While solar battery chargers do work, they have some important limitations. They are not nearly as powerful as standard car battery chargers, so you shouldn't expect to charge your battery as quickly as you would with a regular charger.
Unlike standard chargers with multiple amperage settings, solar battery chargers typically put out a very small amount of current. For that reason they are more useful for maintaining a charge than reviving a dead battery. You can use a solar car battery charger as a trickle charger, but you’ll be left cooling your heels if the battery is completely dead.
Solar battery chargers work by converting energy from the sun into electricity that can be stored in a battery. This is accomplished via a photovoltaic solar panel, which is the same basic technology used in residential and commercial settings to provide on-site power.