A solar car battery charger converts ultraviolet light, usually from the sun, into an electrical current which it then uses to charge a car battery. Most inexpensive solar car battery charger models are of the very low output sort. They are classified as trickle chargers, not real battery chargers. These chargers supply a maximum of one Amp to charge your car’s battery. This charge rate is more of a battery maintainer than an actual charge. They are designed for those cars that sit for longer periods of time, not the completely dead battery. A completely dead battery will take a minimum of a whole day to take enough charge to maybe start a car. There are models that supply up to ten Amps, and this will be sufficient to charge a dead battery in a few hours. However, these models cost several hundred dollars.
For more info, see How to Use a Car Battery Charger