Ah, it's actually 18 Volts to "command" the PCM to accept a new tune I think.
As usual, my SWAG was not even close. Thanks Jack! 
The programmer has a "transformer" (sort of) that raises battery voltage to this level to accomplish the task. Everything else that was said about a healthy battery is dead on. The only place I change my tune is in my garage, with the headlights off, A/C off, radio off and battery tender plugged in. On an 09+ truck, the electric fans should not be on unless the engine is hot, so you probably don't have to pull the fuse.
Slight clarification, only the 10s come standard with electric fans.
But, something that was not mentioned is that the SCT seems to take a super long time to program the PCM! I've heard 40-45 minutes on another forum! That's a long time to be sitting with the ignition ON and the engine off. The CTS programs my 2005 truck in less than a minute. I have not heard if the programming time is longer for 09+ trucks with the CS/CTS.
My CTS takes about 1-1/2 minutes to program my 09. Note this is the Edge canned tunes. Not sure if the custom tunes will take longer.
With a 40 minute programming time, I would definitely have a battery tender or 2 Amp battery charger connected!
Yep - I do, even though the flash time is short.
Since I tow, I really appreciate the digital readouts of Transmission Fluid Temp and Cylinder Head Temp. Also, since our trailer has two batteries and a refrigerator, I want to be able to see the alternator charging voltage. The "idiot" gauges on the dash are pretty much on/off devices that really don't tell you anything useful.
One more thing - it is dead simple to disconnect and remove a CS/CTS for safekeeping if you are going to leave your truck parked in a high crime area. And, as was said, it does not have to be connected at all once the tune is loaded. The tune stays in place until it is changed by a programmer! Even removing the battery does not effect it.
Hope all this makes sense!
- Jack