Sounds pretty close to normal for me. I have an OBDII ScanGuage and "charging voltage" is one of the things that I continuously monitor. The voltage regulator will switch to charge the batteries and then shut off so as not to overcharge and destroy the batteries. How often the regulator switches depends on how much current you are drawing from your battery. My system charges at 14.4 volts and drops to 12.6 when not charging. It has been doing this for several years and the battery is doing fine.
You may want to check your batteries to make sure that they have enough electrolyte but I think your system is operating as it should.
