This is common with car chargers. Someone electronically minded said it was the setup of the little ring on the pin. I think they had the specs on that little ring. It was either Android Tablet Forum or the Acer Forum. Maybe yours got buggered.
I also have an A100 - since I have 2 chargers and they are the same, I'd plug in the other working one. If that doesn't cure the problem, then it's internal and back to Acer.
I'd buy another charger anyway. If you intend to get an inverter and use it in the car, you won't be misplacing the original charger.