It can have several causes... Some 3-stage converters,,, I suspect, "Pause" from time to time to test battery voltage.... My Progressive Dynamics does the opposite, it kicks into "Overcharge" every 20 hours or so for a few minutes, This will be seen as a change in 12 volt lights.
The other day I was standing here and suddenly the lights dimmed, one light (A 120 volt) Went out completly.. Then before I could get outside to check they brightened back up.
Turns out Duke Enegery had a crew working here in the park.
Home is where I park it!