This post will show you how to schedule an off-grid lighting (or a fan, pump etc) to come on and off on a daily schedule without the hassle of programming an Arduino or other micro-controller.

We pair a Drok relay board (real-time-clock, and relay module to be more specific) as the timer and switch in our off-grid grow-light system. The light will run from an always-on IoT Battery Pack like our V50 and the battery will charge from a Voltaic solar panel.

You will be able to use this template to design your own unique projects.

Applications Where a Timer is Useful

• Chicken coop lighting
• Grow lights
• Small water pumps
• Animal feeders
• Fans
•  

Equipment for Power and Switch

1. Drok 5-60V Real-Time Relay Board
2. Voltaic V50Battery
3. Voltaic 6W Solar Panel
4. Voltaic USB Touchlight
5. F-3511 cable (stripped) (2)
6. M-3511 to M-3511 adapter
7. USB to M-3511 adapter

The Drok board requires no code so it is quick and easy to program. It has a nifty on-board battery, so that it never loses memory of our set schedule and it has a 24hr clock which even keeps track of the day of the year.

Assuming your project is off-grid you will need an Always On battery, solar panel, and USB Touchlight (or any combination of our USB power banks and 6V solar panels which are all available on our website). The Always On battery is extremely important as most power banks will shut off if there isn’t anything drawing power. Our batteries are designed to stay “Always On” and are ideal for long term, low power applications.

You will also need a few specific wires and connectors to complete the setup (linked below). We also have waterproof cases and mounting brackets for a full solution.

Deciding on a Timer Schedule

In our case we want the relay to open from 4am to 6am and 6pm to 8pm, to give our plant some extra light in the long New York winters.

The Drok board can set up to 5 daily time periods.

The time periods are designated PE-1 to PE-5.

So we will be using PE-1 for 4am-6am and PE-2 for 6pm to 8pm.

The Drok board also has 5 distinct modes, P1 to P5, to chose from. We will only use the daily timer mode, P1, which is probably the most useful. It will switch the relay at a specific time everyday.

To interact with the board you need to supply it 5V via the Micro USB port. After the inital setup, you do not need the Micro USB connected if you setup the wiring as indicated below.

Step By Step Setup

Firstly set the time and date for the board (detailed instructions here).

Then follow the instructions below using the SET and the UP and DOWN buttons (detailed instructions here):

1. Enter the PE-1 time period
2. Set the P1 mode
3. Set OPE to 4am (or whatever times you need for your project)
4. Set CLE to 6am
5. Confirm the PE-1 time period
6. Enter the PE-2 time period
7. Set the P1 mode
8. Set OPE to 6pm
9. Set CLE to 8pm
10. Confirm the PE-2 time period
11. Done! And you could add up to three more daily time periods.

Read about the other modes on the Drok website. They include turning the relay on and off on specific days in each month, or for a certain time period on a certain day, or to even pulse the relay on and off. There is also an on-board buzzer that may come in handy.

Wiring Diagrams

Voila!

That’s it. You now have an off grid solar-system set on a timer.

Now you could swap out the light for a small water pump and turn this from a grow-light system into an off-grid aquarium if you had the inclination. Or perhaps an ultrasonic transducer for your next camping tent humidifier!

Getting Creative with the Drok Board

Since the sunrise and sunset times change so dramatically over the course of the year you will need to reprogram your Drok board every so often for a grow-light system. There doesn’t seem to be a way to program a shifting daily schedule.

However we thought about this and came up with a clever way to essentially use your solar panel as a light sensor:

1. Ensure relay is set to Normally Closed
2. Set your PE-1, P-1 to be open all day. OPE: 00:01 and CLE: 23:59
3. Wire your panel to the Input + and Input –
4. Wire your panel to the battery as well (you will need a splitter)
5. Wire your load just as before

Now when the sun sets the light will come on automatically.

So as you have seen the Drok relay board is a simple but useful tool with tons of real world applications.

Further Reading

The next step to developing smarter more capable off-grid systems will take you into the world of the Internet of Things (IoT). Here is an IoT article from Voltaic:

From April 10 to 25, I partnered with Grupo Jaragua, the Dominican conservation NGO monitoring the bird’s nesting sites in the country, to track the movements of the petrel as they search for fish in the Caribbean Sea. For this, I used a combination of light-weight GPS loggers and solar powered base-stations. The loggers record the bird’s location every 30 minutes and transmit the data via UHF to the base-station whenever the bird comes back to its nest to feed its only chick. The logs are stored in the base-station’s 32Mb flash memory.

A Diablotin black-capped petrel ready to be tagged.

Given the remoteness of Diablotin’s nesting sites (30 miles from the coast, ~7,000ft above sea level, in the Sierra de Bahoruco National Park, Dominican Republic), I needed to rely on gear that was allowed on international flights, could easily be transported and adapted in the field (with limited electricity), withstand fog, rain and humidity, and was as cheap as possible. To power the base-stations, I was limited to Li-ion powerbanks (allowed on regular flights, easy to transport and affordable – unlike car batteries). Voltaic’s battery packs have been designed with low-power and IoT devices in mind. Their “Always On” feature, kept power to the system even when the Mataki logger was drawing very little to no current.

GPS tracking hardware:
– 3.5g Mataki-LITE (Debug Innovations)
– 150mAh Li-ion battery (TinyCiruits)

Base-station hardware:
– 1040 Micro Case (Pelican
– Mataki-CLASSIC (Debug Innovations)
– 916mHz antenna (Taoglas)
– 5V-3.7V step-down regulator (with micro USB input), with 1000µF capacitor
– V44 USB battery pack (12,000mAh, 44Wh: Voltaic Systems)
– 5.5W solar panel, mounting bracket and cables (Voltaic Systems)
– Sugru and 2-part epoxy (waterproofing)

The base-station is protected in a pelican case, with the lid blacked out to keep the devices’ LEDs from appearing at night.  Since the battery’s 12,000mAh weren’t sufficient to keep the base-station running for the tracking period I needed (~ 1 month, at 480mAh per day), I paired it with one of Voltaic’s 5.5W solar-panels. The circuit is minimal: the solar-panel recharges the V44 battery, which delivers 5V-2A in “Always-ON” mode. The 5V-3.7V step-down regulator is connected to the base-station, in parallel with the 1,000µF capacitor.

Base station on right before mounting. Lid is blacked out to prevent light from LEDs appearing.

I enclosed the base-stations in a 1040 Pelican case. The battery barely fit inside the case and I had to shorten the male 5.5×2.1mm input plug.  I covered the hole and the cables (for the antenna and solar-panel) with Sugru and waterproofed them with 2-part epoxy: this allowed me to adapt the base-station more easily to the local conditions. For good measure, I also added a handful of desiccant packs into the case. Finally, I deployed the base-stations in open canopy, as close to the nests of the Black-capped petrels we tagged as possible. I tried to position the solar-panel to face the midday sun, a time when the morning fog had already burnt off but the afternoon clouds not yet rolled in.

Deployed GPS Base Station with solar panel.

When we picked them up after a month and a half in the field and the base-stations had worked perfectly for the whole time. The battery levels stayed fairly consistent at 4.8V and with an individual
range of about 0.5V.

I’m glad I could rely on Voltaic’s gear for this study: thanks guys for your support and expertise!

And what about the Diablotin, you ask? Well, they flew between 1,200 and 2,800 miles in a week: a devil of a bird!

Bio: Yvan Satgé is a seabird biologist at the South Carolina Cooperative Fish Wildlife Research Unit, Clemson University. He is currently collaborating on studies of Brown pelicans in the southeast US and Gulf of Mexico, American flamingoes in Yucatan, Mexico, and Black-capped petrel in Hispaniola.

If you want to talk with someone at Voltaic about how to power your project from solar, click the button below.

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If you’d like more detailed assistance, you can talk to a Voltaic expert.

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Plan for December

You should size your system to generate enough solar power to keep your system running during the worst months of the year. In the northern hemisphere, this is December. Further complicating things, this period of low sunlight also coincides with low temperatures which will reduce battery capacity. In parts of Africa, the worst performance will occur in the rainy season.

Online Tool for Measuring Solar Irradiance

Our favorite tool is the Photovoltaic Geographical Information System provided by the European Commission. To use it for remote IoT applications, do the following:

1. Select Location on the Map

2. Select Off-Grid Tab and Enter Panel Information
For this example, we’re assuming a 2 Watt panel and entered 2 for “peak PV power”.
For battery capacity, we first start with an artificially low number of 0.1, a discharge cutoff limit of 0 and consumption per day of 0.

3. Click “Visualize Results”
We now see a graph of “Energy not captured” which maps how much power we would have generated per month if we had a battery included.

Maximizing Performance with Panel Angle

With traditional household systems, you are trying to maximize power production over the course of a year. With an IoT device, we are less interested in total power production then keeping the system running. Therefore in many cases, we suggest tilting the solar panel at a steeper angle to optimize for winter sun.

Here’s an example for a 2 Watt Brooklyn, NY in December and June. By angling the panel steeper, we give up 12% power in June in exchange for producing 8% more power in December.

Multi-Location Estimates

In many applications, systems will be placed in a range of geographic locations or move from different location. In these instances, you should plan for the worst case scenario. To continue with our example of a 2 Watt panel. Here’s how much power the system generates in June and December in Miami, Florida, Brooklyn, New York, Seattle, WA and Stockholm, Sweden. For consistency, we’ll angle the panel at the location’s latitude.

Locations closer to the equator will tend to have more consistent power production throughout the year. Locations further north will produce far less power during winter months. A panel in Seattle in December, on average, produces 39% less solar power than the same panel in Brooklyn. That panel in Stockholm only produces 17% of the Brooklyn panel.

Plan for Worst Days & Power Losses

The Photovoltaic Irradiance model is a good starting point, but does not replace real world testing with your panel, device and battery.

The first gap is that the power production are daily averages over a month. The model doesn’t show the stretch of ten days of non-stop rain and heavy clouds that seem to appear every December in New York.

Nor does the model show power losses in batteries. Just because the solar system generates 10 Wh, doesn’t mean that there is 10 Wh available to your device. With lithium batteries, we typically assume 50% of that power is lost going into the battery and back out to the device. That power loss comes from heat as electrical energy is transferred to chemical form and back again, as well as from regulation.

You can evaluate our full range of small solar panels or submit a quote request for a custom solar panel.

Apogee Sensor from a Solar Panel and Battery

Here is what seven days of readings look like in a backyard.

The sensor consumes 52mA or 0.26 Watts continuously which amounts to 6.2 Watt hours a day. Most power banks shut off at this current level, but the Voltaic batteries have an Always On function which keeps the output activated no matter the power draw.

The Setup

To power the system, we paired the 6 Watt panel with our V44 USB battery. In unshaded conditions, the panel should provide sufficient power throughout the year. The battery will run the logger for 6-7 days without any sun. Having the buffer allows the system to continue running during multiple days of heavy cloud cover.

We placed the battery in a waterproof Nanuk case.

Case, sensor and solar panel were all mounted on a pole. The panel attached via our universal bracket and the sensor via Apogee’s Solar Sensor Leveling Plate.

The plate has a handy level that allows you to know when it is pointing straight up.

Data Logging Results

The system powered by the 9 Watt panel and V44 ran for thirty days without interruption and did not appear to go much below 3/4 battery capacity. The limiting factor in the end was the 10,000 readings. We logged every 60 seconds which results in about seven days of readings. To store data over longer periods of time, be sure to reduce the logging rate to account for your desired time range.

Apogee Sensor from Battery Only

Across the US, Schuyler Smith conducted a similar experiment. He connected a SQ-520 sensor to the V44 battery on its own and placed the system in both the front yard and the backyard. Based on the results, he determined that the backyard was a better location for his garden bed. You can read the full post here on Apogee’s site.

The bottom line is that it is relatively straightforward to power the Apogee sensor off grid.

Evaluation of Three Lithium Ion Solar Charge Controllers

Charging batteries or powering devices through a solar panel is very different than having a continuous supply of DC current, such as through an AC adapter. Solar panels’ power output (Voltage X Current) vary based on the amount of solar intensity and temperature. We visualize this characteristic of each solar panel through something called an IV curve, which shows how much current a solar panel can provide at a specific voltage and specific solar intensity (irradiance). Take the graphic below for example:

This is a generic set of IV curves for one panel, where the colored lines are the different solar intensities. From an IV curve, we can derive the power output (since P = IV) and the maximum power of a solar panel is right at the bend in the IV curve, marked by a star on the graph. This is the point where the system should operate to get the most out of the solar panel. MPPT stands for maximum power point tracker and as the name suggests, its goal is to track the MPP in all light conditions because it shifts with irradiance – the black line.

Adafruit vs. Sparkfun Sunny Buddy vs. TI MPPT

The Adafruit Solar Lipoly Charger doesn’t have MPPT, while both the other charge controllers tested do. Adafruit’s design notes give some documentation on why: higher cost, comparable efficiency, etc. Here is a comparison chart of some of their relevant specifications:

Test Setup for Lithium Solar Charge Controllers

The experiment has two variables: irradiance and panel size. Voltaic’s 1, 2, 3.5 , 6, and 9 Watt panels were used. Each panel is connected through a USB multimeter (we used the YZX ZY1270 and ZY1266) into the solar charge controller, then through another USB multimeter and into a 3.7V lithium polymer cell. We measured the current and voltage in each setup to see how each solar charge controller performs with a specific panel size and under a specific light condition. From these measurements, we can calculate the power output and the efficiency of the module. 

Voltaic has a full range of solar panels for prototyping and deployment.

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Results

In the tests in bright light, the TI module outperforms the rest. However, it is important to keep in mind that past the 3½W panel, the Adafruit Over the Shelf (OTS) and Sunny Buddy hit their programmed current limits and cannot provide any more power. Similarly, the modified Adafruit board hits its 1A limit with the 9W panel. If the current limit for the Adafruit is enhanced to 1A, its power output is doubled at the 9W panel. It’s important to keep in mind the limitations embedded into these boards when comparing them to one another. 

In medium light, the TI module still performs well, but it’s interesting to note how the two different Adafruit chargers vary in these two situations. In bright light, the modified version (1A max) beats the off-the-shelf (500mA max), while in lower light it’s reversed. The current limit is no longer a factor and so this is a fair light condition to test the controllers in. The Adafruit and Sunny Buddy have comparable performance.

In both situations, note how the trend among the panels remains consistent, with all the power outputs varying in consistent ratios from panel to panel.

The 1, 2, and 3½ Watt panel results from the dim-light graph are unreliable because the power output was so low that the USB multimeter readings were out of their accuracy range. Although current is likely flowing into the battery, the amount was hard to measure as the USB multimeters themselves draw current. We estimate that the input to the battery was less than 6 mA. In this additional low light condition, the Adafruit OTS continues to do well as the current limit never comes into play. The Sunny Buddy performed more poorly than expected, but it’s a possibility that the potentiometer should’ve been adjusted in the dim-light setting to account for the lower MPP voltage.

Solar Charge Controller Recommendations

Adafruit Solar Lipoly Charger

The Adafruit is the cheapest, but it does have certain modifications you can make to improve the efficiency and power output. One limitation of the board is that it has a set battery float voltage, at 4.5 volts. This is certainly enough to charge most single-cell lithium-polymer batteries, but it isn’t optimal in all situations. Secondly, there is a current limit that is 500mA off the shelf, but modifiable up to 1A if a 2kΩ resistor is connected across the ‘PROG’ pins. Because of the current limitations, it restricts the power output and efficiency of the Adafruit board in situations with greater irradiance and/or larger panels. Yet at the same time, we see that modifying it reduces its power output in lower light conditions.

Because of its low price, consistent power output, and clear documentation, the Adafruit Solar Lipoly charger is a strong choice for those looking for an easy off-the-shelf solar charge controller that needs little to no modifications and is an all-around performer.

Sparkfun Sunny Buddy

The Sunny Buddy has similar limitations to the Adafruit board. Its default current limit is at 450mA, but it can go up to 2A. The pro of the Sunny Buddy is that there is an adjustable voltage input regulation setting which can be changed by turning an on-board potentiometer. Essentially, the MPPT point is set manually beforehand and the hook-up guide shows how to do this. This customization is a form of MPPT tracking but as the Adafruit design notes stated, it didn’t result in an increase in performance, just in cost. It’s worth noting that also similar to the Adafruit design it has a 4.4V battery float voltage.

The Sunny Buddy has a better power management and battery charging chip, the LT3652. The board is also mostly unpopulated, allowing for more custom connections beyond the barrel jack and JST connectors. For someone willing to delve into the LT3652 datasheet, this is a strong board because of its larger range of features, such as termination schemes and fault detection.

TI bq24650EVM

Finally we have the TI board, which allows modifications on both the input and output voltage sides. It has the most technical freedom since it is an evaluation module, though using it can be cumbersome. It requires diving into the datasheet to figure out which resistors to use for which panel voltage or battery float voltage, and then connecting them onto the evaluation module. However, the results are remarkable, with the highest power output in almost all situations. The benefit of this board, besides the obvious, is its flexibility, allowing a range of solar panels and the ability to charge many battery types, even multiple cells in series. It has a high efficiency and a whopping 8A maximum current.

The bq24650EVM (evaluation module) is not for beginners. Despite its stellar power output and its ability to accommodate almost all solar charging combinations, it is both expensive and complex.

Conclusion

Powering outdoor electronics project through solar isn’t always easy, but with one of these solar charge controllers it’s definitely easier. Combined with a good panel, a strong solar-powered system could be optimized to achieve maximum power in a variety of light conditions and thus increase its longevity and reliability. We hope that this article has been informative to both newcomers and veterans of solar-powered projects, and good luck!

If you want to talk to a Voltaic expert about continuously powering sensors or other IoT devices from solar power, schedule an IoT Consultation here.

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As more and more low cost sensors appear, the reach and applications can be tantalizing. The question that crops up is whether the sensors are any good and can provide reliable data. Led by David Hagan, researchers at MIT, Virginia Tech and Hawaii State Department of Health recently compared nine custom built sulfur dioxide sensors against regulatory grade sensors operated by the Hawaii Department of Health. The paper is published in Atmospheric Measurement Techniques here.

If you’re interested in deploying large sets of electrochemical sensors, it is worth reading how they use a hybrid algorithm that allows extrapolation beyond the training set of data.

The system consisted of an Alphasense SO2-B4 sensor, RHT sensor and a DC fan. The power was provided by a Voltaic 9 Watt solar panel and 4,000mAh battery. The system sent data to the researchers via a Particle 3G modem.

Sensor schematic – from paper in Atmospheric Measurement Techniques

The basic advantage of low cost air sensors is that you can deploy more. If you’re looking at measuring air quality in New York City, for example, a few sensors won’t provide the detail needed to identify where and when the quality of air degrades. The newer sensors also tend to be smaller which allows them to be mobile or seamlessly integrated into the landscape. However, according to Hagan, “accurate calibration of such sensors poses a major challenge.”

Air Sensor Setup

Here are the sensors during setup. This is the same location as the higher end sensors.

And in the final deployment:

Thanks to David Hagan for sharing and to Atmospheric Measurement Techniques.

Solar for IoT and Remote Sensors

If you want a one-on-one conversation with someone from Voltaic about running small systems offgrid, you can schedule a consultation:

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Unfortunately, solar charge time is not as simple as just dividing your battery capacity (measured in Watt hours) by the power of your solar panel (measured in Watts). Even in perfect conditions, you get loss due to:

• Voltage drop of solar panel or Maximum Power Point being lower than rated peak panel voltage
• Energy used by circuit to buck or boost voltage
• Energy used to convert electrical energy into stored chemical energy – with excess lost as heat

We go through two common battery chemistries and give you some rules of thumb for each.

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Lithium Ion Battery Charge Time from Solar

Voltaic carries a full line of IoT Power Banks and small solar panels. Since the charge cycle slows down considerably after the battery reaches 90% capacity, this calculation assumes full is about 90% of complete capacity.

Battery Capacity (in Watt hours) / Panel Power (in Watts) X 2

We perform better than this on our own panels and battery combinations on clear sky days, but this is a more realistic estimate for less than perfect days.

Example: 6 Watt Solar Panel charging a 4,000mAh, 3.7V Battery – Time = 14.8Wh / 6 Watts X 2 = 4.9 hours

Tip: Get a “USB Multimeter” from Amazon to verify your charge rate.

If you are connecting to an off the shelf battery pack, there are a number of reasons that the charge rate could be worse. We look at those in our post: “Can Your Panel Charge My Battery Pack.” Adafruit and SparkFun both offer Lithium Ion charge controllers that can work well with solar panels.

Lead Acid Battery Charge Time from Solar

The effective capacity of a lead acid battery is about 50% of stated capacity, i.e. you shouldn’t discharge it past 50% full. They also have a lower charge efficiency than lithium ion. For the purpose of this calculation, we’ll assume that the battery capacity is the true capacity.

This is our favorite waterproof charge controller for lead acid batteries.

Battery Capacity (in Watt hours) X 2 / Rated Panel Power (in Watts)

Example: 10 Watt, 18 Volt Solar Panel charging a 12V, 10 Amp hour Lead Acid Battery (120Wh) from 50% full to Full – Time = 60Wh x 2 / 10 Watts = 12 hours

Environmental Factors Will Likely Increase Charge Time

The solar charge times above assume a 25 degree Celsius day with the panel pointed directly at the sun. Some quick rules for estimation:

Heat: Power output of a panel will decrease by about .005% per degree over 25 Celsius.

Angle: As the panel rotates away from the sun, power output drops.

Clouds: Any haze or clouds will slow down charge time, often significantly. See: Solar Performance in Cloudy Conditions.

Solar for IoT and Remote Sensors

Want to learn more about how Voltaic solar panels can work for your next IoT project? Contact our experienced team today to set up a consultation.

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Today, Safecast is now working to provide “citizens worldwide with the tools they need to inform themselves by gathering and sharing accurate environmental data in an open and particpatory fashion.” Solarcast is part of that evolution. It is designed to be self-powered and deployed anywhere for long-term outdoor use. It includes dual radiation sensors, dust particulate sensors, a humidity sensor and can communicate via cellular or LoRa. Solarcast uses our 6 Watt solar panel to power the system.

From our perspective, the technology is great, but we are impressed by the commitment of Safecast to nurture and grow a community of people who are passionate about science, measuring their environment and, ultimately, taking action to make our world safer.

Perhaps our favorite feature is this snowboard mount that allows Solarcast to be mobile. Lots of people have ski / snowboard racks on their cars. Put a Solarcast on the rack, position the car where you want to monitor air quality and voila, you’re in business!

For more on powering sensors from solar power, read our Solar for IoT guide. Love citizen science? Learn more about how you can volunteer with Safecast here.

This post will walk you though how to protect your Raspberry Pi while powering it from a solar-powered system, and provide some tips for reducing the power consumption. Our desired goal is to power the Raspberry Pi with only a small solar panel (which you’ll see is not easy considering how power-hungry these boards are), so we’ll provide you with the know-how and tools necessary to reduce the power consumption.

If you want a one-on-one conversation with someone from Voltaic about running small systems offgrid, you can schedule a consultation here:

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Items needed to run a Pi continuously from a small solar panel

• A power management system – Witty Pi, Sleepy Pi, Solar Pi Platter or the system described below
• A waterproof solar panel – most of our customers can safely use a 6 to 9 Watt panel
• A battery pack – our V44 with Always On mode (doesn’t shut off in low power situations) is the most popular choice

Protecting your Raspberry Pi – Autonomous Shutdown

One of the main hazards associated with running a Raspberry Pi from a battery is the potential to damage the Pi if it is abruptly turned off. The operating system must be safely shut down before power is removed from the Pi or the SD card (or the Pi board itself) may be permanently damaged. Therefore, our Voltaic USB batteries may not be the best solution for autonomous Raspberry Pi projects since they provide a constant power supply until the capacity is drained, then turn off without warning.

One way to protect the Pi from abruptly turning off is to monitor the raw voltage of a lithium-polymer battery and have the Pi autonomously turn itself off when the voltage drops to a specific threshold.

Monitoring Battery Voltage

You can monitor the voltage of a bare Li-Po battery with the SwitchDoc Labs SunAirPlus Charge Controller or the Adafruit INA219 volt / current meter combined with their Solar Charger and Power Boost with a USB port. The voltage for typical Li-Po batteries is 4.2V when full and 3.7V when nearly empty, so you’ll want to shut down your Pi board at around 3.7V so that it doesn’t completely drain the battery and damage the SD card.

There is a wealth of information for programming your Adafruit INA219 volt/current meter (no programming is necessary for the Adafruit Solar Charger) as well as information at SwitchDoc Labs for programming your SunAirPlus to detect voltage changes. Wiring diagrams are also included through each company.

Monitoring the battery voltage can be much easier with the right user interface to collect your data. Below is an example of the RaspiConnect interface that allows you to monitor your battery voltage in real time from anywhere in the world. Picture widgets can also be included to observe your time-lapse photography as it happens or record video.

RaspiConnect interface for monitoring your battery voltage in real time

Autonomous Shutdown

To shutdown the Pi at 3.7V, see examples of these shutdown commands with a push button using a while loop or an interrupt command (this can easily be modified for detecting a voltage threshold) at raspberry-pi-geek.com. Anyone familiar with Raspberry Pi’s can insert a “sudo shutdown -h now” command when the voltage goes below 3.7V

Note: Your Raspberry Pi board cannot read the voltage directly since it exceeds the maximum voltage of the GPIO pins (3.3V). Therefore it is best to translate this information to the Pi through an Arduino board. The Arduino can control when the Pi gets power through a transistor or latch relay, thereby removing power after the Pi has shut down and providing power again when the voltage of the battery increases.

Simply monitor the voltage of the battery through an Arduino and have it send a unique signal to the Pi board(as simple as True/False, High/Low, 1/0) when the voltage dips below 3.7V so the Raspberry Pi can shut itself down. About one minute after the Pi board has shut down, the Arduino can stop providing power to the transistor or latch relay and remove power from the Pi altogether, that way when power is reapplied the Pi will automatically turn on. More information is available at John Shovic’s Project Curacao page.

Reducing Power Consumption

Raspberry Pi’s consume a lot of power by DIY microcontroller standards, and unfortunately they do not support a “sleep mode” like Arduino boards to reduce power consumption between important activities. You can still reduce the power though by cycling the Pi on and off for set periods of time.

24 / 7 Operation – Cycling the Pi on and off

If you want to keep your Pi board up and running around the clock, you may want to consider how often you actually need the Pi turned on. If you are collecting sensor data or taking time-lapsed photos, it would be advantageous to turn the Pi completely off between data collection and turn it on only when necessary (the less often the better!).

Keep in mind, running a Pi for 24 hours per day can consume a TON of power. Even the smallest, most power efficient Pi board out there (the A+ board) consumes at least 24 Watt-hours per day from constant use, so you’ll need at least a 9W panel to maintain this board in bright sunshine.

Earlier we talked about programming the Pi to autonomously shut itself off, but it is a little harder (translation: impossible) to have it turn itself back on. This cannot be done without another device that stays on indefinitely, and a small Arduino can be the perfect solution. By controlling the power supply to the Pi with a latch relay or a strong transistor, you can have the Arduino awaken your Raspberry Pi at set intervals. All Arduino’s have internal watchdog timers that can be customized to any length of time you want, from milliseconds to hours. Only a small board is necessary, as its only job is to turn on the Raspberry Pi after a fixed period of time (and possibly monitor the voltage of the battery since it can receive sensor data at 5V without scaling it down). They should be put to sleep when inactive to reduce power consumption further.

Note: Technically the internal watchdog timer only lasts 8 seconds before it wakes up the Arduino, but you can set a fixed number of sleep cycles that the Arduino must wake up from before it actually does anything. So if you wanted your Raspberry Pi to turn on every 10 minutes, you would simply set the appropriate number of times it should cycle through the 8 second sleep mode to wait the full 10 minutes before the Arduino allows power to return to the Pi. Code snippets of this are found on our Voltaic GitHub page and through Adafruit tutorials, and examples of this in action are seen in our Solar Air Quality Sensor to wake up an Arduino Uno every 5 minutes.

This can also work simultaneously with the Raspberry Pi’s automatic shutdown feature, so even if it should still stay powered on for a few more minutes but the voltage drops too low, the Pi will safely shut down.

Time-Specific Operation

Turning on the Raspberry Pi for only certain times of the day can also dramatically reduce the power consumption. Perhaps the data you want to collect or the pictures you want to take are most useful to you during a certain period of time, in which case you can program the Arduino to only turn on the Pi during that time of day using a Real Time Clock. Have the Arduino send a special signal of your choosing to the Raspberry Pi when your desired time interval has passed so that it can safely shut down and remain off through the night.

3rd Party Pi Management Boards

Since we first wrote this guide, several products have come on the market to help manage Raspberry Pi’s power consumption.

Witty Pi 2 by UUGear: This product adds a real-time clock and power management to the Pi. UUGear has even written a tutorial to create a Pi-based time-lapse camera using our 6 Watt solar panel and V15 Battery Pack with Always On mode.
Sleepy Pi 2 by Spell Foundry: We have at least one customer using this board for smart power management of the Pi, but no solar tutorials just yet.

Sizing your Solar Panels

It may be useful to briefly compare the power consumption of different Raspberry Pi boards so you can appropriately size your panels.

Raspberry Pi Model A+ and Model B

Note: Every Pi script is different, therefore they all have different power requirements based on the additional accessories and sensors. The numbers presented here are approximations based on power consumption averages with no additional components. The best way to know how much power your Pi board actually uses is to monitor the voltage and current with a USB amp-meter from the battery or some other method.

Multiply the current by the voltage (USB power supplies are 5V) and by the number of hours you plan on running the Pi and you’ll have your daily power consumption in Watt-hours. This is the benchmark from which you’ll compare the output from the solar panels you are interested in. Multiply the rated wattage of the panel by the number of hours of sunshine you think you’ll receive each day and then discount for power loss along the chain by 40%. For example, if you think you’ll have bright sunshine for 6 hours per day, the 9W panel will produce 32 Watt-hours.

There are a lot of variables, but we generally recommend choosing a panel size that produces, on a good day, 2-3 times as much power as you think you’ll need on a daily basis.  If you so if you are using the Pi model B+ and your unique project requires 40Wh per day, we recommend using enough solar panels to generate at least 80Wh per day (equivalent to the 18W Charging Kit exposed to 4.5 hours of sun). This is why it’s critically important to reduce power consumption!

Remember! Don’t only count the number of hours the sun is in the sky, count the number of hours the sun is pointed directly at the solar panels. If there are trees or buildings that cast shade over your project for certain hours of the day, that shortens the amount of time the panels are pointed directly at the sun.

Solar Power for Raspberry Pi: Conclusion

With the appropriate software built into the Raspberry Pi to protect itself and some medium-large solar panels your Raspberry Pi project can live on indefinitely. Combining a Raspberry Pi with an Arduino can add many benefits to your project, such as safety and extended battery life during bad weather, though it is certainly not necessary.

If you can accomplish your project goals without using a Raspberry Pi at all (perhaps by using a BeagleBone or Arduino) it may be in your best interest to do so because of the technical obstacles presented here (high power consumption, risk of corrupting the SD card). Though if you are already very familiar with Raspberry Pi’s or want to use some of the powerful RaspiConnect interfaces to track your data in real time, then it is certainly possible to sustain your project with solar power.

For more on remote systems, check out our solar powered Arduino guide.

In this guide we look at the options you have when deciding on solar panels for small motors starting with how to begin select the right panel for your motor. In general, we found to get the best performance to cost/size ratio when the voltage of the panel slightly exceeded the voltage rating of the motor and a peak current 25-50% higher than the max current of the motor.

Please repeat the tests for your projects and let us know what you find.

We experimented with four simple motors:

• DC Toy Hobby Motor (4.5 – 9V, 0.25A)
• HSD-8025 Brushless DC Cooling Fan (12V/ 0.12A)
• DAGU robot DG01D Mini DC Gear Box (4.5V, .25A)
• Liquid Pump – 350GPH (12V, 1.5A)

And we used a suite of panels for testing including: 2 Watt, 6 Volt / 3.5 Watt, 6 Volt x 2 (paired in series to get 12V) / 6 Watt, 6 Volt / 9 Watt, 6 Volt / 9 Watt, 18 Volt / 17 Watt, 18 Volt panel.

Shop Solar Panels

Making the Connection

You can simply twist the positive and negative leads from the solar panel to the motor. We recommend using our 1 Foot Extension so you don’t have to hack our cable.

To make things easier to swap panels and take measurements, we wired each of the small motors and our extension cable with exposed leads to jumper wires.

This DAGU Motor already had jumper wires.

Basic Behavior

When a panel isn’t connected to anything, it has a relatively high voltage and zero current. Our 2 Watt, 6 Volt panel has an open circuit voltage a bit over 7V. As soon as you connect to the motor the voltage drops and, if you meet the minimum threshold of the motor, current flows and the motor spins.

In this video, you will see motor speed up when pointed directly at the sun and slow down when panel is angled away from the sun or shaded.

2 Watt, 6 Volt Panel Results

With a 2 Watt, 6 Volt panel and less than ideal conditions, the motor spins, but the motor draws less voltage and current than its’ specification. In good conditions, the voltage and current climb slightly above the spec of 0.25A and the total power through the motor is about 3/4 of the rated specification of the panel. When we connect to a larger 6 Watt panel, the motor reaches a maximum current of 0.43 Amps, well above the spec of the motor and less than half the rated output of the 6 Watt Panel. Our 2 Watt or 3.5 Watt panel seems like the best fit if you are expecting reasonably good conditions.

DAGU DG01D Results

We see similar behavior with the DAGU DG01D, but it has more resistance than the toy motor and needs more power to reach its max. A 2 Watt panel can get it turning, but it is too small to get much speed going and we’re only able to get about 30% rated power from the panel. In this case, we’re exceeding the rated current with a 6 Watt panel in perfect conditions, but in imperfect conditions, we are able to make the motor move consistently. We’re also getting 75% of the rated power into the motor. Bumping up to 9 Watts, it is easy to exceed the max current specification of the motor in a range of conditions.

Low Voltage Kills Power

We tested also tested a 12V motor, 0.1A motor with 2 x 6V panels in series and parallel (6V and 12V). With the lower voltage setup, the voltage, current and total power output were less than with the panels in series, the power output increased by about 4X. The lesson is that if the panel is well below the rated spec of the motor, it may spin, but you will be wasting a lot of power.

Real Work – Doubling Power on a Pump

If you are below the rating of a motor or pump, increasing the amount of solar power, will increase the power through the motor. As a quick example, we connected a single 17 Watt panel up to a water pump and it moved water pretty quickly out of the bucket through a 3/4 “ hose. However, that panel outputs 1A max, below the maximum rating of the pump. Adding a second panel increases the current and voltage and increases the through put of the motor.