- Tag Your Green Contest 0 – Introduction
- Tag Your Green Contest 1 – Preliminary Design
- Tag Your Green Contest 2 – Designing a Solar NiMH Charger
After looking into a number of different options for making a solar device charger, here are my current thoughts.
I am not quite satisfied with the Adafruit Solar Minty Boost I referred to earlier; I would rather have a device that used NiMH batteries, and that could also serve as a stand-alone NiMH charger, rather than the LiPoly battery used in the Solar Minty Boost. I can’t prove it, but I have a hunch that NiMH batteries will be cheaper, and have a simpler recharging circuit than the LiPoly, and will also be more readily available to folks. I’ll do some further investigation on this point.
I will be starting out using some of these 6.7V solar cells from Solarbotics. There’s an interesting note about splitting the panel into separate cells and running them in parallel to get half the voltage and twice the current. I want to keep the solar panel voltage low, enough to charge the batteries, but less than 4.5V so I don’t over-charge the LT1302 step-up converter chip. If I can get ~80mA from each of these tiny panels, I could use 6 of them to get 480mA, or maybe 7 or 8 to get 500mA in less than full sunlight. If I can get this much current, I might be able to recharge devices directly with the solar panel output, instead of the batteries. It will take more experimentation and analysis to determine this.
Anyway, as a starting point, I’ll be following the Minty Boost design, and adding solar cells to recharge the batteries and (possibly) drive the step-up converter to charge devices directly. I’ve ordered the parts for the Minty Boost (I expect to make my own, modified circuit board). Next, I’ll put together a first cut of the schematic, and when all the parts come in (including the solar panels) I’ll start putting it together for analysis.
This is a bit of a weird project for me to do, since I don’t actually have any USB-chargeable devices that I could use it with, so I’ll have to find some friends willing to let me test it on their own devices.