I recently bought a 1998 Lexus LS400 having issues keeping charge. Car would drive but dash would cut off and on so I ran a battery test and told me it was the alternator failed. So I changed the alternator and battery and was running great until a couple hours later same issues started again and now won’t even start unless with a jump.

 

Obvious question: is how old is the battery?

Non obvious opinion: doesn't even have to be that old to have failed. I had a 4 year old battery get killed after I ran the car out of power two times. Just twice, that's it. 2nd time it wouldn't even jump start. With a new battery it was instantly fixed.

Draining a lead acid battery (aka requiring a jump) to zero is very damaging to the cell and visiting there a few times can kill it entirely.

 

The battery and alternator are both new made this year

 

• Where did you get the battery and alternator?
• Is the alternator rebuilt or?
• What test did you perform that indicated the alternator was bad?
• Typically, the battery needs to first test good (full charge/able to hold a charge/able to provide required power), otherwise electrical tests are pretty meaningless.
• Make sure you're not leaving a door ajar. The door needs to be completely close or the dome/courtesy lights will remain on.

 

What I would do is when the system is operable, just put a meter on the battery when the car is at, idle write down the results, then bring the rpms up some, and write down results again. You'll know if the system is charging properly just by looking at the test results. Unless there is some kind of intermittent problem. But the first thing you can try to rule out is if your alternator is working properly, you could also test off the alternator directly as opposed to the battery.