Electrical – Solar powered Arduino project

arduinosolar cell

I have some ideas for Arduino Nano projects I'm throwing around that I would like to power only with a solar panel and I'm wondering if I am going about it the right way. I have: A 7.2V 100mA solar panel, a couple of AAA rechargeable batteries, my NANO(s), and external components. This seems like a complicated project to a noob, so I'll try to list just a few concerns.

  • Is the 7.2V solar panel compatible with five 1.5V rechargeable batteries?(7.5V when put together in series). I'll just connect + to + and – to – and it will slowly charge?
  • Should I be concerned about my power supply slowly dropping voltage every night for weeks or months (hopefully years) and charging in the morning, and messing with the circuitry of the nano? I would assume that at night it would die off after a couple of hours, but that may depend on how much power my external components are dissipating.
  • This may have to be found out experimentally, but does the solar panel produce enough power to at least keep the Nano on and charge the batteries?

I'm thinking it should be a simple circuit, solar panel→batteries→Vin & GND (5V regulator is built in, of course). But if the low voltage while charging and dying off is a problem, I was thinking about using a low-voltage indicator circuit to tell when the power drops below 6.5-6.9V:

Hand drawing:

YouTube video: https://www.youtube.com/watch?v=Z0cOO0TNjNA

and some logic gates, maybe a NOT gate and replacing the positive side of the LED from the circuit above with the input for the NOT gate, with the output going to Vin. Hopefully a transistor NOT gate can withstand the 7.2V.

Please tell me what I should change and anything I should be aware of. Thanks in advance.

Best Answer

The solar panel should not be connected to the batteries for the following reasons:

  1. Overcharging can reduce the life of the battery.
  2. If the output voltage of the solar panel drops below the battery voltage, then the battery voltage will feed into the solar cell.
  3. If the battery is empty, the solar panel may charge the battery too quickly (with too much current) which can reduce the life of the battery.

Instead, you should use a battery charging circuit. Many off-the-shelf boards can be found at online & local electronics shops. Eg This battery charger from Sparkfun. Some of these boards turn off the voltage when the battery voltage gets too low - to prevent damage from over-discharging the battery.

I would also be cautious about having an LED indicator for when the battery is low. This will drain the battery even further and can damage the battery if it over-discharges. Instead, have a LED on if the battery is OK - although this will cause the battery to drain quicker.

As a rough guide, the Arduino UNO only requires 20mA to operate. Providing that your external circuitry doesn't draw much current either, you can charge the battery and run the Arduino at the same time. Perhaps, the LED indicator could indicate that the solar panel is charging the batteries.

As Dmitry suggests in his answer, using the low power modes when on battery alone reduce your power consumption and hence increase the life until the battery goes flat.