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I am trying to build an off-grid automatic watering system, and I was thinking of many options and the most reliable one that I found is to use a power bank which is charged by a solar panel.

So, I have a solar panel that outputs 5V, and I can charge a standard power bank with it. In addition to this, I bought a USB pump which I can also power with the power bank and controlled using an Arduino board. The problem is that the power bank doesn't support charging and discharging at the same time, and I thought that I could use an Arduino board to control this, but I have no knowledge on this. Therefore, I need help.

Is it possible to have a system like the one sketched below, where the Arduino board controls the input/output of the power bank?

enter image description here

The black path is my current path, but I was wondering if I could do the red path so that I can have both chargings from the solar panel and discharge due to the pump and Arduino board.

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Is it possible? Yes. Is it easy? Maybe.

A relay shield or similar would allow you to break the power connection from the solar panel to the power bank input, wait 5 seconds and then connect the power bank output to the pump.
Once you are done with the pump you could disconnect power from it and reconnect the charging power.

That is all fairly straight forward, maybe a dozen lines of code and some very simple wiring.

The issue is where does the arduino get power from? Or more accurately how does the arduino maintain power during the transitions between different power sources. How much of an issue this is depends a lot on how quickly the power pack transitions from charging to outputting and back and how you want the system to behave overnight or other times when there pump is idle but there isn't much sun light.

Also have you done a power budget? How much power does your pump take and how much power does your solar panel output under normal usage? Based on those two how long per day can you run the pump for assuming an ~80% efficient battery pack.

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You should be able to solve it with two diodes, preferably Schotty diodes, they have a low forward drop voltage.

enter image description here

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  • Simple, but I'm not sure a 5v solar cell itself can run the pump... – dandavis Aug 1 '17 at 17:07
  • Well if the voltage from the solar panel drops below the voltage from the power bank, then will the pump/arduino be get its energy from the power bank. – MatsK Aug 1 '17 at 17:25

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