[RichardJ]

Before you take it anywhere, just remove the Bat wire & plug connector from the Alt and see if the battery still drains down in the same time period. A bad diode may not stop it from charging close to normal and most flaps alternator testers can't detect the difference. If battery stays charged, the Alt is probably bad.


OR:
Remove battery cable from battery and place an Amp gauge between battery post and battery cable. You should be seeing a discharge reading, unless it's a dirty battery top as MR48 suggested. Remove Alt cables and if the Alt is the culprit, the discharge reading will go away. If the discharge reading stays, then start removing fuses and accessory power, one at a time.

OR:
Remove battery cable from battery and see if it sparks when you touch cable back to battery post. Remove Alt wires, then see if battery cable still sparks. Take your own chances with sparks around a battery.