If I'm correct, the Ego-C Upgrade in its unregulated mode runs the battery at 4.2. - 4.3V, then as Nubbie stated, it will drop in voltage output as the battery drains.
Most ego passthrus are charge as you vape, not true passthrus, unless it has the cord directly coming from the battery itself. So, if the battery is dead, if you plug it in to a power source, you won't be able to vape right away. It'll have to charge a bit first.