I did exactly as you just stated and it wont charge and the led on the charger blinks red.
I have owned the aventure for 1.5 years - the charger was stolen while my daughter was out with friends with panier etc... I have been trying to get a new charger to no avail until Wednesday Aventon sent one out. plugged charger to wall, plugged St3 carefully and firmly into battery and it was green than went directly to steadily blinking red- 6 hours later - no charge- still blinking red