I have a 2010 RX 450h. I'm the second owner. For the last year and a half I have had the battery die about 4 to 6 times. I went to our local Lexus dealership service department and they said that there was nothing wrong with the battery but the AAA people say that the battery is not functioning at full power, my uncle (not an expert in anything but a dabbler in everything) says that it might be the alternator which is not supplying the battery with energy. Anybody have any bright ideas on what I can tell the Lexus dealership when I go back?

Sidenote: I think our Lexus dealership service department is not very good because in my opinion they should have been more curious about why my battery died so often.

There is no alternator in the RX450h. The only accessory that you might be able to see which is driven by the serpentine belt on the 2GR-FSE engine is the water pump.

I'm not entirely sure how the 12 volt auxillary battery is charged, but presumably this is controlled by some ECU.

Is the battery in your vehicle a Lexus/Toyota genuine one? If not, perhaps it's an aftermarket standard lead acid flooded cell wet battery. The battery needs to be an AGM, and even if it is, it needs to be (in my personal opinion) fully charged up with a smart charger / battery maintainer, as the stop start driving does drain the battery and the ECU doesn't do a great job in keeping it charged up.

You can get your existing battery load tested, and it may be in a weakened state - if you do replace the battery be sure to get the right battery. The current Toyota part number is 28800-31290 the original battery was a Panasonic S55D23L. You might find that most aftermarket battery suppliers do not have a functional equivalent of the Panasonic S55D23L.

Have a read of this thread: https://www.clublexus.com/forums/hyb...-rx450h-6.html

12V is charged off hybrid battery via special electronic device, inverter. Good, healthy hybrid battery is base for properly charged 12V.
It is not clear what is meant by "dies". Dies how? Is not charging? Can't hold charge? Shortens?
Those batteries are deep cycle batteries. They are special kind designed to be a large 12V storage for long discharges, feeding computers, mostly, with back up power. It can be as simple as oxidized battery cables and, as complicated, as any electronic piece along the battery charging path. It can be battery drain. It can be sign of bad or going bad traction, hybrid battery.
This needs to be scoped by a good electrician specializing in hybrids.