I've just swapped my RX450h for a 64 plate GS300h and it is a superb car to drive!
That said, when I was sitting with just the battery on inputting phone speed dials, the power went off (battery flat) after around 5 minutes.
AA did test and said a new battery was required (saw the test machine), but Lexus dealer says that after a charge, the battery is fine.
They claim that just sitting with only battery power can deplete the battery fast. Is that correct?