A 20Amp charge rate is very high, i havent seen any 12v chargers that charge at that rate, though my battery is 40ah, my charger charges at 4ah.
I would charge at 12amp overnight, 20ah would be more of a fast charge and might not be very efficient in fully charging the battery. 12Ah charging would be better, and will not harm the battery, 20ah is more likely to if you leave it on too long.