Solar Charging For Load Connected NiMH Battery

Discussion in 'The Projects Forum' started by josep01972, Jul 12, 2014.

  1. josep01972

    Thread Starter New Member

    Jul 11, 2014
    Hi All,

    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 the solar cells is that I really wanted for the charging to occur while the circuit is connected 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 want to be able 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 5.5. 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 Schottky diode is there for when night comes along.

    The Attiny software operates so that the device goes to sleep often through the execution loop (only about 17% awake) I believe this is what makes the trickle charge work, since the current requirements of the circuit will be very small most of the time thus allowing for the current to flow to the battery during the sleep time. During awake time, the current requirements can get as high as 70mA so I think during the day, on the awake portion of the execution cycle, the panel should provide 35mA max and the battery the rest ( Is this a correct assumption?) During the night, the battery provides all the current needs, of course.

    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 solar 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.

  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    You won't need the schottky, as the solar panel voltag is much higher than the battery voltage so nighttime reverse leakage current will be practically nil. Also your NPN will act like a diode.

    However you have another problem with the design, that a lot of power is wasted through a 200 ohm resistor into a 6v zener.

    First thing you need to do is check those figures. The solar cells will produce full voltage long berfore they produce any significant current.

    That value of 16mA output current will ONLY occur in full blasting sunlight with 100% illumination, for about 3 to 4 hours around noon, if there are no clouds.

    If this is expected to work under clouds that will drop to maybe 3 to 6mA, for about 4-5 hours a day.

    If it is indoors, expect far less than 1mA. And even at 1mA it will make full rated panel voltage.

    What I suggest is to connect a few different resistor values to the panel array, and get some hard facts on how many volts/amps the panel makes under your typical operating conditions.

    With such a small panel you can even use a pot as a load, it should handle 16mA fine and lets you adjust the load to get more data.

    Or, just connect your panel through a multimeter on 20mA ammeter range, to the 6v zener diode. That will show how much current it can source into 6v for your different lighting conditions. :)
  3. josep01972

    Thread Starter New Member

    Jul 11, 2014
    Thanks for the reply. In the bright light under the lamp I do get full charge. I do seem to get around 12mA in the setup. No clouds mostly, but some shade.

    Is it true that during the sleep cycle both the battery and panel are contributing to current?