inanimate objects never have problems nor will they have experienced problems. all test have been done at idle, how about at around 2000 rpm (the most used cruising speed). maybe your idle speed is too low. charging rate depends on the state of charge of the battery, the speed of the field coil magnetically interacting with the stator, the resistance of the circuits and the capacity of the alternator. so, suffice it to say: if the battery appears to be okay and the alternator is the right size for the car, the constantly changing factor is the alternator speed.