The voltage from a solar panel will fluctuate throughout the day.
If AC power is being used, the voltage is usually stable.
The voltages coming into the regulator inputs are controlled so that the battery won’t be overcharged (and thus ruined by boiling out the electrolyte). If the battery connected to the regulator is good, the highest voltage you will likely see is just above 14 Vdc in the extreme cold, but normally it should be around 13.2 Vdc.
If you have a nearly dead battery (to be checked with a voltmeter) or a battery with shorted cells, you will see a very low battery voltage. The lowest voltage you will see on the data logger data is usually about 10.0 V because the data logger will shut down near that level and then wait for the voltage to increase to an 11- or 12-volt level.
The ripple voltage is a few millivolts flowing into the battery, but the battery should filter out most of that noise, providing a pretty stable voltage.