You have to drive hybrid ever so often to recharge traction battery. I keep forgetting how often, but it's in weeks, I believe. You can't place charger onto hybrid battery.
here you go:
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 10 miles (16 km). If the hybrid battery (traction battery) becomes fully discharged and you are unable to start the hybrid system, contact your Toyota dealer.