Velocity12 - you can have an autoparts store or a mechanic do a load test on your battery and your alternator to make sure they are working. They should also be able to check to make sure that there isn't something (gone bad and/or shorted) that is placing an extra drain on your battery. Some of them will do it for free.

Typical numbers for a lead acid battery system:
Typical battery - 12.6 V (plus or minus 0.3V)
Recently fully charged battery - 13.4 V (plus or minus 0.3V)
Full charging voltage from the alternator - 14.2 to 14.6 V (you won't see that voltage on a depleted battery though, it will pull the alternator's voltage down)

 

It's a Toyota engine, so a Lexus mechanic is not needed. I go to the same guy I have been going to for years to work on the SC. The only thing that would get me to the dealer is some electrical component issue or something with the hard top.

 

+1 JohnnyCake, you could idle for ten hours and the battery would still be charged