I have the same problem with my ls400 if it sits for more than a day the battery is dead. I have had the whole charging system looked at and it always checks out even lexus said it was fine. I did some checking on my own from there I found that at an idle when warm and in gear the brand new tested good alternator is not charging over 12volts. but at 100 rpm or more it is fine i am still trying to find a solution to this. if anyone has an idea please et me know. I hope this helps you figure out your problem if you do please let me know.
