How to make a solar powered battery charger?

Question

A home base DoItYourself for residential purpose.

Answer 1

buy a large solar panel, put in a diode to keep the current from flowing backwards, and connect to a controller panel.

Answer 2

The simplest way is to get a battery and a solar panel of a slightly higher voltage than the battery*.  Good point Bob B; get a diode, too.  Then put the solar panel in full sunlight and connect the positive ends and negative ends** and the battery should charge.  The charging is done when the battery's voltage is what it should be.

This method should not be dangerous with small batteries (AA, AAA, 9V), but don't use it with a power outlet, and don't use it to charge, for example, a car battery.  Still I take no responsibility for any damage to people or property.

* Test the voltage of the solar panel under the amount of sunlight you plan to use it in.

** This is how I charged my cellphone battery for about 2 months before I got my cellphone fixed.  (I didn't use a solar battery; I connected the battery directly to the charger.)