This flashlight is very simple to build. It uses a cellphone li-ion battery . Its rechargeable. It should only draw about 900mA from the battery. It lasted for about 74 minutes before it went too much below the light bulb voltage. I measured it while the light was off after it went dead it read 3.27v and while running little bit above 2v. It is best practice to not leave it on after it dies because they cannot go below 2v.
Circuit diagram 
On the circuit D1 protects the USB port from any inductive spikes. D2 blocks the battery voltage going back to the power supply, plus drops the voltage to a safe level since most cellphone batteries have built-in protection circuit.
Here are the items you will need
1x cardboard paper-towel tube
1x Solder iron,scissors nnd hot glue
Circuit Simulator