Don't charge in the rain. I became concerned, because I know that water conducts electricity. I'd rather not. Actually water doesn't conduct electricity very well. If it did, every time it rained all the power lines would short out.
Salt water is an exception.
It turns out people are better conductors of electricity than water. So if you reach into or touch water that is covering an electrical appliance, the electricity finds an easy path to ground through you, shocking. This is why our battery chargers have a grounding conductor, and so should your extension cord and outlet. Remember this, if an electrical appliance falls into water do not reach in to get it. Unplug it first.