Your good everything is OK. I just spent more than 15 hours logging voltage from a 14 Impala and it does the same thing. My 05' does the same thing. The only time you will see it hold 13.8 is when the lights are on. The reason for this is that the lights are rated for a certain amount of output at a specific voltage and current. You would constantly notice that your lights dimming and brightening if the voltage fluctuated as much as it normally does. If you drive your car you will see the voltage jump from resting voltage of between 12.4-7 depending on condition of the battery. The voltage will go up to between 14.1-8 for a time then it will settle down to 12.7-8 and as low as 12.6. Then after a while it will increase to 13.8 for a while then drop back into the 12's. The PCM controls the alternator and has it put out only what it needs to to charge the battery and whatever excess it needs to keep it at that level. The reason for this is 2 fold, one your not cooking your battery, so it extends the battery life and 2 it cuts down on excess energy wasted running 2 volts higher than it needs to operate the cars electronics.
Jeff
