Lately I have upgraded by 12v battery system by adding an extra 150W solar panel to the existing setup, totaling my 12v system to 320W (STC). I just have a small limited number of circuits/load on this system and was wasting a lot of power through the Xantrex charge controller configured as a diversion load. On a sunny day, the charge controller was diverting power to a power resistor before noon, wasting power as heat for the whole afternoon.
To make better use of this power, I installed a 12v 1000W grid-tie inverter. The idea is to divert the excess power to this inverter once the batteries are full and use only the charge controller as a fail backup if there is no grid power.
The circuit is fairly simple and I tried to keep it that way! I always follow the KISS principle (Keep it Simple Stupid).
The circuit below is used to switch on/connect the inverter to the panels/batteries once a threshold voltage is reached and then disconnect the inverter after a pre-determined amount of time. I did NOT use a low voltage disconnect and opted for a timely disconnect simply because this will cause the inverter to connect/disconnect too frequently.
Basically, a reference voltage is compared to the 12v battery voltage using a comparator. This will bias a transistor, switching on a relay, connecting the inverter to the grid.
Supply for the circuit is taken from a 24v supply rail (another set of batteries & panels in my case) and this is stepped down to 15v using the 7815 linear voltage regulator. This 15v will power the whole circuit. Capacitor C2 is used as a charge reservoir to maintain a steady 24v supply to the 7815 voltage regulator. Diode D7 (Green) signals that there is a supply voltage on the circuit board.
I have used another voltage regulator 7810 to get a 10v supply. This voltage is divided by resistors R1 & R4 which will give me a 5v signal voltage. This voltage is made steady thanks to another reservoir capacitor C3. Capacitor C6 is used as a charge reservoir to maintain a steady 24v supply to the 7810 voltage regulator
The 12 volt detection is carried through Diode D4, split by potentiometer R2 and smoothed by capacitor C4. Any quick variations in the 12v system are simply filtered out through R2 and C4, thus providing a pretty steady signal to the opamp. This will eliminate any 'quick' variations to the supply rail such as switching on heavy loads or panels cloud effect or even the switching on itself of the grid- tie inverter.
The opamp is an LM324, and I'm using just one opamp out of the four available. The opamp is configured as a differential comparator.
The opamp feeds the base of a bipolar transistor Q1 - TIP122 which in turns powers the relay. Diode D2 protects the transistor from any back emf generated by the relay coil.
Diodes D3 and D8 safeguard the circuit from accidental reverse polarity.
The non-inverting input of the opamp is held at about 5v thanks to the R1/R4 voltage divider. The inverting input is connected to the 12v signal detection. As the battery voltage rises, it slowly starts charging capacitor C4, a 10000uF capacitor. Once the inverting input exceeds the non-inverting input, the opamp output switches on. Orange LED D6 is on. Capacitor C5 starts charging through diode D5 and this will provide a base current to transistor Q1, switching on the relay and connecting the grid-inverter to the grid.
Once the inverter is on, the batteries will start draining heavily and thus the signal voltage will lower down, The non-inverting input will become more then the inverting input and therefore the opamp output switches to low. This will not happen suddenly thanks to the R2/C4 configuration, thus providing some hysteresis. Also, once the opamp output switches to low, transistor Q1 will remain on for some time thanks to the D5/C5/R5 configuration, providing approximately 5mins of ON time.
The above is a photo of the finished circuit.