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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.

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  • \$\begingroup\$ Please attach any images to this post, or at least make it available where it can be shown without having to run javascript just to show a simple image. \$\endgroup\$ – pipe Apr 1 '16 at 7:05
  • \$\begingroup\$ if I understand correctly you're planning on completely draining the batteries every night, until your circuit fails. This will deep discharge your batteries, which they won't like and lose capacity very fast \$\endgroup\$ – Christian Apr 1 '16 at 7:51
  • \$\begingroup\$ Then I will probably include the low voltage circuit to stop them from completely draining \$\endgroup\$ – Gᴇᴏᴍᴇᴛᴇʀ Apr 1 '16 at 15:54
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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.

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  • \$\begingroup\$ What do you think of the circuit I was thinking about building to turn off the Nano when the voltage was too low? (last paragraph) \$\endgroup\$ – Gᴇᴏᴍᴇᴛᴇʀ Apr 1 '16 at 16:02
  • \$\begingroup\$ I would measure Vin (through a voltage divider) using one of the Arduino analog inputs. I would then have the Arduino turn on the LED when it was above/below a certain threshold. If you pulsed the LED output, the LED would consume less current as it is only drawing current during the ON phase of the pulse. \$\endgroup\$ – sa_leinad Apr 1 '16 at 16:32
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The power provided by your solar panel will be more than sufficient for the arduino itself. If you use low power modes, you may reduce your consumption drastically, below 1 mA. Check out LowPower library.

If you use NiMH batteries, you can probably get away with connecting them directly to the solar panel (perhaps via a diode to prevent reverse current). Such batteries can tolerate small overcharge currents without getting damaged.

The only weak point of your project is the regulator: zener diodes typically need to sink about 5 mA to work properly. Personally I would avoid the regulator altogether and power the Nano by 3 NiMH cells: I'm quite sure Atmel chips can cope with voltages in 3.6-4.5V range without any regulation. Incidentally, you can also use a cheaper and less powerful solar panel for this project.

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