from this post:
Can a portable solar system be used to charge your existing vehicle battery and then continue using the vehicle as your primary source of power? If that is possible it might eliminate the need for secondary battery (weight and space).
Yes. I describe that usage in the Shallow Cycling rvwiki article. Trebor and Sternwake on CRVL have both done it successfully.
I'm guessing that might shorten the life of traditional vehicle battery, IDK.....
We don’t know.
My reading suggests that starter batteries average about 36 months of useful life. The main failure modes are:
sulfation due to chronic undercharging
loss of water
In single-battery systems the first cause of failure would be greatly diminished, as the battery would be getting to 100% state of charge instead of just the 80% most normal driving achieves.
If the battery has removable cell covers then watering the batteries will greatly diminish that failure mode.
In summary, I suspect (with no proof) that shallow cycling a starter battery will not diminish starter battery life.