Summary

The solar cell we use is rated for only 80mA, (because we ordered a kit) so we're in the clear.

 

For future projects with this design and concept it would be interesting to test if super capacitors charge both themselves and the phone itself faster. 

Solar USB Charger

Aubrey Carter / Ludnell Lubin / Josh Chapman / Alex Donlandson
 
 

Solar Power

Batteries require a minimum voltage in order to charge, but raising the voltage will not cause the batteries to charge faster. The general rule is to provide 1.5 times as much voltage as your battery needs (for us 3.6V x 1.5V = 5.4V), but with solar we want some wiggle room. We want our solar panel to meet the minimum voltage, even on days with a few clouds, so a 6V panel does a good job of making of for the Suns' inconsistency. When charging AAs using a wall adaptor, we're able to charge them at high speed due to smart chips that constantly monitor the battery. Since our circuit is "dumb," with nothing to monitor the batteries, we trickle charge the batteries using the 10% rule. AAs can be safely charged as long as they don't have more than 10% of their capacity thrown at them at any one time. This means our 2,000mAh batteries should only get 200mA of current.

Electronic Parts

Soldering Iron

 

Solder

 

Wire Cutters

 

Wire Strippers

 

Screw Driver

 
 

USB Project

USB charging requires 5V at around 500mA of power to charge most gadgets. To ensure our product receives enough Volts we use a USB Charging Circuit, which boosts low voltage (2V) DC up to 5V DC. Our increaseda voltage lowers the milliamp level. To avoid this tradeoff, we could use larger solar panels and larger batteries, but this would result in a larger, heavier, more cumbersome and more expensive charger.  

Battery Power

Solar power is a bit inconsistent due to the inconsistent nature of the sun. Using batteries helps stabilize the flow of power, and also lets the charger store power for later. In this project we use three rechargeable AA batteries. Rechargeable batteries put out 1.2V of power and using three in series gives us 3.6V. Using two batteries will cause our voltage to drop below 2V too quickly, and our USB Charging Circuit won’t be able to operate. Four batteries would require a much larger solar cell to charge so three is the obviously the best choice.