Solar Panel & Battery Boxes

I make solar panel and battery boxes that are available here




Discussion and sales links below:
http://cgi.cottonpickers.plus.com/~cott ... hp?f=4&t=3
http://cgi.cottonpickers.plus.com/~cott ... hp?f=5&t=4

The 2 watt panel and 4 nimh holder is the most popular, but I also make larger 1000ma panel options capable of running the pi overnight. They are linkable to increase the current.

Micromart has reviewed the basic model this week if you are interested in seeing how they got on with it.

Links are broken.

Corrected links:

http://cgi.cottonpickers.plus.com/~cott ... hp?f=4&t=3

http://cgi.cottonpickers.plus.com/~cott ... hp?f=5&t=4

My Pi draws 2 watts, how can it run off a 2-watt solar panel that's also trying to keep batteries charged? Or do you charge it all day and then run it for 3-5 hours at night?

The pi draws about 2 watts, the panel is 2 watts, and the batteries store about 8 watts. So when its sunny you can run your Pi for hours, or yes, you can leave in the sun to charge the batteries and then run at night for a few hours. The panel isn't big enough to do both at the same time. But for many projects its enough to take the pi out in the sun and run it pretty much all day. Its meant to be a simple way to get your Pi solar.

I do make a 'linkable' version so you can plug in a second unit to both double the charging capacity (up to 4 watts) and double the battery storage. I also make larger versions right up to a 5 watt panel and 8D battery pack capable of running the pi for about a week depending on where you are in the world! but for most folks this or the linkable one is a nice compromise with size / flexibility etc. Its not perfect, but I like it

where do you buy that solar panel?

I live in Scotland. Whats the 'sun'?

Note the newly released model A Raspberry Pi used 1/3 of the power of the model B
...so it will run for 3x the time on one of these.

I'm thinking about powering my Raspberry Pi web server (which needs to run 24/7) from solar power. Let's assume that it's a model A, which consumes about 0.65 W in average, if I'm not mistaken.

I live in an area where the average sun hours worst case occurs in December, about 60 hours of sunlight for the whole month, or 2 hours of sunlight per day. We need to calculate based on the worst case if we don't wan't to run out of power for the Pi during the winter (the average in the summer is about 9 hours of sunlight per day).

So the Pi consumes 0.65 W for 24 hours. That's a total of roughly 16 W a day. With 2 hours of sunshine in December, that means that we need an 8 W solar panel. Make it 10 W just to be sure.

The battery needs to store energy for at least 5 days in advance (there might not be any sunshine for 5 days), that means that it must be able to store 80 W. But the deep cycle batteries recommended for solar applications should not be drained below 50% of their capacity, so we need to double that to 160 W in order to ensure that if there will be no sunlight for 5 days, the Pi will still not drain a fully charged battery below 50%. Assuming that it's a 12V battery, which are most common, 160 / 12 = 13.3, so we will need a battery rated at 13.33 Ah. 12 Ah might do the job, or I could go for 20 Ah, which is the next available size.

Obviously, the solar panel and the battery would have to have triple capacity for a Raspberry Pi model B. Also, these are calculations based on the worst case which is still realistic. In the summer there will definitely be plenty of solar power for the Pi. In the winter, if the sun does not shine for a week, the Pi is going to get thirsty

What do you think, are these calculations realistic?

Thanks,
Andras

If it is a web server it is going to need to connect to the outside world. So for a start your power consumption figure is wrong, because it won't take account of the 150mA or so required to run a USB wifi adapter. Even a 100mbps USB-Ethernet adapter is going to need something in the region of 100mA to work at full speed.

So your 0.67W needs to be at least doubled to make use of a wifi adapter, and a little less than that for an Ethernet adapter. One would wonder if the Model A is the right choice for your application. The answer would only be yes presumably if you intend to only use wifi connectivity.

I assumed that a WiFi dongle uses nearly 0 power, but it seems that I was wrong. I'd still go with model A, because model B has a built-in Ethernet adapter and that makes it eat 3 times the power compared to model A. I don't want to use an Ethernet connection. The idea is to make a web server which is completely off the gird, with no mains power connection, no network cable plugged into it, nothing. Completely independent. But I guess you're right, the WiFi will double the power requirements...

Thank you!

Well, I guess I'd wonder what a web server with no connection to anything is _good_ for, but that's probably a rathole.

With the exception of your units (watts times hours gives watt-hours) your calculations are approximately correct.

I had a couple of golf cart batteries (Trojan T-105 225AH) in series with a couple of 20 watt solar panels float charging them for a weather station for a few years, and the concept works well, but you still need to maintain them (I had hydrocaps, which helped) and check up on them occasionally. Maybe your webserver could log the battery voltage, so you could keep an eye on your state of charge, but that's more circuitry and more power consumption....

You'll also need a charge controller for the days when you make more than you use, so your panels don't cook your batteries. And it should probably be temperature compensated, as I'm guessing this is in a cold part of the world.

Lots of things to think about, but dive in and get started and you'll learn a lot!

is the solar panel produces 2 watt at best and the pi draws to what the pi will last as long as the batterys last really a 4watt panel is needed to keep the batterys charged and run the pi

[quote="dasimpson"]is the solar panel produces 2 watt at best and the pi draws to what the pi will last as long as the batterys last really a 4watt panel is needed to keep the batterys charged and run the pi[/quote]

OK, now that makes no sense at all.

You need to start with your load (2W), and the number of hours you want to run it without sunlight (168, say a week), that gives you 336WH of battery.

And then don't discharge your batteries more than 50 percent, so 762WH.

And then take into account the efficiency of the voltage regulator from your battery voltage down to the 5V that you are feeding the RPi with. SWAG from some random selection at DigiKey is 85%, so 790WH. A 12V battery would be about 66 amp-hours.

Now you need to charge the battery. You really need some idea of the distribution of the sunlight in that "average December" is really 2 hours a day, or is it 6 days of 10 hours per day and then darkness the rest of the month? 8*) But let's take 2 hours a day as the average. In 2 hours you need to return the power used that day (plus a little to recharge the last week's discharge, in case it was dark last week). [Note, I'm assuming it's "Peak Solar Hours" we're talking about, as that's most likely, but you'll want to confirm that.]

So 2W times 24 hours is 48WH, divided by 2 hours gives 48 watts of solar panel DIVIDED by the charge efficiency of the charge controller and the batteries.
http://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery points to
http://www.osti.gov/bridge/purl.cover.j ... 266357.pdf which says something like 50-90% efficiency, so double the size of your panels.

Run the calculations with better or different numbers if you have them, but 50-100W of solar panel and 50-75AH of 12V battery are going to be the right ballpark.

i was more meaning the first post
but to your comments yeah your looking at 2 hours a day hour befor solar noon and after that when the panels will produce there most
were i am in the north west of england on a winter day with the angle set for best results at solar noon you be lucky to get 75% rating of panels
so 80w panel and 105ah battery with 15amp charge controller i think be the ballpark
also using a UBEC (Ultimate Battery Eliminator Circuit) rather then a linear regulator will results in for fewer losses
for a little more infor http://www.wattflyer.com/forums/showthread.php?t=39138

I can confirm the above post with real world rpi-solar experience....I have a headless RPI server running on 45 watt solar and an RV/Marine deep cycle 12v battery....I think it is around 70 or 80AH...Where I live, 45 degrees North, the days are too short and too dark to keep the battery charged from November until about now.....

I thought my setup would be extreme overkill, but, sadly, like many before me, grossly underestimated the bulk of equipment required to run a tiny computer 24/7 365.....by the way 2 watts may actually be what the pi is consuming, but if you measure the power required from the battery to power the 5v supply and the pi, you will find it is at least double that. My system draws an average of 6 watts to run the RPI and Arduino UNO, a USB wifi dongle, and a webcam...

Good luck! it is fun learning the reality of solar power and real world usage....

solar is not what is seems thats for sure i guess you could always chuck a wind turbine in also are water turbine if your have running water this is starting to sound more like an apocoleptic servival system lol

bblood did you align your panels to get the most out of them in the winter seasion
when i align panels i always align to this
For winter, take the latitude, multiply by 0.89, and add 24 degrees
normaly this is when we need the most from are panels and i leave them like this all year round

I too live along the 45 degrees North parallel
When I said that I want a web server with no connection to anything, I just meant no wires to any kind of large scale network (like the internet or the 220V grid), WiFi is obviously needed.

I guess you're right, there would be some power loss in the conversion from 12V to 5v, but I'm hoping that the system will not draw more than 3W in the worst case with a RPi model B and much less with a model A.

BTW, I already do have an experimental solar system made of a 10W solar panel, a 12V 12 Ah deep cycle battery,a charge controller and a home made lamp consisting of 3 Cree XP-G power LEDs. In the summer there's joy. I can have my lamp (which draws 700 mA at 12 V) working for about 4 hours during the night. In the winter, however, the battery is barely charged. I've been testing this system for nearly 2 years now, so it's clear to me that if you want enough solar power during the winter, where I live, you need to oversize the system. You need to plan for at least 5 days without sunshine, maybe even 10. In the summer however, much of the power would be wasted because the charge controller would just switch it off.

i am 53 degrees North
for you to get best winter results you need 64 degrees angle on the panel from horizontal and facing solar noon

I've never really thought of how important the panel's angle is, but I've done some reading and it is! Thank you

no problem i hope you now have better results