more on battery vs capacitor in my design

Skyko

Interesting other thread on capacitor vs battery since I was considering the same thing in my portable solar charging solution.

The problem: The Brunton Inspire LiPo battery/USB output module will not accept power in below about 500mW. At about 4.8V and 90mA from a solar panel it shuts off and does not charge the internal battery (the charging icon goes off and the 4 blue indicator leds come on).

4.8V and 90mA is small, but it *is* power and on a cloudy day it might be all you get from a small portable panel draped across your backpack. Over a 6 hour period that is over 2.5W lost if the Brunton isn't charging, which is enough juice to get several hours of runtime from my combo gps/cell/ebook reader Nexus One.

I have thought about a possible solution that is simple and lightweight and am looking into a latching power storage circuit between the solar panel and the Brunton Inspire device. If I went with a capacitor and wanted the duty cycle to be reasonable, say dumping a charge into the Brunton every other minute, I would have to use about a 10F 12V capacitor, or an array of capacitors meeting that value. This would provide about half an amp into the dc-dc converter for almost a minute before the cap voltage has fallen too low. A 10F 12V cap is huge and outrageously expensive if you can even find one. (think $50+)

Perhaps something that makes a lot more sense is using a 7.2V 250maH NiMh 9V battery, costing perhaps $5 or less. This would provide several minutes of half an amp at 7.2V into the DC-DC converter and should allow the Brunton Inspire to charge (5V at 100mA into the converter seems to provide enough out of the converter to start charging the Inspire).

The only downside is the NiMh battery is going to see quite a bit of charge/discharge, though probably never being charged to full and discharged completely. The circuit will also need to bypass the NiMh when the panel is getting full sun, and prevent any overcharging of the NiMh buffer battery.

Nothing is ever simple is it?