Perhaps you can experiment. Maybe get a few resistor values and try them. The Vf can vary a bit depending on how the LED was manufactured. If you would find the possible range of voltage that the LED would operate at when powered at 1.5A (like 3.6V-4V for example), then you can enter them into the calculator to find a range of resistor values to try. Then you can buy a few resistors with a values that will fall in this range. Depending on the source voltage, a resistor that will work when the LED Vf is a generic 4V may give you lower or higher current levels when the actual Vf of the LED is 3.81V, for example.
If you have any spare linear voltage regulators laying around (like the LM317), you can find the voltage going to the LED that makes the LED draw 1.5A. Then you will know the true Vf of the LED. Maybe a LM317 set in current limiting mode can be used in the light. The efficiency may suffer some, but can it be any worse than using a resistor?
I wish it could be simpler (unless there is some easy process that I am missing). A current regulating circuit is ideal, but I feel your pain when finding one that can handle higher currents. Perhaps a bunch of those current regulators from DX will work (the ones that give you 350mA per parrallel attached IC). If you could attach just one more in parrallel to the board that already contains 4, you can get up to 1.75A. 1.75A isn't too bad on the LED, but I understand how the max input voltage will be a problem.
Have you asked if the Downboy can be modded to output 1500mA? I thought it could. With George's D2DIM between the batteries and the Downboy, you might be able to dim the current regulated circuit (using good 'ol PWM). Not sure if this was addressed in your other thread.
Finally, have you thought about using only one Li-ion cell or maybe two in parrallel (make sure they are matched, and/or balanced when charging)? Then you can use a more common driver (like the DX regulator mentioned before). Hmmmm...
I wish I could help more. I cannot wait until more manufacturers and dealers make higher power driver circuits for single LEDs. I thought that Dat2zip may release a beefier Downboy or something. Maybe I was seeing the max current specs of the SOB at the Sandwich Shoppe. I suppose you will find something. I predict that LED emitters will be made to handle higher currents (or even be spec'ed at them) more in the future (like the 'ol K2). With LEDs becoming more efficient, they will run cooler and more efficiently at higher currents. Companies/Dealers/Developers have to release some type of higher current drivers eventually. Good luck, and please keep us all posted. This will be useful for many in the future.
-Tony
