1. Low Current: The current produced was too low to charge some devices in a reasonable amount of time. I harvested the original cells from solar garden lights and the specs were quite a bit lower than I initially thought.
2. Intermittent Charging: Because it was generating power real-time and feeding it directly to the USB port, the charging capacity was only at its maximum when the Solar Shrub was in direct sunlight. If a cloud passed over, or the cells were shaded even temporarily, the voltage dropped below 5V and the device stopped charging.
3. Difficult to Assemble: My strategy for the original Solar Shrub was to build “stems” with solid-conductor wires reinforced with wire clothes hangers. These stems were then pushed though holes in a base, bent to 90 degrees, and tie wrapped into place. This turned out to be extremely difficult to assemble, the stems were not as flexible as I would have liked them to be, and I had no easy way to replace a bad stem if needed without completely disassembling the shurb. Plus the solder joints between the solar cells and the solid conductor wires tended to break easily because the wire was too rigid.
4. Vulnerable to Weather: One of the first questions I got about the original Solar Shrub on Instructables was, “Is it waterproof?” My answer was no, but it got me thinking. What if the Solar Shrub was outside charging and it started to rain?
So with these problems in mind, I set out to design a better Solar Shrub!
Introducing Solar Shrub II
Higher Output Current - I achieved higher current by purchasing 8 high-quality solar cells rated at 2-volt x 80mA. Then, with help from Joshua Zimmerman at BrownDogGadgets.com, I added a USB booster and internal lithium battery.
Combo USB Boost + Lithium Charge Controller + Lithium Battery - The combo board boosts the voltage from the solar cells to provide a steady 5V to the USB port. It also charges an internal lithium battery which supplements the cells when they are not in full sunlight. Once the battery is charged, I can even carry the Solar Shrub II indoors for charging devices in the comfort of my home!
Pluggable Stems - In order to make the stems more flexible and replaceable, I decided to use stranded wire instead of solid conductors and attach the stems to the shrub base using 1/8" phono plugs and jacks. Now I can easily plug and unplug the stems as needed for quick and easy assembly and/or replacement.
A Sealed Water Resistant Enclosure - Instead of building the Solar Shrub II into a flower pot like the original design, I built this one as a self-contained plastic “capsule” with all the electronics safely inside. The stems plug into jacks on the top of the capsule which also contains a USB cable with female connector. This design allows weather protection and much more portability. It can be placed into any flower pot or container you wish.
For the latest info on the Solar Shrub Project, please visit my blog SolarShrubs.com.
Step 1: SketchUp Drawings
Here are some of my preliminary drawings.
Step 2: Step 1: Materials and Tools Used
Materials for Power Circuit:
1 - Combo USB Boost + Lithium Charge Controller (BrownDogGadgets)
1 - 1,300mAh Lithium Polymer Battery (Li-po) (BrownDogGadgets)
1 - USB extension cable (Male to Female)
1 - USB cable with micro end
Materials for Leaves and Stems:
8 - 2V x 80mA Round Solar Cells (Futurlec #SZGDDIA58)
8 - 1/8" (3.5mm) Stereo Panel-Mount Audio Jack (Radio Shack #274-249)
8 - 1/8" (3.5mm) Stereo Phone Plug (Radio Shack #274-284)
1 - Roll 24-gauge speaker wire (Radio Shack #278-1301)
10 - Feet 14-gauge solid wire (Green)
1 - Roll 3/16" x 8 Ft. Green Shrink Tubing (Harbor Freight #66766)
Materials for Enclosure:
1 - 4" PVC Cap (Lowes)
1 - 4" Blank Cover (Lowes)
1 - 1/4" rubber grommet
1 - Tube silicon sealant
Step 3: Step 2: Building the Power Circuit
I desoldered and removed the full USB output connector so I could solder my USB extension cable directly to the board. But first, I had to drill out the PVC cap, insert the grommet, cut the male end off of the USB cable, and pull the cable through the hole in the cover. I actually drilled 9 holes in the cover; 8 to accommodate the phono jacks plus 1 for the USB cable.
Then, I soldered the USB cable and battery leads to the circuit board and added a little hot glue to the connections. I also hot glued the battery itself to the bottom of the circuit board to keep the two secured together.
Step 4: Step 3: Assembling the Base
I wired the 8 phono jacks in series-parallel to get the desired voltage and current capacity and then wired the micro USB cable to the solar cells.
I wired four series pairs of cells in parallel with each other. Each series pair of cells produces 4V x 80mA. Four pairs wired in parallel produces 4V x 320mA.
The phono jacks have threaded posts, so I attached them to the bottom of the PVC cover using the included collar nuts.
Finally, I plugged the mini USB cable into the combo board input port, placed everything inside the PVC cap, and sealed it up with silicon sealant.
Step 5: Step 4: Building the Leaves and Stems
Each stem consists of a length of 2-conductor speaker wire with a 2V round solar cell soldered to one end and a 1/8" audio plug soldered to the other (matching polarity of course). I also included an equal length of 14 AWG solid conductor wire and green shrink tubing for stability with just the right amount of flexibility.
First I soldered the solid conductor to the metal shield of the phono plug. Then, I soldered the speaker wires to the + and - terminals of the plug. I added a piece of small diameter shrink tubing to one conductor to ensure electrical isolation between the terminals.
Next I slid the green shrink wrap over the entire assembly and added the phono plug's screw on hood. I had to cut about 1/8" off the narrow end of hood to allow it to slide over the shrink wrap.
Finally, I soldered the solar cell to the other end of the stem and heated the shrink tubing. And voila! One stem down, seven more to go!
I used a variety of lengths of wire from 6" to 10" to make the shrub look more like a real plant.
Step 6: Step 5: Putting it all Together
First I set it up in direct sunlight to see if it would chage my iPhone 4S. Success! It's hard to see the charging screen in the bright light so I snapped several pics.
After charging for a while, I took the Solar Shrub II inside for the "real" test. Can it charge my iPhone without sunlight (after the battery has a charge)? Again, Success! As you can see, the shrub is charging my iPhone indoors with only it's internal lithium battery!