Rain water is very good for your Car Battery because rain water don't contain any mineral on it. Rain water is proceed from natural condensation. Rain water may dissolve gasses from air, and if gasses contain acidic material so rain water can be acid rain. But normally rain water have pH from 5 to 6.5. This condition is very good for Car Battery. But if you have use other kind of battery like dry cell it don;t use water anymore.
Here is the video that use rain water to refill my Car Battery:
