It's not a GXP thing, it's a 04+ thing. The older cars did it as well to a lesser extent. The normal routine for the car is that with a charged battery it will start and charge at about 14V (unless the temp is very low then as high as 15). After several minutes of driving the voltage will slowly drop to about 12.5V and hold. If you are on a steady cruise you will see the voltage stay at 12.5 for about 20-30 minutes then it will rise to 13.8V for about 10 minutes and it will continue until you stop. At night the battery is monitored and depending on it's SOC and state of the battery itself and will go into reconditioning mode when it feels the need. When batteries start to go bad the system will go into that mode more often. The only time you will see the battery run at a constant 13.8-14.4 is when you have the lights on. The lights emit a set amount of light based on voltage and current so the voltage is set to maintain the same brightness. As far as batteries go any quality battery is fine. They will actually last longer since they are not constantly held at such a high level of charge which actually causes batteries to fail.
Jeff
