Sounds to me like OP is saying the battery dies quickly when it’s connected. It’ll retain enough charge for 24 hours to start the car, but not for 36 hours, at which time it needs a jump start.
OP then recharged battery and disconnected the positive terminal to see if the battery would hold charge while disconnected. After 48 hours OP reconnected the terminal and was able to start the car.
Ergo the battery is holding a charge when disconnected, but when connected there’s a parasitic drain that’s running it down in less than 36 hours.
That’s my interpretation. Correct me if wrong.
PS…thanks Tester for the modern test protocol. My vehicles were built before F.R.E.D.s were invented.