Voltage regulator for battery maintainer?

I bought a few of these to keep my unused vehicles batteries charged. It has many good reviews. Most people don't use voltage regulators even though it says one more likely should. Do you have any recommendations for some basic regulators that i can attach to these, so it won't over charge? Nothing fancy.

https://www.harborfreight.com/15-watt-solar-battery-maintainer-64251.html?utm_source=parcellab&utm_medium=email&utm_campaign=ArrivesToday