Using resistors with LEDs

I am going to put some headlights on my skateboard, so I bought a bunch of 10mm ones from LCK on a member's recommmendation. The foreward voltage is 3.2-3.7. However I don't really know how to hook them up.

The website says to use a 82 ohm resistor with a 9v battery, but the LED PRO program shows that I should use a 270 ohm. So I'm a bit puzzled.

I know my ARC AAA doesn't use any resistance at all, so how can I tell what battery configuration will let my run them unregulated? LED PRO also says I should use negative 14 ohm resistance with a 3v battery, and I've had one of the LEDs strapped to a CR123 for about an hour now and it's fairly bright and hasn't burnt out. Can I just run say 3-5 of them in parallel off a CR123 or maybe 3 NiMh AAs with no resistor at all?

I don't know anything about those particular leds, but given the numbers you posted:

Assuming average Vf is 3.5V and that the 9 volt battery is at 8.5 volts under load, that gives you a 5 volt drop accross the resistor.

5V /82 ohms = 60ma

5V / 270 ohms = 18.5ma

The general formula for direct driving leds is:

V(battery) - Vf (forward voltage of the led)
------------------------------------------ = Current (A)
Ohms


A CR123 cell under load is generally in the range of 2.6 to 2.9 volts, so if the led has a Vf greater than that, it would not conduct any current. (Theorectically) - In the real world, you will probably see the led illuminate with 2.6 volts across it, it just won't be anywhere near it's full rating.

(With no resistor, you risk running large amounts of current through the led, way past it's rating. You'll most likely be thrilled and amazed at their brilliance, for a few seconds, until they burn out)

The website is being smart. It's telling you to put two LEDs in series and use an 82 ohm resistor. This will work out really well, as less power will be wasted in the resistor, and you will use half as many resistors.

LED pro is telling you what value to use with just the single LED hooked up to the 9V source. In this configuration, you will need a resistor for each LED, and most of the power in the battery will be dissipated in the resistor.

I would go with two LEDs in series and the 82 ohm resistor.