Well , I make a very (very !) rough calculation:
20 amps loaded in 48 hours. To make it more easy: in 40 hours. 20 divided by 40 gives 0,5
So let me guess: your loader provides max. 500 mA - which is 0,5 A
So: use a more powerful loader. Lets say: 1 A loader:
Then the (very rough calculation) is 20 Amps/hour (which is the same as 20.000 mAmps/hour) divided by 1 amp is 20 hours , you see: you have cut the time by half. But please note: this calculation is very rough , but gives you the direction in which you have to go.