This is another attempt at building my own battery charger and I want to run this design by the community before implementing.
Here's how I officially assign the parts:
G2 is a 9 or 12 volt solar panel that has a 20 to 100ma rating. C1 and C2 are 22uF each.
R4 is possibly 10K, but I might need to change it.
R1 and R2 is the op-amp's voltage divider. R5 is the output current limiting resistor. I might make it 1 watt 50 ohms. D2 is actually a 1N4007, not 1N4148. I might also make D1 1N4007 as well.
The opamp shown is LM358 but I might need to change it.
Here's why I think the design works but I post it here so I can be corrected if I'm wrong and/or missing something.
I configure R1/R2 such that R1 is a very low value and R2 is a very high value. This sets the voltage that I want the battery to end up with in the end.
When solar panel is next to full light the target voltage appears at the +ve pin of an op-amp.
Since fixed voltage regulators work best when its GND pin is connected to actual GND, I connected the GND pin to R4, a pull-up resistor that sets the GND pin to a high voltage level if the transistor doesn't turn on. This means that if the NPN's base is low, then the regulator shuts off and charging doesn't happen. This is why I connect -ve input to the battery being charged through the resistor R5. I'm considering connecting the -ve pin directly to the battery itself.
There will also be times where the solar panel does not receive light and that the battery is fully charged. Would my design work in all cases or could my design be improved?
The opamp shown here is LM358 but its bad for gain. Is there a substitute opamp I can use that's commonly available suitable for this purpose?