how to lower a power bank USB 5.1v /2.1amps output to 4.5v to power a 150 lumens LED flashlight
simulate this circuit &ndash; Schematic created using CircuitLab

This should operate about 
350 mA for a 1W LED  (initially) and then decay towards 10% of this after 24 hrs.

However AAA Alkaline batteries are expected to decay in &lt; 2hrs at this rate and the product page indicates 24 hrs of service ( including dim service)

LED's of this type are rated up to 150 Lumens per Watt and 0.35A*3.2V=1W  THe AAA batteries often have an average ESR of 1 Ohm or less and thus also limit the current to the LED.



You  can expect the Rs for those diodes to make a subtle difference initial brightness and shorter battery life with the 3A diode. Self heating for the 1N400x should be ~ 1/4 W initially. You may compare  the curves of Forward Voltage drop to see the difference,  The 3.5A diode has a lower bulk Rs and thus will make the LED slightly brighter intiially ( maybe not noticable) and slightly shorter operating time. 

My formula for silicon diodes is \$V_f= 0.6V_{(V_{th})}+\dfrac{I_f}{I_{rated}}*0.4V\$ for 1V at max current with 25% tolerance.  For white LEDs the threshold is ~ 2.85V and Rs depends on the Mfg tolerances for rise above this and 1/Pmax.