Measuring solar power with an Arduino

Question

I'm pulling together a project for my class where students build a solar panel structure/frame that moves with the sun to capture more power than if the panel were just flat on the roof.

I can handle controlling the motors, but I wanted some help to make sure I safely/accurately measured power.

I have an Arduino (the model isn't especially important), a 12 V deep cell battery, and 6 V solar cells. Obviously I don't want to hook the cells straight to an analog pin, but I came up with the schematic below. The wire to the battery will go through the CT (the inductor) to get current in order to calculate power.

Would the battery still charge even if the cells are only producing, say, 8 V? Am I overlooking anything or is there any circuit protection I need to include? Maybe a computer fan as a load on the battery and to cool everything? Any help is appreciated.

Answer 1

Just a few points:

• You can't measure DC current with a coil; you would need a Hall-effect sensor;
• The charging current and the voltage you measure will depend on the state-of-charge of the battery, making it difficult to compare measurements;
• You can't fully charge a nominally 12 V battery with a 12 V solar panel; a 12 V lead-acid battery with the panel you mention in the comments should work;
• You need at least a diode between solar panels and battery, and possibly a proper charger, depending on the chemistry and capacity of the battery and the maximum charging current; maximum charging current for a 12 V, 9 Ah lead-acid battery is about 1 A if you stay on the very safe side, so the panel you mention should be OK; just don't overcharge the battery;
• You need to be sure the battery voltage doesn't exceed 12 V by too much, or the voltage divider will put more than 5 V on the Arduino's input pin and fry it. Nominally 12 V lead-acid batteries are usually charged to about 14 V, and the solar panel you mention can get there. This would put 5.6 V on the Arduino input pin with the resistor values you use in your voltage divider.

Also have a look here: Measure power of solar cell with an esp8266.

Answer 2

Would the battery still charge even if the cells are only producing, say, 8V

Only if the battery is below 8V.

Am I overlooking anything

Yes. Current transformers require a load resistance and bias voltage to convert a current measurement into a voltage the ADC can measure.

Answer 3

Use something like a 1NA169 'high side' current sensor, like this Sparkfun module instead of a resistor or inductor. Much more accurate, and it can be placed on the 'high side' of the power circuit - very handy.

Answer 4

I have done something similar on my boat. This has a 12v car battery and a solar panel charger that can generate upto 20v in bright sunshine. I used a 3:1 potential divider so that the voltage supplied to the Arduino input is never more than 5v.