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I have tried apps, but they are really bad at reading the color codings on resistors. I would suggest either measuring with a multimeter or just read the color code using a chart like this one:http://nearbus.net/wiki/images/7/7d/Resistor_color...And yes, higher resistor value is less bright. Most commonly used resistors are 1k - 10k.
Many more expensive battery packs offer 110/220v, like the Goal Zerio Yeti 150 so you can plug in just like with the wall.http://www.goalzero.com/p/164/goal-zero-yeti-150-p...Many cheaper packs also have 12v supply (like the Goal Zero Sherpa 50)http://www.goalzero.com/p/151/sherpa-50-power-packand you can find 12v to 110/220v inverters in many forms on amazon.I would not suggest modifying medical equipment yourself as it can quickly become very expensive and dangerous if something breaks.Your charger draws 50 watts from the wall, that is out of the realm of USB but totally possible from either of the batteries linked above.
To get your super simple circuit done, you will need some resistors.The basic setup would be to connect a 500 - 2000 Ohm resistor to one of the legs of the LED. Connect the other leg of the LED to your USB wire and the last leg of the resistor to the other part of the USB.And LED has 2 pins but only works in one direction. If there is no light when you try it, don't be afraid, simply switch the legs and it should work.If you pick a very large resistor, there will be a bit less light. Don't do anything below 500 or it will blow.
Some portable USB chargers give more power, but yes.