It acts like the alternator is bad what's a shot says it's good what else could

What is your question?

It'll act like the battery is completely dead but when you put jumper cables on it immediately starts and I can run it a mile and a half before it shuts off while I'm driving

What makes this problem better or worse?

Jump start

How long has your 1990 Lexus LS400 had this problem?

It just happend