Super bright Blue LED

You technically won't overload the circuit, your LEDs will just get dimmer and dimmer and dimmer until they won't light at all.  I had three super brights on my DEI alarm a few years back with no problems.

LED outputs typically consist of voltage, the LED, and a resistor,  The resistor will limit the amount current that can be drawn, so that is why you can't overload it. 

I'm sorry if I implied that an LED is a resistor.  I was simply trying to state the necesarry parts in the electronic output of the alarm (voltage, the LED, and a resistor).  Without the resistor the LED would draw the maximum amount of current availible from the power source and burn itself out rather quickly.

This is why you need the resistor, but a side effect of the resistor is that it will limit the maximum availible current availible on the output.  Because of this you shouldn't have to worry about overloading this circuit and damaging it.  It just simply won't output any more current no matter how many LEDs you attach to it.  Sorry for any confustion!

You should always use a resitor with an LED, even when using the 'correct' voltage.  It is more important to drive an LED with the correct current, and the resistor will allow you to do that.  Also, if you are driving an LED with an unregulated power supply any spike in voltage will most likely destory the LED, if no resistor in in line.  Furthermore, LEDs are voltage rated with a tolerance.  Just because one LED can handle 5vdc from a batch doesn't mean that the next three will handle it.  Throw a resistor inline and you won't have to worry about the voltage tolerance.

For more information about this check out:

http://www.superbrightleds.com/led_info.htm

I'm 99.9% sure that ALL comercially availible products utilize resistors when dealing with LEDs.

Haha, I wasn't referring to that part of the page, I was referencing:

DO NOT use LEDS without a current limiting resistor in series with the LED. The forward voltage rating is TYPICAL and can vary from part to part, so while some LEDS may work fine just connected to a battery of the proper voltage, other will be overdriven and destroyed. ALWAYS USE A RESISTOR IN SERIES WITH THE LED OR LED

There is a link on that page that says something like "why do I need a resistor" and it goes on with further detail.

You lost me on the whole kit thing though, what kind of kit are you referring to? 

I would connect the LEDs in parallel, not series.

I never meant that the original poster needed to use a resistor with this application.  I was trying to state that the DEI unit has a resistor built in to it, to drive the LED that comes with the unit.  This is where the 2v output comes from (the voltage drop over the resistor).  The original poster was worried about overloading the circuit and  I was simply trying to explain that it is it impossible to overload an LED drive circuit because they have current limiting resistors on them (evident from the 2vdc on the line). 

If you read the deeper in to the link I posted they touch on the subject of driving LEDs with current, not voltage, as current is much easier to control and more stable when it comes to unstable voltages.  On the site linked to they too recomend to never use an LED without a resistor.  You don't 'have' to use a resistor, but you should.  Most discreet components can be used without resistors, but its not recomended to do so.  In fact, the first circuit I ever built and installed in a  car didn't have a single resistor or capacitor in it, and it worked great (it was a sequencer consisting of a 7805, a switch and a microcontroller directly driving, through internal resistors, three super bright LEDs).  In the circuits that I design today there are countless components added to filter and protect the overall circuit.  We use transistors to protect the microcontroller, resistors to limit current in to the transistor, which in turn limits output of the tranistor, resistors on the output side of the transisors also help limit the total output of the board.  We also use 470ohm resistors on all our LEDs.  Sorry for getting off subject a little there, just trying to illustrate my point that even though the resistor may not be needed, it should still be used.

I've never seen a scanner that you have to use a resistor with, all of the ones I've installed worked off of straight 12vdc.