Ignoring the usb charger and just focusing on light. There are a multitude of mechanically powered lights. Personally I have several of them. IMHO the most effective and efficient are the Shake Type originally marketed under the name "NightStar jP Shake Flashlight" The best are relatively expensive USD $60+ I have an original NightStar JP that is more then 10 or 15 years old, It has spent hundreds of thousands of miles sitting on the floor of my vehicle next to the drivers seat and still works (just checked)
The reality of mechanical energy to light is usually less impressive (shorter light time) then advertised. There are currently several "shake lights" on the market and some made by the same NightStar company. All the models I have tried have been a disappointment, even the NightStar JP which is the best I have seen or used really only gives a small amount of light, and 2 to 4 minutes of it for a a few seconds of shaking. On a clear night, with just the stars for light, I would not use it to light my path.
The magnet is strong, and can be a risk to any sensitive electronics placed near it. But the magnet also works well to keep the flashlight in place next to the metal base of my car seat.
Having said that the best is disappointing; if my NightStar JP was lost, I would buy another. It does not give a lot of light, and it does not last long. But it always works. It is possible to walk and shake it at the same time. When it is dark and overcast and you literally can not see your hand in front of your face and you don't remember when the last time you checked your flashlight was, a quality mechanical light (that uses a capacitor NOT a battery) can make all the difference.
If you are concerned about ease of use, weight, cost and brightness of light, one or two small LED, battery operated lights, with a supply of spare AA or AAA is probably going to give you more light, for less money and weight then a mechanical light, for trips of several days.
Other answers address, the practicality of spare batteries for short term electronic requirements.