Outdoor power

[squarepie]

For my weather station project, I need to house my model B pi outside. I have no viable means of running cables outside, I'm going to use a WiFi adapter to communicate with my web server. The problem I have come across is that I need a cheap, waterproof of powering a model B pi with a WiFi adapter 24/7 without an external power source. The obvious answer is to use solar, but this doesn't work at night and I have no idea as to go about doing this. Does anybody know how to do this within a reasonable budget (£30 max)?

[Tage]

you could use a solar panel to charge a 12V SLA battery, and power your Pi from that battery using a USB charger adapter. the problem is the cost of the solar panel and the fact that the Pi draws more power than the panel and battery can provide. So you need to keep the Pi shut off most of the time and only turn it on for short periods to keep your solar panel and battery cost low enough.
you do not need to buy a charge controller if you add overvoltage and undervoltage supervision to you Pi. for example, use a simple comparator and two GPIO pins to monitor the battery for overvoltage and undervoltage. if your battery is overcharging you keep the Pi on longer, if the battery is getting low on voltage you have to decrease the amount of time you run the Pi.

[squarepie]

you could use a solar panel to charge a 12V SLA battery, and power your Pi from that battery using a USB charger adapter. the problem is the cost of the solar panel and the fact that the Pi draws more power than the panel and battery can provide. So you need to keep the Pi shut off most of the time and only turn it on for short periods to keep your solar panel and battery cost low enough.
you do not need to buy a charge controller if you add overvoltage and undervoltage supervision to you Pi. for example, use a simple comparator and two GPIO pins to monitor the battery for overvoltage and undervoltage. if your battery is overcharging you keep the Pi on longer, if the battery is getting low on voltage you have to decrease the amount of time you run the Pi.

That would be good be good although as it is logging atmospheric data, I need complete uptime for the pi. Can you get more powerful solar panels?

[Tage]

an alternative to increasing the solar panel size is to use two deep cycle batteries and exchange the battery with regular intervals for charging.

[richardski]

Raspberry PI power consumption figure here:

http://www.raspberrypi.org/archives/260

Model B owners using networking and high-current USB peripherals will require a supply which can source 700mA (many phone chargers meet this requirement).

Allowing for a 1000 mA current draw, to cater for losses in a 12V to 5V regulator, means that you would be drawing in the region of 1 Amp per hour and a caravan type deep discharge 100AHr battery would give you approximately 100 hrs of run time. Any solar or wind power electricity you can supply would extend your run time and if you throw enough money at it then the whole station could be totally powered by sun and wind combined, as per some active road signs. Not a cheap option and very tempting to thieves if in a remote location.

Richard

[squarepie]

How about this solar panel http://uk.rs-online.com/web/p/battery-c ... r/7067918/ ? I could attach it to a standard 12v car battery and I think it has a high enough rating to power the pi.

[plugwash]

With a reasonablly efficient DC-DC converter the 12V current should be about half the 5V current.

[Tage]

my gut feeling is that you need about 60W panel and a 100Ah 12V deep discharge battery to run the Pi continuously year around.

[squarepie]

my gut feeling is that you need about 60W panel and a 100Ah 12V deep discharge battery to run the Pi continuously year around.

Could I wire two 30w panels together to achieve this? Also would this be able to cope with limited sunlight? E.g if there was a particularly cloudy day?

[plugwash]

my gut feeling is that you need about 60W panel and a 100Ah 12V deep discharge battery to run the Pi continuously year around.

My gut feeling is it wil depend massively on where you live, I would expect a much smaller pannel to suffice in calafornia than in scotland.

[aTao]

If it is acceptable to upload data sporadically (every 20 mins, or an hour) then your cheapest solution is low power data collection system (probably a microcontroller) and only turn on the RPi for communications.
You might be able to get the RPi to go quiet while not actually transmitting, avoiding a power-up boot.
Either way any $ you spend on reducing power consumption will easily be offset by the savings you get from a lighter solar/wind setup.

[jdb]

Is the weather really going to change so dramatically in 5 minutes? At least, if Micheal Bay isn't directing it.

I would suggest a microcontroller with an RTC coupled to the Pi. The microcontroller is responsible for switching the main input power to the Pi and for the most part, keeps the Pi switched off. Every 20 minutes or so, the uP switches on power to the Pi and keeps it on for ~5 minutes. The Pi boots, connects to wifi and performs its measurements. 60 seconds before the uP switches off the power, it sends a message that the Pi is about to be turned off and the Pi shuts down cleanly. The cycle then repeats.

If a model A plus wifi dongle consumes approximately 2 watts, then you would use an average of 5/20 = 0.25W for the Pi. The uP can be asleep for almost all the time - waking only when its RTC tells it to poke the Pi. Such chips use microwatts of power.

[Tage]

my gut feeling that a 60W solar panel is needed is based on the assumption that there will be about 2 hours of sunlight each day, but the panel will not be providing full power. then there will be weeks when the sun only shines perhaps one day, then the cloudy weather returns for another week. etc. during that brief day of sunlight there will be very little charging of the battery, so a quite large battery is needed. you can come up with any numbers based on if you are an optimist or a pessimist. each location on earth will have different numbers of sunlight hours per day, and you need to be aware that during the winter months the number of sunlight hours per day on average is much lower.
when playing with the calculator and trying to estimate the panel size and battery Ah rating, I find it much easier to calculate in Watt hours.

it would be interesting to get some comments from people that have actually set up some type of solar powered off grid system, even at different power levels.

regarding the question about if two or more panels can be combined, the answer is yes. exactly how it is done depends on the panel and the battery, but if you have a 12V battery and so called 12V solar panel you would parallel connect panels.

if you have an MPPT charge controller you could use higher voltage and it may then sometimes be better to series connect two panels for 36V input instead of 17V. from what I have seen when looking at prices for solar panels it appears that the type that is used on rooftops for very large systems are the cheapest when comparing cost per Watt. I am guessing this is because they are sold in large quantities.
these panels are about 200W-300W so for your case it is probably overkill. but if you can get a 200W panel for the same cost as a 60 or 100W panel, why not. you would save a bit on the battery size as you can charge faster. but you then need also a charge controller.

[Leo Rest]

This is all extremly good information for designing an off grid supply but the OP wanted to do it for £30. Considering the costs of panels and batteries this is not achievable for the price, never mind MPPTs.
Another point is that it is not good to fully discharge a lead acid battery. Most of the calculations I've seen work on the assumption of stopping at 50% charge. With that in mind you now need batteries twice the size which pushes the £30 budget even further out of reality.
I don't want to dampen the enthusiasm of the discussion I just wanted to let the OP know his budget is way too low to allow a solution.

[rpdom]

Another point is that it is not good to fully discharge a lead acid battery. Most of the calculations I've seen work on the assumption of stopping at 50% charge

Which is why someone recommended a "deep discharge" caravan (or Leisure) battery. They are designed to handle lower peak current than a car battery (no starter motor to turn), but can be safely discharged well below 50% with no damage.

[pluggy]

My solar panels average around 10% of their peak power on a year round basis, but there is a huge difference between summer and winter. They make about 15 times more in a good June than a bad January and on a bad day in January they'll do literally nothing. You'd need huge over capacity on the panels and a serious battery to keep a Pi going during the winter months without something else topping up the batteries. I'd be looking at 100W of panels and a serious leisure battery with something to stop it overcharging the battery in summer.

[JoeDaStudd]

How up-to date does the data your transmitting need to be?
The wifi adapters suck a lot of power so if you could cut it down to only connect/transmit once a day or every 6 hours you would be able to get away with far less power then real-time information.
If this isn't an option it might be worth looking at POE using some custom wiring. There was a kit and some DIY projects linked on here a while back which split a standard ethernet cable at either end and allowed you to rig up your own power system. Given the added range it might be possible to use it for your project, which would save the usb wifi adapter as well as buying a more expensive solar/battery setup.

[poing]

There have been a few attempts to power the Pi remotely 24/7 but AFAIK they all failed sooner or later; you need a lot of panels/turbines to run a Pi the year round. Anyway the way to go is probably a Model A (uses less power) with a Gerduino board. The latter uses little power, has a RTC, can power the Pi up and down on specific intervals and can probably interface with the weather station components.

[Tage]

I am aware of the low budget for the project which is why I suggested two batteries that are manually charged and swapped out at regular intervals. The battery should always be a deep discharge battery as been pointed out. a car battery will not last when it is being repeatedly discharged more than 20% or so. batteries for golf carts are commonly used in motorhomes and trailers and are relatively cheap.
If I was involved in some school project with solar power and Pi, I would approach the companies that install and repair large solar power installations and ask if I could get some damaged or retired panels at a low cost of free. during every installation there is some fallout, panels are damaged for different reasons. with three bypass diodes normally being used in each panel, this means that 2/3 of the panel is still functional even if a cell is totally destroyed. such a panel can no longer be used in an installation but is perfectly good for hobby use or different small projects. the maximum power point will be at about 50% of the open circuit voltage which is typically 36-44V. so a damaged standard type panels can be used for charging a 12V battery at up to 8A.

[squarepie]

With that in mind you now need batteries twice the size which pushes the £30 budget even further out of reality.
I don't want to dampen the enthusiasm of the discussion I just wanted to let the OP know his budget is way too low to allow a solution.

I can see your point about my original £30 budget being too low. I will have to find additional funds for this project.

The battery should always be a deep discharge battery as been pointed out. a car battery will not last when it is being repeatedly discharged more than 20% or so. batteries for golf carts are commonly used in motorhomes and trailers and are relatively cheap.

How about this as a deep discharge battery?http://www.ebay.co.uk/itm/Ritar-RT12120 ... 0247735564 How long would it last if I left the pi running constantly?

I would suggest a micro-controller with an RTC coupled to the Pi. The micro-controller is responsible for switching the main input power to the Pi and for the most part, keeps the Pi switched off. Every 20 minutes or so, the uP switches on power to the Pi and keeps it on for ~5 minutes. The Pi boots, connects to WiFi and performs its measurements. 60 seconds before the uP switches off the power, it sends a message that the Pi is about to be turned off and the Pi shuts down cleanly. The cycle then repeats.

How would I go about doing this? I have no experience with atmel micro-controllers which in this case I think would be this obvious choice and I have a limited understanding of PICAXE micro-controllers although I'm not sure if they are compatible with RTCs.

[Tage]

a 12V 12Ah battery has about 12V*12Ah=144Wh capacity.
if your Pi and its gadgets draw 4W then the battery will last 144Wh/4W=36h if it is fully charged.
in real life if you want to get some decent lifetime from the battery you should not discharge it totally. it will last longer if you recharge after discharging 50% of the energy. and it will last longer if you try to keep it near fully charged state as much as possible and not allow it to remain at low state of charge for weeks and weeks.
if you don't use a lead acid battery you should recharge it at least every other month as there is a self discharge and if left empty for a long time it will die.

[Leo Rest]

Another point is that it is not good to fully discharge a lead acid battery. Most of the calculations I've seen work on the assumption of stopping at 50% charge

Which is why someone recommended a "deep discharge" caravan (or Leisure) battery. They are designed to handle lower peak current than a car battery (no starter motor to turn), but can be safely discharged well below 50% with no damage.

The 50% figure was for a deep discharge battery. I believe 80% is safer for a car starter battery. Fundamentally lead acid does not cope well with running flat. A well designed leisure battery will survive some abuse but it is still going to shorten its life whatever the manufacturer says.

[Ravenous]

Just for info, there's an example of a good birdbox monitor that was posted here:
http://www.raspberrypi.org/phpBB3/viewt ... 63&t=41542

His original site (in Swiss, but plenty of diagrams - note the 80W panel and 70Ah battery):
http://earlybird.christian-fahrni.ch/informationen/

Even this might not stay up all year round though. The real limit is the panel size in winter, for those of us in temperate zones anyway. I guess if the battery charge starts to decay you would have to buy and add more panels to the system! (I assume they'd all just wire in parallel and a battery charge circuit would prevent damage due to overcharge.)