Hi - I've just skimmed this thread (giggling a bit, since I own one one of these little critters). Most of what I'm about to post has already been covered, but it's a copy/paste from one of my files, so just skip the redundant parts. I'm feeling too lazy to edit.
The eGo-C Upgrade an interesting model Joyetech came up with prior to the Twist. Its sometimes called a
variable voltage device, because the voltage does vary, you just cant control it like with a Twist or Spinner.
The battery has 2 modes: Unregulated (the button is yellow) and Regulated (the button is white/blue).
In the unregulated mode (Joyetech calls it "Battery Voltage" mode), the battery comes off the charger at around 4.2v and will fire at that output. The voltage then goes down as the battery discharges. I've been told it goes down and hovers around 3.7-3.8v for awhile before dropping lower, but I have no way of testing that
In the regulated mode, the battery fires constantly at ≈ 3.3v. I think it's pretty much a gimmick, or concept over function fail. The point of VV is to be able to adjust the volts to where you want them to be. This battery does not do that.
To make the voltage even less operator variable, in the regulated mode the button is white when fully charged, then turns darkening shades of blue as it discharges. In the unregulated mode the button stays yellow. If one is vaping in the unregulated mode, there is no way to tell what voltage you are (roughly) at without switching to the regulated mode and trying to guess if the button is cobalt or cornflower blue. I just use mine in the regulated mode like a standard eGo.
To switch modes, click the button 5 times to turn it off. Hold the button down until it turns white/blue or yellow, whichever is different from the last mode you were in and it will blink 3x in that color. Click it 5 more time to turn it back on.