Me and a guy I work with each bought the same SUB wall charger, USB car charger, and retractable USB cable package from the same seller on ebay.
His car charger worked perfectly
and mine blinked the notification LED
His wall charger blinked the notification LED
and mine worked perfectly
Oh yeah, tried all of them with different cables also.
Since I already had a bunch of wall chargers I gave him mine and threw out the car charger since I already had one or two that at least kept up with my GPS.
So the bottom line is, there are good odds you can end up with defective chargers even at the high price of $3.99 shipped
Now in answer to the OP. If it starts to blink the LED in the first few minutes it is definitely a defective charger. If the battery gets hot and it starts to blink it is either a defective charger or battery or just charging under hot conditions like the phone sitting in the sun.