Again, that is really going to depend on a lot of factors. The strength of the charger, the levels of sunlight in your area, the batteries size and current charge level. The good thing with solar chargers however is that they are pretty low powered – that’s why they are perfect as trickle, or maintenance, chargers. So don’t leave them on permanently, as they have no overcharge protection. But it’s unlikely you’ll do major damage if you disconnect after a few hours in the sun. Also bear in mind that there are some models that come with charge indicators, and they will tell you the batteries’ current charge level. If you want peace of mind then, consider one of those models. Related Post: Best Car Audio Batteries