2001 Hyundai Elantra battery keeps going dead checked alternator, starter, battery checked good.
If the starter operation, charging system and the battery were tested correctly and are indeed good, there must be a parasitic draw on the battery. That means an electrical device is on when it shouldn't be, or a solid state component is defective and using too much current with the ignition off.
Check all the basics like the interior lights, under hood light, trunk light, glove box light, etc. There may be a bad switch or something keeping them on. After that it requires an ammeter to measure the amount of current being drawn (up to 30 milliamps is normal). Then isolating which circuit is causing the problem.