Solar Panel Charging with Arduino

arduinobattery-chargingsolar cell

I built a solar panel charge controller from instructables with an easy to read schematic and for some reason my battery charges up to a certain point and then loses charge again. That leads me to ask a couple of questions given that i am very new to this.

My Setup:
10W solar panel, 12v7A Lead Acid Battery, Arduino Nano and all components from above schematic except for the load at this point.

More details on solar panel specs: Solar Panel 10W • Vmax P = 17.2V • Imax P = 0.57A • Voc = 21.6V • Isc = 0.83A • 396x348x25mm

  1. Before I connect my solar panel my panel's voltage sits around 20volts (with volt meter) but straight after I connect it to the circuit, the voltage drops. Is this because there is now current and that reduces the voltage?
  2. At a certain point (around 12.4 volts) my battery stops charging and actually starts losing voltage. Could this be because the current supplied from the solar panel is not enough to charge the battery? If not, any other ideas?
  3. Does it make sense that the Arduino, solar panel, batteries and output voltage ground are all connected together?
  4. At what voltage would I actually charge my batteries when in bulk charging mode (99% pwm)? Does it have to be more than the current voltage of the battery and by what margin?

Thank you very much for the assistance and please bear with me, I am new to this.

Best Answer

  1. It's common for a voltage source to drop in voltage when current is drawn. This is down to the source resistance inside the voltage source. When you measure the output voltage of the panel without it connected, then you a measuring the voltage of the panel without any current passing through its internal source resistance. When current flows, it passes through that source resitance and there is a voltage drop across it, so the output voltage now drops.

How much the voltage drops when under load depends on how much current you are trying to draw and the internal source resistance of the panel.

a) It could be natural and a function of the solar panel itself. b) You could be drawing too much current from the panel and the voltage drops greatly.

  1. Battery discharging: it could be that the battery is pushing power back into the panel. You can stop this by putting in series a suitable diode. You'd have to measure voltages and currents to work out what it is going on, measure the battery voltage with the panel disconnected at this point. An ammeter in series will tell you which way the current is flowing and hence which voltage is greater than the other and this will help you work out what is going on.

  2. Answer: Yes.

  3. I can't answer, I don't actually know what's defined by 'bulk charging'.

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