Need some advice with solar charging circuit please

[KTP]

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!

[KTP]

I have been thinking some more on this and would like to incorporate the ability to charge two AAA NiMh batteries in my solar hiking solution as these are used in our led headlamps and FRS radios (when we take those).  It would be great to incorporate these two NiMh cells in the charge storage/dump circuit to charge the LiPo module when conditions don't meet it's minimum power input requirements (as mentioned, about 500mW minimum).  I would need to boost the 2.4 volts from the two 800maH NiMh AAA cells to 5V so it can charge the Brunton Inspire LiPo module through it's USB port.  Opening up a cheap DC-DC car cig lighter converter (on clearance at radio shack for $1.47  I see it uses a 8 pin MC34063 dc-dc converter IC, configured as a step-down converter.  The data sheet shows this part can also be configured as a boost converter (step up) but it seems to only work down to 3V 

The data sheet is here (with nice circuit layouts too):

http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF

Perhaps there is a similar chip that will work for ~2V input...I know there are a lot of single cell LED flashlights that must use some sort of boost converter.

[xani]

Seems that u have to make your own battery module (charging phone from LiPo battery which is charged from NiMH which is charged from solar panel is kinda.... werid, and you will lose a lot of power "in meantime". IMO just get cheap 5Ah 12V Sealed Lead Acid battery (these weigth about 1,4 kg), charge it with solar panel (u prob could away with just putting diode in series, but simple charger circuit wouldn't hurt) and use that 12/5V converter to power up phone.
Edit: Or do some kind of LiPoly charger but that will prolly be much more complicated

[Simon]

I agree, forget the comercial storage thing its making the problem in the first place. use a lead acid or bank of AA/AAA NiMh batteries (or other type) to directly store the panels power, then charge anything else from that, so say you stop at the hotel you can charge the phone off the battery, you could to make it more efficient have the sets of batteries for your phones charging off the panel too via a DC-DC stepdown converter

[djsb]

Hi,
Maybe this is what you need.

https://powertraveller.com/

David.

[KTP]

It turns out the car cig lighter adapter dc-dc that came with the Brunton Inspire LiPo module was more of what was limiting the lower power charging capability.  I tried another generic dc-dc adapter that seemed to have a lower dropout voltage and it caused the Inspire to start charging with as little as 4V at 50mA input (200mW).  If I am getting less than 200mW from my 14 watt panel the day is pretty dang overcast.

The LiPo module is 4 ounces and contains a USB boost converter inside.  I don't think any lead acid solution can come close to that power density.  In hiking it is all about ounces, not cost.

[Simon]

surely so you need some of the later technology rechargeable batteries that I probably haven't got around to needing yet