ECU Power Source Circuit [04/2018 - 08/2020]: Description
WARNING: This page is about a different car, the 2020 Toyota Avalon and 2019 Toyota Avalon. However, it is still accessible from the selected car via links, so may be relevant.
If the power switch is on (IG), the hybrid vehicle control ECU applies current to the MREL terminal to turn the IGCT relay on. This supplies power to the +B1 and +B2 terminals.