this. Here, you see the orange power cord that plugs into a wall outlet. Harder to see, but coming out with the orange cord are two 18 awg cables that will go to the battery bank terminals. The outlet on the front of the box is the receptacle for the battery charger The red switch is the reset button and the white wall switch is an optional SPST switch to disconnect the battery power from the circuit board.

The metal box houses the circuit board, left, and solid state relay, right. Fastened to the cover plate are the switches and receptacle.

This is the grid charger voltage switch circuit built on a printed circuit board (available on last page).

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