It doesn't take long for the Sun to damage any plastic and solar garden lights are no exception. This spring I decided instead of replacing the old solar yard lights and try restoring the plastic cells do to oxidation. This was not allowing the batteries to fully charge.
Check out my other project I just published https://www.instructables.com/id/Bucket-Solar-Light-Storage-Container-SUPER-EASY/ Found a way to use damaged solar light that their globes broke do to the Sun's UV rays but the cap that contained the solar cells are still good.
UPDATE: Check out another Solar Light I Made using Old Solar Garden Light https://www.instructables.com/id/DIY-Solar-Lamp-with-the-help-of-Mother-Nature-CO/
Step 1: Item Needed for Restoring
I used RAIN X because it is a well known brand. I am sure other brands of Headlight restorer would work. Just follow the instructions and be surprise of the results. Now I get a fully charged battery. You can see the results in my photos.