From my handbook..
Charging the hybrid battery (traction battery)
As the gasoline engine charges the hybrid battery (traction battery), the battery does not
need to be charged from an outside source. However, if the vehicle is left parked for a
long time the hybrid battery (traction battery) will slowly discharge. For this reason, be
sure to drive the vehicle at least once every few months for at least 30 minutes or 16 km
(10 miles). If the hybrid battery (traction battery) becomes fully discharged and you are
unable to start the hybrid system, contact any authorized Lexus dealer or repairer, or
another duly qualified and equipped professional