Friday, December 2, 2016

5V Solar Power Supply for Raspberry Pi - first results

The introductory article was about describing the feasibility of the project.
Let's see some preliminary results.

The measurements

Until now I did run the system with an external voltmeter to see if the solar panel was able to keep up with the system.
It seems that so far the answer is no.

The graph shows the battery level measured in few days.
Note that basically there was no real load to the battery, only the Dc/Dc converter, so very few mA were drawn from the battery.
The days were pretty cloudy all the time, so no full sun for long time. In the best scenario (see the latest days) the battery level barely reached the 2.9V, well well below the minimum of 3.5V, and again I stress the fact there was no real load on the system.

The solar panel used so far is a 6V 2W, capable to provide in full sun a current of 330mA.
Definitively not enough to keep the battery charged AND powering something else (the Raspberry Pi has an estimated drawing current around 300-350mA).

Until there was sun the system did run happily, but as soon as the solar panel was not in full sun, the system had to use more and more the battery to power up the project and of course there was no battery charging there.

The battery is a nominal 3.7V 6600mAh and can last just few hours when not charged, so the system starting with a loaded battery in a sunny day can run almost all the day, but that only the first day.

What to do ?

There are few things to try.

  1. Use a bigger and powerful solar panel.
    A bigger solar panel, still 6V but maybe 6 or 9 W, probably can give more energy even with less light.
    In full sun the power should be enough to charge the battery AND power the system.
    Adafruit has a 6V 5.6 W solar panel, capable to give up to 930 mA in full sun and presumably more juice even in less than ideal conditions.
  2. Implement an intelligent power management
    We can assume to don't have the system running 24 hours a day.
    An external circuit (like the RPOf) can monitor the charge of the battery and shutdown the Raspberry when the battery level is below a specific threshold.
    When the battery threshold level is above a specific value the circuit can power on back the Raspberry.
  3. Reduce as much as possible the Raspberry Pi current needs.
    It can be done disabling every thing not needed, like the HDMI port or the LEDs.
    Even better adopting a Raspberry Pi 3, eliminating also the need to power the USB ports (currently the USB ports are in use for the WiFi dongle).
Very probably all the solutions will be needed.

Sensor modification

To better handle the solar management, a modification is needed on the light sensor.
Currently the light sensor is based on a photoresistor.
It can indicate if there is light or dark but is not enough to determine the "quality" of the light.
In other words the value returned is not enough to determine if the light is enough to generate ppower via the solar panel or not.
To do so a better light sensor is needed.  So in the near future I'll use a solar light sensor rather than the generic light sensor.

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