Which ON position keeps 12V battery charged?

Discussion in 'Prime Main Forum (2017-2022)' started by IABoy987, Jul 7, 2025 at 9:23 AM.

  1. IABoy987

    IABoy987 Member

    Joined:
    Nov 25, 2011
    199
    79
    0
    Location:
    Iowa
    Vehicle:
    2018 Prius Prime
    Model:
    Plug-in Base
    Went to fireworks show and listened to programs and fireworks music on car radio. When we went to start, the display said 12V battery low, see owners manual. Luckily we got a 12V jump start from someone and on our way.

    I had been told that putting car in ACCESSORY, not ready to drive position (two button pushes) allowed the big battery to keep "little battery" charged. Was this information correct, or should we have put it in RUN ready to drive mode to keep 12V fresh?
     
  2. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    26,411
    17,304
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Only in READY does the car keep the 12-volt battery charged. Both ACC and ON drain charge from the 12-volt battery. ON drains it faster.
     
    IABoy987 likes this.
  3. IABoy987

    IABoy987 Member

    Joined:
    Nov 25, 2011
    199
    79
    0
    Location:
    Iowa
    Vehicle:
    2018 Prius Prime
    Model:
    Plug-in Base
    Thanks!!
     
  4. CharlesH

    CharlesH CA HOV Decal #5 on former PiP

    Joined:
    Nov 27, 2005
    2,796
    1,162
    0
    Location:
    Roseville, CA
    Vehicle:
    2017 Prius Prime
    Model:
    Prime Advanced
    Yes, when you want to park with the radio or other accessories on, just leave the car in READY mode. The engine won't run unless the main battery charge gets too low; the engine will run a little while then turn off. Same if you want to park for a while with the A/C on to keep cool.
     
    otatrant and IABoy987 like this.
  5. IABoy987

    IABoy987 Member

    Joined:
    Nov 25, 2011
    199
    79
    0
    Location:
    Iowa
    Vehicle:
    2018 Prius Prime
    Model:
    Plug-in Base
    Please excuse my dumb question as I am not in my car to check. :( Is ready when you depress brake pedal and push start button, or after turning off car, just pushing start button three times without depressing pedal? Would it matter either way with this scenerio, as long as big battery was charging 12V? My guess is pressing pedal and button is correct answer.
    Doing startup so many times (push pedal then button), you don't pay attention except when car does not start.
     
  6. ChapmanF

    ChapmanF Senior Member

    Joined:
    Mar 30, 2008
    26,411
    17,304
    0
    Location:
    Indiana, USA
    Vehicle:
    2010 Prius
    Model:
    IV
    Yes, you hold the brake and push the button to get to READY.

    The word READY lighting up on the dash is how you know you're there.
     
    IABoy987 likes this.
  7. IABoy987

    IABoy987 Member

    Joined:
    Nov 25, 2011
    199
    79
    0
    Location:
    Iowa
    Vehicle:
    2018 Prius Prime
    Model:
    Plug-in Base
    Thanks!!
    Learn something new everyday!
     
  8. otatrant

    otatrant Active Member

    Joined:
    Sep 5, 2023
    195
    130
    10
    Location:
    San Francisco
    Vehicle:
    2023 Prius Prime
    Model:
    XSE Premium
    Here is a good summary from google ai.

    upload_2025-7-7_15-56-17.png
    upload_2025-7-7_15-56-30.png
     
    IABoy987 likes this.
  9. IABoy987

    IABoy987 Member

    Joined:
    Nov 25, 2011
    199
    79
    0
    Location:
    Iowa
    Vehicle:
    2018 Prius Prime
    Model:
    Plug-in Base
    Thanks for the list.
    I had not read of My Room mode or missed it in the owners manual. Not quite sure where it might be used (at a campsite with 120V outlet nearby??)
     
  10. otatrant

    otatrant Active Member

    Joined:
    Sep 5, 2023
    195
    130
    10
    Location:
    San Francisco
    Vehicle:
    2023 Prius Prime
    Model:
    XSE Premium
    My uses for it are:

    1. keeping the car internal temperature cool to allow electronics inside (ie my Anker Solix) operating at below its max temperature.
    2. Sitting in the care while plugged into a charger and using the 12V outlet to power a laptop computer and Starlink.
    3. One other advantage of room mode is it continually draws power from the charger so if you need to maintain a charging state to park in certain parking spots it will keep the charge from completing until you return to the car and turn it off.
    4. One other use would be if you want to charge your car but will not be around to unplug it when it is done and are worried about draining your 12V auxiliary battery you can turn off most of the interior electronics (ie media screen, climate system) and the automatic headlights. Room mode maintains the 12V auxiliary battery charge while plugged in to the EVSE.