Autonomous computer connected via GSM


#1

:red_circle: :construction_worker_man: work in progress :construction_worker_woman: :red_circle:

Bonjour,

The goal is to setup a solar powered computer connected to the net with a SIM card and run a web server behind tor. The machine should be up at all times but it’s not catastrophic if it is down for a day or two, as long as it’s not a recurring pattern and only happens in exceptional circumstances.

I know nothing on that topic: all valuable information comes from @obinou and all mistakes are mine :wink:

Cheers

Raspberrypi 3A+

Hardware

Estimations

  • Raspberrypi 3B+ needs 2.5W (and powers the SIII via its USB port) and operates at 5v therefore 0.5A (2.5W = 5v * 0.5A)
  • We assume the worst case scenario with amorphous solar panel is 24h at 0%
  • In the worst case scenario the battery will be drained 12Ah ( 0.5A * 24h )
  • The battery must therefore be 2 x 12Ah = 24Ah (because a battery advertised for XAh must not be discharged less than 50%)
  • The solar panels are sized according to the worst day of the year, 21 december with 5h of light and 19h with no light
    • 5h x 2.5Wh = 12.5Wh provided by the solar panels
    • 19h x 2.5Wh = 47.5Wh provided by the battery
    • The battery needs to be charged during the 5h, so the solar panels are actually required to produce a total of 12.5Wh + 47.5Wh = 60Wh during this day, that is 60Wh / 5h = 12W
    • Assuming amorphous solar panels only yields 20% when the weather is very cloudy, 12W in the worst case scenario requires 5 x 12W = 60W

Mounting

  • SIII connected to the computer Raspberry USB port for charge and network
  • Raspberry connected to the power bank
  • Power bank connected to the MPPT
  • MPPT connected to the solar panels

NUC

Hardware

Estimations

  • NUC needs 15W (and powers the SIII via its USB port) and operates at 12v therefore 1.25A (15W = 12v * 1.25A)
  • We assume the worst case scenario with amorphous solar panel is 24h at 0%
  • In the worst case scenario the battery will be drained 30Ah ( 1.25A * 24h )
  • The battery must therefore be 2 x 30Ah = 60Ah (because a battery advertised for XAh must not be discharged less than 50%)
  • The solar panels are sized according to the worst day of the year, 21 december with 5h of light and 19h with no light
    • 5h x 15Wh = 75Wh provided by the solar panels
    • 19h x 15Wh = 285Wh provided by the battery
    • The battery needs to be charged during the 5h, so the solar panels are actually required to produce a total of 75Wh + 285Wh = 360Wh during this day, that is 360Wh / 5h = 72W
    • Assuming amorphous solar panels only yields 20% when the weather is very cloudy, 72W in the worst case scenario requires 5 x 72W = 360W

Mounting

  • SIII connected to the computer USB port for charge and network
  • computer connected to the MPPT (? cable ?)
  • Battery connected to the MPPT
  • MPPT connected to the solar panels

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References


#2

I write this post from a solar powered laptop. And this home installation saved me a few afternoon of work during tempest when electrical network is locally broken (as I have 4G internet).

Two advices:

  • have 100% or 200% margin in solar panels capacity (55w is the peak power under best circumstances for the 1st considered panel, best circumstances are midday hours during winter. It will produce an average of 45% depending on where you are on earth, a few hours a day)
  • battery capacity is even more important, because regularly you have 2 or 3 days of rain in a raw, and solar panels will produce nothing under those circumstances. A full week of battery capacity would achieve the wanted exceptional only power outage.

Batteries

Acid-Lead batteries have an average of 60% of working rate.
Lithium is around 90%.
Some Acid-Lead batteries are to start engines (huge short load), other are more appropriate to long slow discharge currents. Bad choice will result in poor perfs and shorter life for the battery.

Régulateur de charge MPPT

Victron MPPT are working as expected. I did buy counterfact false chinese “MPPT” sigled things…

Solar panels technology

  • Amorph solar panel such as the 55w one have around 7% yield rate.
  • Polycristallin are around 12% (to keep simple).
  • I recommand (and I use) monocristallin (with peak rates at 17%).

It means that for 100w, a monocristallin panel will be around 3x smaller than an amorph one. And polycristallin are working better in dim lights.

We still have margin to invent better panels.