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I am planning to build a weather station powered by solar power. I want to use an Arduino Nano because it's low power requirement. I want to connect some basic sensors to it (temperature, humidity…) as well as an anemometer, a wind vane and a rain gauge. The weather data should be sent wireless via 433 mhz to a raspberry pi, which acts as a web server. I do not have power cable access to the Arduino so solar is the only option.

Which solar panel should I use? I was planning to use 2 2.5W 5.5V panels which charge a Lithium-Ion battery which powers the Arduino using a step up converter. Assuming I have 2-4h of direct sunlight per day and I want to query the sensors every 5 minutes, how big of a battery should i choose? I don't want it to go out of power and want it to last a couple of days without sun.

  • How large is the load? – Ignacio Vazquez-Abrams Jul 9 '15 at 17:22
  • I don't know. I heard the Arduino nano draws very little power so I would say about 40-60mA under max load once every 5 minutes. It takes very little in sleep mode like 5mA. I dont own one yet so I am just assuming this based of data I read on the internet – qwertz Jul 9 '15 at 17:26
  • I think 2 panels is way overkill. With 8 hours of daylight and 50% efficiency that will result in 4Ah. That's 166mA you'll have available constantly. With that amount of power available you wouldn't even have to consider putting the arduino to sleep. – Gerben Jul 9 '15 at 18:06
  • I don't have 8 hours of daylight. The direct daylight ranges between 1h in the winter to 6 hours in the summer and I also want it to be substainable in the winter. Also the lithium charger accepts input voltage from 4.5V to 5.5V . – qwertz Jul 9 '15 at 18:08
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I won't be able to tell you which solar panel or battery to use, but I can tell you how to figure that out.

Battery

For the battery there are a few different ways to figure this out:

  1. The most common way to do this at the professional level is to build the device and run it attached to an energy meter and measure it for an hour or two performing its usage, and use that to calculate how much it will use over a 48 hour period. That being said that type of test equipment is not really a hobbyist piece of equipment and probably not something you are wanting to obtain for this project.
  2. You can try and use the datasheets for all of your components to determine their idle and active current draw is, add them up for idle and active, adjust the values based upon the efficiency of your step up converter (so if it only 75% efficient, divide them by .75), guesstimate how long your sensor read process will take, multiply your active current draw times the voltage and times the time it will take (in hours, so if this is a few seconds, it will be a really small decimal), multiply your idle current draw times the voltage and times .083 hours (5 minutes), add those two together multiply it times 12 (12 - 5 minute periods per hour) and then by 48 (two days) and that will be an estimate of your energy usage for two days. Now this will be a very rough estimate as the datasheets only provide nominal values, and their actual usage in practice may be different, which leads me to the final option...
  3. A hybrid between the two above solutions. Set up your device and have it read the sensors and transmit continuously. Then use a multi-meter to measure the current usage of it actively being used. Repeat the measurement again but this time with it running on an indefinite timer to simulate being idle. Then take those two numbers and use the above formula substituting the datasheet estimates with your actual measurements to get a much more accurate picture of what your energy usage will be.

Once you have your energy usage (this will be in milli-Watt-Hours or mWH) pick a battery voltage you will be using (this is based on the charge circuit you are using, typically, single cell lithium ion batteries are 3.7 or 4.2, with multi-cell batteries being multiples of those), divide your energy by the voltage to get the power (in milli-Amp-Hours or mAH) and you'll want to pick a battery greater than that. You'll also want to double check the data sheet of your step up converter and see what the drop out voltage is, and then compare that to the datasheet for the battery and see at what capacity it hits that voltage. You'll want to make sure that it is at least 80%. Otherwise you'll have to pick a different battery with a larger capacity, or a different step up converter with a lower dropout voltage.

Solar Panel

For the solar panel, what you will want to do is get a battery charger designed specifically for use with solar panels. This is because when it says 5.5v solar panel, that is it's nominal voltage in full sunlight, however, it most often will not be that and will vary greatly throughout the day. What this means is that solar panels are basically on unregulated power supply, and most battery chargers require a regulated power source. With a solar specific batter charger, it is designed such that it can handle voltage variation and sudden dropouts. Usually these chargers come with a recommendation as far as voltage and current requirements and those specifications should form the basis of your solar panel selection.

So for example, if you have a battery, on the datasheet it will list what its charge current is, and you'll want to make sure you have a charger circuit that can supply that amount of current. Then you'll want to take a look at the datasheet for the charger circuit, see what current it requires and then make sure that the solar panel can provide that current.

  • Wow thank you very much for this detailed answer. So I guess I'll have to calculate all the power draw of each component and figure out the battery size depending on it. About the solar panel: I found a lithium-Ion chargin circuit for it which requires an input of 4.5V to 5.5V, which is perfect for the solar panel I guess. How can I then figure out the solar panel power I need? Basically I have 1-5h of direct sunlight per day. – qwertz Jul 9 '15 at 20:23
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    You will be unhappy if you attempt to connect the solar panel directly to the charger. You need a MPPT regulator, which will allow you to maximize the power drawn from the panel. – Ignacio Vazquez-Abrams Jul 9 '15 at 22:09
  • I've made an edit, see the bottom. I hope you are catching the general theme here: check the datasheet. Electronics work, especially power design, is more math than it is design. You just need to make sure that at each stage of power that you are providing enough current for the next device and that you are leaving enough extra room for power loss. Things like going from one voltage to another or charging a battery are not 100% efficient, so you have to add in some extra for tolerances. – Jake C Jul 9 '15 at 22:12
  • Also to reinforce what I mentioned at the beginning of the Solar Panel section, and echo what Ignacio commented, you really should use a charging circuit designed specifically to work with solar panels. Here is one as an example sparkfun.com/products/12885 (this does not constitute an endorsement of this product). They can be a little more expensive than regular chargers, but not too much and totally worth it. – Jake C Jul 9 '15 at 22:17

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