if we connect the adapter directly into the battery then the charging current should be very high
howcome it takes a day?
for the sake of this metaphor try thinking of power like plumbing...
voltage is like water pressure.
amperage is like the pipe's diameter.
there's no point having a wide pipe capable of carrying a lot of water if there is no pressure to push the water.
if your power supply is the same voltage as the battery it is like there being no difference in pressure between the 2 supplies.
it would be like connecting 2 taps with the same pressure this a bit of pipe. no water would flow even though the pipe is capable of carrying a lot of water.