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 wrote: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.
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?Tage wrote: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.Tage wrote:my gut feeling is that you need about 60W panel and a 100Ah 12V deep discharge battery to run the Pi continuously year around.
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.Leo Rest wrote: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
I can see your point about my original £30 budget being too low. I will have to find additional funds for this project.Leo Rest wrote: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.
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?Tage wrote: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 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.jdb wrote: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.
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.rpdom wrote: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.Leo Rest wrote: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