Hi guys,
I recently found an interest in building (or buying) a solar charge controller for keeping my new Nexus One smart phone charged while on extended backpacking trips (1 to 3 weeks). The Nexus one is an amazing all in one tool, 5 megapixel camera, full sat gps, ebook reader, wifi (maybe for browsing at the hotel before or after a hike), and of course phone. Of course as good as the battery life is, on a two week hike using a lot of tracklog gps and taking pics you either need a separate backpack full of batteries or figure out another method of keeping it charged.
I bought a 14 watt Powerfilm rollable solar panel that weighs abot 14 ounces and produces peak power at ~15.5V and 900mA. Open circuit voltage is higher, at around 21V. it is a good candidate for solar charging the N1, but since the N1 wants a usb level voltage of 5V so a dc-dc converter is in order. Fortunately this is a common item with cig lighter adapters accepting 12-24V input and producing 5V at 1 amp or more output with good efficiency. But there is a slight problem.
If I hook the panel up to the dc-dc converter and then feed that into the phone, the phone is going to power on every time the solar panel is shaded/unshaded. I plan on hiking with the panel draped on my backpack and there may be a LOT of powercycles to the point where any gain in solar power would be offset by the phone powering on and off. Also it would be nice to be able to use the phone while hiking (gps, picture taking) without having a cable attached).
Enter the Brunton Inspire LiPo battery/usb output module. This neat 4 ounce device has a 11 watt hour LiPo (or possibly LiFePo) battery and some form of boost converter which outputs 5V at up to 1 amp. It charges from usb level voltage and in my tests will accept a charge of as little as 4.9V at 90mA, or about 1/2 watt. It comes with a car cig lighter adapter that takes 12-24V and outputs 5V at 1 amp. Almost a perfect match for my solar panel setup, and will allow me to store charge during the day and charge the phone and other flashlight batteries at night. Except...
I found a much smaller, lighter 6 watt Brunton foldable CIGS panel on clearance at REI for $24. It works great but in cloudy conditions doesn't quite meet the half watt minimum to start the charge cycle on the Brunton Inspire. It *does* produce some power in almost total overcast conditions, but it is around 300mW. In full Seattle sun I have gotten as much as 4.8W out of it, which is impressive for the winter.
This long post leads me to the circuit I would like to design. I want to capture the small amount of power from the panel and dump it into the Brunton Inspire battery at regular intervals in the most efficient way possible. If I am getting 400mW for 6 hours that is over 2 watts that I would like to have stored into my Inspire instead of just wasted because it didn't meet the minimum threshhold. I was considering some form of a latching circuit with a largish capacitor holding the charge and then dump and reset, but if I want the duty cycle on the Inspire to be reasonable (say 1 minute on charge or greater) then I need something like a 10F 12V cap or larger, which is expensive and not small. I am now considering a small NiMh battery, perhaps a 9V 250mA cell to store the low incoming power and then dump it into the Inspire. I need to be able to bypass the battery when the sun is full (and the temp storage is not needed). I can certainly make a two transistor latch, but I am still unsure if I am going about this the right way. Any opinions? Thanks for reading!