I'm powering my LilyPad Arduino with a 3V coin cell and want to take analog input. Does the analog reading of 1023 always represent 5V regardless of Vdd (which is 3V in my case), or in my case would 1023 represent the 3V Vdd value?
If the analog reference is default, 1023 will be the power supply voltage (what ever the supply voltage is). So for 3v3 power 1023 is 3v3. Measure the actual supply voltage to be precise.
Do you mean lithium coin cell like CR2032? They are only rated for 2mA continuous
or so - this could prove problematic anyway...
The ADC is ratiometric w.r.t AREF - whatever AREF is is the full-scale. When set to
Vcc its relying on the supply being stable.
Maybe you should read this: