Hello,
As according to our discussion Charging current is decided as 1A to charge the battery and voltage is 4.3V. I build circuit as shown here https://postimg.cc/GTbYfXVv
Please have a look.
I got 3.9V as the output voltage but I want 4.3V. Also, I got 1A as output from the circuit to charge the battery. What to do to get 4.3V as the output?
What represents the battery and solar cells in that circuit?
All I see is a 5V linear regulator, connected to a 5V supply, with a transistor short circuiting the output and the regulator limiting the current.
You'd be better off selecting the adjustable LD1117 and using resistor values to give 4.2V out. Don't aim for 4.3V, as the battery will overcharge if the component tolerances make the actual voltage higher. Refer to the schematic on page 24 of the data sheet. If R1 = 120R and R2 = 300R, the output voltage will be 4.2V.
https://www.st.com/resource/en/datasheet/ld1117.pdfThe only issue with the suggestion above is what happens when it's dark? The battery will discharge via R1 and R2 and could also try to flow back through the regulator into the solar panel. A Schottky diode could be connected between the regulator circuit and the battery, but it would drop a small voltage so the battery will take much longer to charge. The voltage of the regulator circuit could be adjusted to compensate for the diode loss, but then there's a risk of overcharging the battery, as the diode will drop a much lower voltage, when the current is low.
Another possibility is a shunt regulator made with a transistor boosted shunt regulator IC, such as the TL431. Refer to the schematic in figure 31, on page 24 of the data sheet. Connect the circuit directly across the battery terminals, with a Schottky diode in series with the solar panel, to prevent back-feeding. If R1 = 8k2 and R2 = 12k, the output voltage will be 4.2V. The transistor needs to be rated to dissipate about 5W, which means it will require a decent heat sink.
http://www.ti.com/lit/ds/symlink/tl431.pdfThere's still the issue of disconnecting the battery, if its voltage drops below 2.8V, in accordance with the recommendation on the data sheet.