Why is my idiot light on a 2007 Acura DMC saying "Battery not charging

I have a 2007 Acura DMX. Today (11/21/2017) I took the car out in a heavy rain. Eventually, I lost power. Pulled over side of the road, waited awhile and started car up again. Idiot light came on to say "Battery not charging" but I could run the car. However, power steering was totally gone and had great difficulty in turning the wheel in either direction. I thought everything had gotten soaked from the heavy rain. It is now 8 hours later. Car is parked in a warm garage in my house. Turned up car again. Again, idiot light came on, "Battery not charging". What should I do in this situation. Thank you.