I need some advice on my situation. I have searched for many days (weeks and months actually) and have not found anything that specifically answers my question.
I want to be able to use solar panels to keep a battery charged during the day for a project utilizing the Attiny. Because of space limitations, I am powering everything using a 4.8V NiMH, 350 mAh battery, which fits nicely in a fairly small project box. I purchased small 16-17.5 mA, 6V solar cells in the hopes that I could used them to trickle charge the battery, but unfortunately the sun only partially shines in the area where the project box is. My voltage readings are only about 3.5 V, and although I did not measure it, I assume that the 16 mA of max current from the solar cell is about half as well. One other thing I thought of after I purchased solar cells is that I really wanted for the charging to occur through the day so I had to make sure that the charging voltage would not go over 5.5 V (I think) since my components are rated at 5.5V max. I also wanted to periodically put the project box under a bright light and generate the maximum voltage charge to speed up the charging process by maintaining continuous light.
To solve this problem, I put put together the attached circuit after a bit of study.
The idea is to maintain a max of 5.4 volts on the component/battery side while also limiting the current to the battery through R2 at trickle levels (I = 350/20) after the battery gets to full charge. The ziener diode at the base of the bjt is meant to maintain around 6V even when the full light is shined on the solar cells so that the emitter side of the bjt never gets above 5.6. I realize at 7V, the ziener will probably not be able to maintain all 6v (I compute around 5.8, I can post my math if it is useful), but it should be good enough to turn on the transistor and source current to the battery at a voltage of greater than 5, less than 5.4. Of course, the mandatory shotky diode is there for when night comes along.
I purposely did not use an LM314 regulor, since I could not guarantee that the voltage at the output of the regulator would be high enough to charge the battery fully. This is because I chose to use 4 of the battery cells (for space reasons) to jack up the voltage and current to 12v, 35mA max at full light intensity (under the lamp), but I measured the typical setup to be around 7v, too low to maintain > 5 V (I think ) for charging and still use the lM314. My understanding is that the input to the regulator would have to be around 9v to get around the voltage values I require, and of course, I still have to add something to keep the clamp the voltage at below 5.5 when in full light.
I don’t consider myself an expert in these kind of circuits (my background is in digital design and software) so I would greatly appreciate any comments or suggestions for something better.