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.