“When I designed the Stella amp a couple of years ago, I realized that I could make the amp run at 3 volts. I knew right then that I wanted to make a solar powered version of the amp. A few months ago, I grabbed one off the shelf and picked up everything I needed from Adafruit to make it happen.”

The original open source Stella Amp kit can be found on Wilson’s site for musically inclined DIY’ers. The solar upgrade just requires a few more parts from Adafruit, one of which is our own 2-Watt solar panel. The end result is a compact and portable amp that can go anywhere without ever needing to be charged.

General Guidelines for Combining Power Sources

• Voltages of power sources should be similar. i.e. Combining 6 and 10 volt panels is probably OK. Combining 6 and 14 volts is a poor match
• Panel voltages should be higher than the charging/load/battery voltage. A panel voltage that is lower than the battery voltage isn’t going to charge the battery. We find that panel Voltages 1.5x the Voltage of the battery typically results in fast, efficient charging.

Once you’ve checked that your solar panels fit the criteria above, you can wire them in parallel. To do this, simply connect the positive connections together, and then connect the negative connections together. Our circuit boxes have a switch that allows for parallel connection of multiple panels.

Lets look at how combining different panels worked out for us in the lab.

The setup

We used a 6V 2W panel and a 10V 1.3W panel (discontinued), connected in parallel using a modified 3-panel circuit box. The device we charged was a completely drained V11 battery pack. The table below shows the open circuit voltage of both panels independently, then combined in parallel, and the performance of each configuration charging the V11.

This means that it is ok to combine panels of different Voltages together as long as they are roughly similar and they don’t fall outside of the specifications of the device you’re charging. The Voltage of the combined panels drop to whatever the load requires to charge and the POWER output of both panels on their own is roughly additive.

Combining solar panels with other power sources

In the case above, with two panels of similar voltage, we see that the power is approximately additive. In a situation where you might combine solar with energy generated from a dynamo or wind generator, you will need to use diodes to prevent the solar from driving the dynamo as a motor and the dynamo’s energy from being dissipated through the solar panel.

We tested the output voltage of our dynamo from a previous post and combine it with a 6V 2W panel to charge a V11. We want to show the difference between connecting the two sources in parallel with and without the diodes

If we combine the max power output from the panel of 1.44W with the 2.4W from the hand cranked dynamo, we expect to see 3.84W going into the battery. Where’s the missing 0.84W? In the case where the panel and dynamo are connected in parallel without the diodes, some of the extra power from the dynamo is actually absorbed by the solar panel. In the case where we have diodes in series with the parallel power sources, the diodes are responsible for most of the missing 0.84W. The reason is that each of the diodes requires voltage to operate, at least .7V, and this drop comes at a cost in terms in power. The amount of energy lost in using the diodes is the current multiplied by the voltage drop of the diode. If we want to minimize this loss, selecting Schottky diodes would be the way to go, as they only “cost” .5V to operate.

To summarize our findings, it is possible to combine multiple power sources in parallel. If the power sources are similar in Voltage and type (i.e. both solar panels), then it is straightforward and you increase the total power produced. If the sources are different in type (i.e. a solar panel and a dynamo), the system will likely increase total power generated, but you need to include diodes in your design and that will cause some power loss.

In a previous post, we showed how you can charge GoPro Hero cameras from our solar chargers. Since this is such a popular device to charge, we did a bit more testing and discovered that you can charge a GoPro Hero direct from one of our 6Volt, 2.0 or 3.4 Watt solar panels. This method is not as consistent as charging from one of our batteries however and we recommend it only if you’re in a pinch. The model we tested had a 4 Watt hour battery so it would take about 5 hours from a single 2 Watt panel to charge in direct sunlight, about 3 hours from the 3.4 Watt panel (there are several loss factors in charging we go through in our tutorial).

In addition to the panel, you’ll also need our Female 3.5×1.1 MiniUSB adapter to connect from the panel to the GoPro.

The advantage of this approach is that it is cheap and you don’t have to worry about waterproofing anything as the panels are waterproof and nearly indestructible. The downside is that it is less consistent then charging from our batteries.

Techy note: the Hero does have a blocking diode so a shaded panel won’t pull charge back from the Hero.

Here’s the video of the car cruising through our offices.

If you want to make your own solar-powered lego car, you’ll need:
-Lego parts, including gears to transfer power from the motor to the wheels
–Lego motor
–2 Watt solar panel

Here’s a few close-ups of the car:

We did a quick mod of the car to put in a female 3.5×1.1mm jack so that we could easily plug and unplug the panel from the motor. You can use our “USB Out Cable 3.5×1.1” if you want to avoid cutting the plug on our panel.

Go Legos! Wanna race?

Go here to see all of Voltaic’s rugged small solar panels.

Pair up the kit with one of our 2 Watt solar panels (also available from Adafruit) and one of their Lithium Polymer batteries.

The charger is tailored for solar and will pull as much power as possible in any lighting condition. The kit includes the charger board, a large capacitor and a JST 2-pin cable for connecting the battery or load. You can also charge the battery from a USB cable if there is no sun.

The board includes indicator lights that tells you when its charging and when the battery is low. There is no temperature monitoring of the battery. You’ll need to keep the battery in the shade (and dry) and add in a thermistor if you want to use it consistently outdoors in hot and cold settings.

Adafruit recommends reading the full tutorial before purchasing.