That's a bit misleading. The bottom line is that the power limit is HIGHER on dc than on ac. The charger will not reduce the rate on dc power if it is low, it will simply quit at 10.5 volts input.

Lets back up to your first post. The 1 hr charge on ac is normal. The 2 hr charge on dc is not. It has nothing to do with input power or voltage and really shouldn't have anything to do with dc power either. To verify this hook it up to a car battery and see how long it takes (or verify the rate)