i have a 1990 LS400...
i keep having the problem where the battery keeps dying...
happened a few times where i'm driving and when i step on the brakes everything seems to shut off.. the radio turns off and all the lights on the gauge lights up as if the car is off and the headlights starts dimming.. BUT the motor is still running...

once i gas it the lights get bright again and everything turns back on until i step on the brakes and it "dies" again but the motor keeps running

when i turn off the car the car wont start unless i jump it... and it stays it remains on (motor is still running) even if the radio is off and the lights are dim... and it even remains on when i take off the positive terminal until i turn off the car

seems like the alternators fine but the battery ends up dead.. and this is the 3rd battery within the last year... and all the times this happens it always happens in the rain

 

Alternator is dying, check the voltage and ampage output. I bet you will get mixed and dropped voltage numbers over a period of time. Also, check your wires for corrosion build up.

 

I had the exact same problem 2 weeks ago.....it is definitely your alternator....your alternator is intermittent......probably have bad diodes in the rectifier inside the alternator along with bad brushes........have your Lexus alternator rebuilt by a reputable auto electric shop....cheaper way to go.....you may also have a bad cell in battery.....check & replace if applicable........

I did these 2 things & all problems cleared up.....

5ofakind

 

Any warning lights on the dash? Otherwise, sounds like an alternator draining the battery. Get it fixed soon, extreme low discharges on the battery are not good for its longevity.