It should only regenerate when a car is in the volume, and you just place these volumes in the world where you want them.
So you also have to add a OnEndOverlap event that triggers a Stop adding health event on the Car. So it stops when the car is outside of the volume and not just when it is at full health.
This is a basic set up there are a bunch of other things you have to make sure that happen or don’t happen. But this should give you a good starting point.