Using battery maintainer

As you approach the car , it wakes up , detecting the keyfob increasing the power demands on the 12V battery .Opening the door activates an electric hydraulic pump to pressurize the brake system, lights up some bulbs , activates the power on switch; wating for you to push the brake pedal and the power on button......this could explain the temporary voltage drop you have observed.
and yes , avoiding ON or Accessory is recommended to prevent draining the 12V battery ...the Prius is a saucerfull of secrets

 

When I picked up the Outback, the salesperson warned me to have the car on whenever I was doing something with the infotainment system. Nearly all modern cars put a large drain on the 12V otherwise.

 

Makes me wondering, does the Prius have HV contactor relays like you see in EVs? When I get in my Model 3, I hear a "clunk" when the relays connects the HV battery to the rest of the system. I don't recall hearing that kind of sound in the Prime.

 

Yes it does ,So the proper way to start the car is pressing on the brake pedal and pushing the power on button .This will put the car in READY state. Lets go deeper to understand what is happening. It is nice to know that before the READY light turns on , the system is checking the integrity of the high voltage cables and circuitry of the car . When all systems check OK, then the connection to the high voltage battery is permitted and all systems are powered on; hence READY state. From here , the 12V system is being charged by the DC to DC converter .