Here is the procedure:
- Do not turn on the car: just wake up the OBD adapter from sleep. This is accomplished on my car by just opening the driver's door.
- Run Hybrid Assistant and let it connect: it will display "Waiting for the car" in the status line.
- After a couple of seconds a panel will open with the 12V measure updating over time.
- Once you bring the car to READY, the panel will close and operations will proceed as usual.
Please note that Auxiliary Battery Voltage reading works even with the car completely off.
The reading is available as soon as the adapter wakes up from standby.
This is one step beyond the measuring method that uses the hidden menu of the Toyota Touch, because it still needs the car to be at least in accessory mode, and the voltage reading is somewhat altered because of the Toyota Touch load itself.