I read that you should "always" have a resistor in series between an LED and its power source. But what if the power source already has its voltage set to a suitable max for that particular LED? Why would you need a resistor then?
So here's what I want to do:
Power a 3 watt LED with a single cell LiFePO4 battery. The only other things in the circuit would be a switch and maybe a fuse (mostly to protect the battery). The LED charts I'm looking at show the LED comfortable and productive in the voltage range between 3.3 and 3.5 volts. This is perfect for a LiFePO4 battery with 1 cell. My charger is a Turnigy Accucell 6, which can be set to always charge the battery to 3.5 volts, or any other voltage I tell it to that is reasonable for LiFePo4. I can get a good chunk of the total energy capacity of the battery just running it from 3.5 down to 3.3 volts, which is the range where the LED works ok, without over-current or over-voltage (according to the mfr charts for the LED).
For a straw-horse LED, we could look at this one, which has nice charts available here: http://www.rapidonline.com/pdf/170197_da_en_01.pdf
In this pdf, page 15 has the chart for Vf vs If, with 1 amp being the max If allowed, this happening at 3.5 volts.
Will this work ok - as long as I am careful to never let the battery voltage go higher than what produces max allowable current in the LED?