The sun panels in the rechargers are very small and takes forever to recharge the batteries even without cloud cover.

You will have to rig your own using the solar panel that most WV, car come with to keep the battery charged.
Here is an example, it can be found in E-Bay for $20 or $30.00



I have one at the cabin I only wish I have more time to spend out there to use it more often.
black bear 84