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We are working on a thesis and we are thinking of using Raspberry Pi for our data logging of our greenhouse monitoring system. We decided to to use a solar panel to power the Pi 24/7 but we don't know what battery pack we should use to meet the power consumption of the Pi. What would you suggest?

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Assuming no power-hungry peripherals and no heavy computations, your RPi will draw about 0,5-0,7A. Since you want to run it 24/7, you should account for these rare cases where it's raining for several days straight without any useful sunlight. If you want your RPi to survive a 5-days outrage (120h), you'll be fine with a battery of 60Ah. This won't be cheap, but if you can afford such a battery, then problem solved.

There are several tricks which can reduce this number to something more reasonable.

  • If you don't mind losing data on rainy days and only want to reliably keep it running at night, you can reduce your estimated outrage time to about 12 hours and use a battery which is 10 times smaller.
  • If you can afford a little soldering, you can make use of DC power converters. A 5V 60Ah battery holds as much energy as 24V 12.5Ah battery, the latter being cheaper and easier to find.
  • You may want to investigate how much power you can save by e.g. disabling HDMI, Bluetooth, etc. If you can keep current consumption under 0,25-0,35A, you'll be able to cut the battery capacity in half.
  • Above all, check if you can use Arduino for your project. Arduino consumes a ridiculous 0.02A when fully powered, and sleep modes can potentially reduce this current by another 100 times. Chances are, you don't even need a solar panel since Arduino would live off a pack of Duracell for the whole year.
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I am not familiar with battery specific supplies but I have worked with Uninterrupted Power Supplies, and I believe you can get solar ones. I would also like to suggest you find one with a wall plug as well to assure reliability when it isn't generating power from solar.

The only concern then becomes can you find one small enough for a Pi? Most UPS are for desktops or servers, which may be greatly excessive. Whichever you choose, it must be able to supply the Pi whilst under load.

Another option you can look into kinda stems off of a project I did back near the end of highschool. Essentially, I disassembled a camping solar clock and a Nintendo DS charging cable and MacGyver'd them together. This proved to be fruitless as the DS received an inconsistent charge, was unable to keep up with the load. This would be another issue you may run into.

EDIT: As a quick search, I was able to pull this up too, which may interest you.

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There is a variety of cheap solar powered 5V USB smart phone powerbanks offered at Ebay. Some of them hold a capacity of 50Ah or even 100Ah.

Those powerbanks include all neccessary functions in one easy installable device. I can't say how long it takes to charge one while under load of a Raspi nor do I know how long the last not beeing charged. But that's easy to measure. Simply power your Raspi by a fully loaded powerbank with a covered solar panel and wait for the Raspy to brown out. You may want to append a timestamp to a text file by cron every minute and afterwards read the file.

The powerbank may have to be installed outside the greenhouse to receive the full solar spectrum.

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