chrisd10
Posts: 5
Joined: Wed Nov 12, 2014 11:57 am
Location: Germany

Solar Powered Pi

Thu Jun 18, 2015 10:42 am

Hello!

So I was thinking about going solar with my pi. The only problem is that im not experienced in the solar field or with batteries.
I have a Pi B+ running a VPN Server, Webcam server and home automation via GPIO and some Relays.

Info:
I want to run the pi 24/7
I live in Germany.
I use WiFi not Ethernet.

My questions are:

What size panel should i be looking at?
What size battery do i need?
Can i connect different types of solar panels eg. 10w and 20w?
To connect more solar panels do i connect them in a row or parallel?

It would be nice if someone could explain how to calculate the values so i can help others in the near future (:
Like i said go easy on me guys, im pretty new to electronic circuits.

A big thank you to everybody in advance! :D

hampi
Posts: 223
Joined: Fri May 31, 2013 11:29 am
Contact: Website

Re: Solar Powered Pi

Sat Jun 20, 2015 4:42 pm

10 W panel and 12 V 7 Ah lead acid battery gave me 55 hours of computer time last week. One week is 168 hours. This system @RPiAOne has been build with cost in mind and should be accessible to most hobby builders. Only very basic and widely available components are used. Presently I am using Raspberry Pi A+.

https://github.com/oh7bf/RaspiPwr12V/wi ... spberry-Pi

User avatar
jbeale
Posts: 3581
Joined: Tue Nov 22, 2011 11:51 pm
Contact: Website

Re: Solar Powered Pi

Sat Jun 20, 2015 7:38 pm

The model A+ RPi uses significantly less power than a Model B/B+/2 due to not having the power-hungry ethernet chip.
See for example: http://raspi.tv/2014/raspberry-pi-a-how ... es-it-need

Model B recording 1080p video takes 480 mA @ 5V or about 2.5 watts.
Model A+ recording 1080p video takes 230 mA @5V or about 1.15 watts

Load:
If you have a constant load of 2.5 watts over 24 hours, that is 60 watt-hours.
sealed lead-acid battery charge efficiency (0.75) * 12V->5V power supply efficiency (0.9) * panel charger loss (0.8) = 0.54
So as a guess, your system needs to capture 60 / 0.54 = 111 watt-hours per day.

Supply:
according to http://www.pveducation.org/pvcdrom/prop ... -radiation
Stuttgart averages between 3 and 6 kWh per day per m^2, depending on the month of the year.
If your solar panel is 15% efficient, that means 450 Wh per day per m^2 in winter, and twice that in summer.
If you need 111 Wh/day input, that's a 0.25 square meter (active area) panel *on average*, not counting more-cloudy-than average days. You need only half of that if you use a RPi A+ instead of a RPi B. Size can be a few percent less if you get a premium more-efficient panel.

> I want to run the pi 24/7

The key question then is how many peak-sun-equivalent hours per day you get. It seems to me that you could have several days where it is cloudy most or all of the time, is it acceptable for the Pi to be down during bad weather? If not, you are going to be looking at a pretty large battery to hold over during the darker days, and similarly more solar panel to be able to recharge that large battery plus maintain the Pi, when it is sunny.

You can also refer to sites like http://photovoltaic-software.com/PV-sol ... lation.php
but again remember that's a yearly average, not accounting for short-term weather

For a 12V battery, 111 Wh is 9.25 Ah, a reasonable size. I have an 8 Ah 12V SLA battery in front of me which measures 15 x 9 x 6.5 cm (6 x 4 x 2.5 inches). But that's if the battery capacity is 100% used in 24 hours. If you get 6 useful hours of sun a day, you're still discharging the battery 18 hours of the day. Remember the deeper the discharge each night, the shorter the overall lifetime of the battery is. Maybe you should plan for double that for longer battery life.

I assumed lead-acid battery because it's by far the cheapest per stored watt-hour but are those even still used, over there in the land of RoHS ?

chrisd10
Posts: 5
Joined: Wed Nov 12, 2014 11:57 am
Location: Germany

Re: Solar Powered Pi

Mon Jun 22, 2015 9:04 pm

Thank you for your detailed reply!

No, I would like to keep the downtime to an absolute minimum. The best would be no downtime at all.
We do actually still use lead acid battery's ;) although I think ill use a deep cycle battery. I'd rather spend more and therefore keep it reliable and keep it powered in the long run.

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