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So i'be been toying with the idea in my head of creating a solar powered weather station with an arduino, two 6V 2W solar panels in parallel, two 5V USB battery packs, and an adafruit module which converts voltages from 3-12V to 5V (to charge my batteries). I'm also working on using some small relays or MOSFETs for the arduino switch between batteries.

The problem: If my solar panel voltage drops below 3V, the adafruit board will supply the batteries with a n unstable voltage below 4.8V. I need some sort of circuit or sensor that is capable of detecting when the voltage from the solar panels drops below 3.3V and which will open the circuit to protect the batteries from unstable voltages.

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You have selected a number of things that don't go well together. It is possible to use relays or mosfets to switch between batteries, and the Arduino could be used to measure a voltage. Or you can make a circuit with a few electronic components (e.g. a comparator). However, no one does it like that. It would be unreliable.

Every solar power charging module can handle darkness as well. Some modules will also create 5V.

This Adafruit module has no 5V output, it only contains the charging part: Adafruit:USB / DC / Solar Lithium Ion/Polymer charger - v2
It is out of stock at the moment, and you need solar cells, battery, a cable, and a DC-DC converter to make 5V.
When it is dark and the battery is empty, I assume the output drops and the Arduino stops working. Adafruit made a tutorial for it that shows the DC-DC converter and also Collin shows a DC-DC converter: Youtube, Collin's Lab: Solar

There are other modules like that. For example the Sparkfun Sunny Buddy.

Take care that the batteries don't get too hot.
It is possible to run some Arduino boards directly from a battery (3.1 to 4.2V). Then you would not need the DC-DC converter.
Buy solar cells that are the best in low light conditions. With enough sunlight, the power is no problem. It is the dark winter days that could be a problem.

  • So I've looked at many solar panel charge controllers and modules, and I found that pretty much all of them are just variable input D.C.-DC Step down converters, taking inputs from 6V or above. That would mean that the batteries would only charge in direct sunlight. However with the adafruit module I found, the input voltage can be anywhere from 3-12V. It sounds like I will try a comparator, which I have never used. – patrick jackson Jun 24 '17 at 14:54
  • The other problem (which is very likely to occur) is that even with 2 solar panels in parallel at 3V there isn't enough current to charge the batteries properly. So i'll just set the comparator to a higher voltage. – patrick jackson Jun 24 '17 at 14:58
  • In my opinion, you are still going in the wrong direction. If the voltage of a solar cell is low, then there is probably not enough current either. If you want even the smallest energy from the solar panel, try a energy harvester module. The solar charging modules have a chip specifically for that, it is not just a DC-DC converter. Don't connect solar panels parallel, set them in series. – Jot Jun 24 '17 at 17:45
  • I see what you're saying with the voltage dropping. However, why wouldn't I put them In parallel? It draws less current from each of them and still keeps them above 6V each in good sunlight right? – patrick jackson Jun 24 '17 at 18:38
  • Maybe you can if they always have the same amount of light, but it doesn't make sense and it has no advantages. In series it is possible to add a bypass diode to each panel. In series the MPPT can be used to the maximum. The solar charging modules use a switching regulator, that means with more voltage and less current as input, they can create a lower voltage with more current. – Jot Jun 24 '17 at 22:53

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