The alternator is charging, it drains my battery.

First make sure that nothing is left on.

There is an old method of pulling each fuse to see which one depletes the load, but that doesn't work on newer cars because thay are designed to have certain computers on for certain periods of time, which could give false readings.

You can remove several relays and then see if the battery is dead the next day. If not the problem is in one of the circuits controlled by those relays. Replace a couple of these relays and try it again until you have narrowed if down to the problem circuit. (I konw this is a long procress, but without fancy/expensive equipment this might be the best option--either way it will take some time).