eMacs have a 3.6V PRAM battery and some sources indicate that when the voltage starts to drop that they may have trouble booting. Will an eMac work without a battery?
No. I measured the voltage across the power pins before and after installing a new battery and observed a change from 0V to 3.6V. The three pins for the power switch are 3.6V, ground and ground, with this potential difference coming from the PRAM battery. Without a battery installed (or with a battery that's severely depleted) closing the switch does nothing and the machine will not power on or show any signs of life.
I just started an eMac that has not run for 2 years. Even at that time the battery was kaput; it couldn't keep time and you had to set the time.
It started just fine for me, several times over and over.
However when I plugged in this eMac, the power strip happened to be in a location that is very dark. I could see a minor arc flash as the plug made contact. Something is powered up 24x7, and it has a capacitor!