How old is your battery? If it has the age of RX, may be it's time to change it. In hybrid cars 12V battery is necessary only to start ECUs, so it lasts much time, but even if it's not stressed by great current requests it gets old like all Pb batteries. To avoid problems, if you need to keep on light or radio or any other service in your RX my suggestion is to put engine ON, so to use Hybrid battery energy (and have your RX ready to go with engine warmed up), as after warming phase , in summer just few seconds, it will go off.