Quick LED question

[Fallingwater]

Can I hook a standard white 5mm LED to a regulated 3v RCR2 cell without any resistance inbetween?
How about an unregulated 3.7v RCR2?
Thanks.

Edit: dammit, I wanted to open this in the electronics board. Could it be moved there? Thanks.

 

[Amonra]

yes you can hook any white led to a 3v RCR2 with no resistance but it will be underdriven. with a 3.7v one it is a different story as they normally come off the charger @4.2v which would be overdriving the led quite a bit and shorten it's lifetime to a few hours maybe( normally a white led requires about 3.4 - 3.6v to be driven to spec ).
the real question is how long do you expect the led to last you.

 

[Fallingwater]

How much resistance would you say I should add to the circuit to get acceptable life out of the LED? I don't need the "100.000 hours" typical life, a few thousand (or even a few hundred) would be ok.

 

[Amonra]

i found this calculator very helpful: http://led.linear1.org/1led.wiz
if you run it at 40mA you should use a 15 Ohm resistor for 60mA you should use a 12 Ohm one