The post discusses a circuit method which may be used to automatically switch and adjust the stronger counterpart amongst the solar panel, battery and the grid such that the load always gets the optimized power for an interrupted error for operations. The idea was requested by Mr. Raj.

Technical Specifications

Your projects/ circuits on are truely inspirational and comes handy even to a layman.

I am also an avid fan of circuits and electronics but lacks any professional knowledge. 
Here is a case you could help me out: 
Suppose I have three sources of power to my home : i) From Grid ii) From solar panels and iii) Battery via inverter.

The main source of power is from Solar panel whereas other two are subsidiaries. Now the challenge is that my circuit should sense the load and in case more power is required than the supplied power of solar panels, it can take the deficient power from Grid, whereas if its vice versa, say more solar power is available then the remaining power is used to charge the batteries or given to Mains ( grid).

Also there is a condition that when NO Grid power or solar power is available the load is taken up by the inverter. Assume that normal household consumes 6 KWH of power daily can be taken as standard calculation for designing the circuit. 

Looking forward to a positive reply at your end.



The Design

6 KWH means approximately 300 to 600 watts per hour, implies that the solar panel, the inverter, the charge controller all should be optimally rated for handling the above mentioned load conditions.

Now as far as dividing and optimizing current from the solar panel directly and/or battery is concerned, it may not require sophisticated circuitry rather may be implemented using appropriately rated series diodes with each of the sources.

The source which produces higher current and relatively lesser voltage drop will be allowed to conduct by the particular diode in series while the other diodes remain shut soon as the existing source begins depleting and goes below any of the other source's power levels the relevant diode will now override the previous source and takeover by enabling its power source to conduct towards the load.

We may learn the entire procedure with the help of the following diagram and discussion:

Referring to the above grid, solar panel optimizer circuit, we can see two basic identical stages using two opamps.

The two stages are exactly identical and form two parallel connected zero drop solar charge controller stages.

The upper stage1 includes a constant current feature due to the presence of the BJT BC547 and Rx. Rx may be selected using the following formula:

0.7x10/Battery AH

The above feature ensures a correct charging rate for the connected battery.

The lower solar charge controller is without a current controller and feeds the inverter (GTI) directly through a series diode, the battery also connects with the inverter through another individual series diode.

Both the solar charge controller circuits are designed to generate the maximum fixed charging voltage for the battery as well as for the inverter.

As long as the solar panel is able to receive peak sun light it overrides the battery voltage and allows the inverter to use current directly from the panel.

The procedures also allows the battery to get charged from the upper solar charge controller stage. However as the sun light begins depleting the battery overrides the solar panel input and supplies the inverter with its power for carrying out the operations.

The inverter is a GTI which is tied with the grid mains and contributes in sync with the grid. As long as the grid is stronger the GTI is allowed to be sedentary which proportionately prevents the battery from getting drained, however in case the grid voltage drops and becomes insufficient for powering the connected appliances, the GTI takes over and begins fulfilling the deficit through the conneced battery power.

Parts list for the above solar, grid optimizer circuit

R1 = 10 ohms
R2 = 100k
R3/R4 = see text
Z1,Z2 = 4.7V zener
C1 = 100uF/25V
C2 = 0.22uF
D1 = high amp diodes
D2 = 1N4148
T1 = BC547
IC1 = IC 741

R3/R4 should be selected such that its junction geneartes a volatge which may be just higher than the fixed refernce at pin2 of IC1 when the input supply is just over the optimal charging level of rthe connected battery.

For example suppose the charging voltage is 14.3V, then at this voltage R3/R4 junction must be just higher than pin2 of the IC which may 4.7V due to the given zener value.

The above must be set using an aritificial 14.3 V external supply, the level may be changed appropriately as per the selected battery voltage

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