I have a robot on wheels that uses 7.2v battery. The battery runs it for good amount of time it is a 2000mah nimh vex battery. I want to supplement and charge it during peak hours. I bought 4 solar panel that can produce 2 V near my window and 3 volts outside. They produce 200mah at full sun. I want to series the solar panel up and have voltage regulator that can regulate it. I was told that the current also needs to be constant and so I ordered a LM317T.
How does the lm317t react when my solar panel hit shade and current goes under 200mah. What voltage should I put on my voltage regulator that I will connect to LM317T. Should I make it 10 V or 9 V?
I really am new to this and want to use solar energy to charge my battery while it is connected to my robot in parallel. What current should be held constant to charge 7.2v 2000mah nimh?
Best Answer
You're not going to be using the LM317 as a voltage regulator in this case. It will solely act as a current regulator, also known as a constant current source. (CCS).
I would expect it to simply pass the current it gets as input in this condition, up to the point where the regulator doesn't get enough current to operate, which will happen somewhere under 10 mA. At that point, you'll get a large voltage drop across the LM317-as-CCS, and current through the regulator will drop off sharply. (There may be some residual leakage.)
I hesitate to guess whether this voltage drop will present as a cliff or a ramp.
It's not hard to test it and find out, though, and you have the equipment, which I do not. (I have LM317s on hand, but no bare solar panels.)
You shouldn't need a voltage regulator.
Think of the LM317 in this circuit as a rubber band between the voltage source and the sink. That is, between the series-connected solar cells and the NiMH battery. When the source voltage rises due to higher insolation, the voltage drop across the LM317 will increase accordingly, soaking up the difference between the battery voltage and the solar array voltage. As the battery charges, its terminal voltage will increase, decreasing the voltage across the LM317.
You should set the LM317 for 200 mA or less, to avoid exceeding an 0.1C charge rate, which is the fastest you should charge a NiMH battery if you do not include safe methods of early charge termination, such as -ΔV detection and overtemp detection.