You can make a solar battery charger for under 5 dollars, all you will need is a solar path light that you can get at walmart. I have even seen these lights at some dollar stores. First you’re going to have to find the perfect solar light for the project. The light will have a battery compartment like the one in the photo below.
When shopping for your light you can easily twist off the top to see if it is the type that has the battery compartment.
Once you have your light you are going to have to do a quick optional modification to make it into a battery charger. The way it is, it is designed when the sun shines on it a circuit will turn the light off and allows the battery to be charged. When the sun goes down and it gets dark the circuit turns the light on, normally this is fine and if you can remove the battery before it gets dark out you’re all set. But if you don’t get the battery out before dark the light will discharge your battery. So to make it so you don’t have to worry about the battery discharging in the dark. Remove the battery and the screws securing the solar panel onto the battery compartment, like the photo below.
Next if there is another circuit board screwed to the compartment remove those screws too.
Next pull the circuit board out carefully to expose the led light.
Next remove the led light by cutting it with some wire cutters.
Next reassemble and it’s all done, place it out in the sun and leave it. These lights are weather proof so you can even leave it out if it rains. You now have the cheapest and easiest solar battery charger.