Low battery warning, what does pressing the parking brake actually do?

Discussion in 'Gen 4 Prius Main Forum' started by JStrenk, Apr 27, 2020.

  1. JStrenk

    JStrenk Member

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    This morning I had the Low 12 volt battery warning and after finally reading the instruction carefully.. (Oh, the PARKING brake !!) I got the Prius started. I may have left it in the ACC condition after lowering the windows a notch.

    I'm kinda laughing at myself now and wondering if the Prius was getting frustrated with me. "NO! Take it out of DRIVE, Put it in Neutral, No, neutral, not Reverse you dummy!." "Oh, just put it back into PARK. Now, step on the parking brake, NO! Not the brake, the parking brake you idiot." "Quit hitting random button! Just get out of the car and start over.. I'm not letting you drive me until you wise up and read the instructions!" (I swear it kicked me through the brake pedal at one time...) "That's right, now that you pressed the parking brake I'll start the car for you ...."

    But I was wondering? What does pressing the parking brake actually do? Is that one of those easter eggs that bypass the minimum 12 volt battery when starting and runs the 12 volt directly off the hybrid battery.

    I can't even fathom how the system had enough power to inform me the 12 volt battery is low.
    With all the dash lights going crazy, it looks like a reboot or something is going on.
     
  2. The Professor

    The Professor Senior Member

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    By parking brake, I assume it means"P"?

    Putting the car into "P" disengages the drive train (engine and electric motor) from the wheels, and locks the front wheels so that they can't rotate. It ensures that the car won't lurch forward (i.e. in D or B) or backward (i.e. in R), or start rolling due to a gradient, while the car is starting. It also ensures that the energy from the electric motor is entirely directed into the combustion engine to start it, rather than to the wheels, or into the generator to charge the traction battery.

    If you've been messing around with the gear selector knob you might have ended up in N accidentally. It sounds like the car was simply telling you to put it back into P to start it.

    Acc mode turns off after 20 minutes, so it shouldn't flatten your battery too much.
     
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  3. JStrenk

    JStrenk Member

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    It actually displayed depress the "parking brake" not just "park".

    [​IMG]

    Usually my muscle memory has me Pressing the brake pedal, hit the start button and put it in drive. When it didn't move, I then looked at the dash and saw the low 12 volt battery warning. So I tried to put into drive before the system was ready.

    After finally corrected all the wrong steps I did I finally got it started. But was wondering, why the parking brake? To keep it from accidentally surging forward when it reboots?

    (reading through this post now, missed it before Low 12-Volt Battery Apply Parking Brake Securely... | PriusChat)
     
  4. The Professor

    The Professor Senior Member

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    Ah, my bad. I see what you mean.

    There's a big thread about it here: Low 12-Volt Battery Apply Parking Brake Securely... | PriusChat

    The short answer is that the P mode (i.e. locking the front wheels electronically) may not operate correctly when the 12V battery is low. So, to ensure the vehicle isn't going to roll away, you must apply the (left foot) Parking Brake.
     
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  5. Salamander_King

    Salamander_King Senior Member

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    Thanks for the explanation. I was waiting for someone with an answer. No, I have not seen the warning on my car yet. But it is good to know what happens and why I have to use the parking brake in case of the low 12v battery.
     
  6. ChapmanF

    ChapmanF Senior Member

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    Yeah, that's a stock message. One thing that happens when the 12 volt battery is very low is that there isn't enough power to operate the motor that parks and unparks the transmission. When that happens, you get the stock message that wants you to be sure you apply the parking brake, since you can't count on the transmission to keep the car from rolling away.

    However, that stock message isn't telling you anything about having to press the parking brake for the car to start. There isn't any such requirement. It's just a warning about being sure to have the parking brake on before you walk away.

    If you tried several times to start the car and it finally did after you applied the parking brake, that's just the way it happened to play out.

    I have found that 12 volt batteries show a certain small degree of 'recovery' at a very deep discharge; if it fails to start the car a couple times, sometimes just letting it rest a couple minutes will get the job done on the next try. That worked for me many times in conventional cars, where the battery had to crank the engine. It takes less energy than that to start a Prius. Also on your first attempts to start, the brake system was probably still pressurizing, which draws a lot of electric power, and that buzzing may have stopped by the later attempt when you successfully started.

    But you have reminded me of Chitty Chitty Bang Bang having a light on the dash that really read "Pull, idiot!".
     
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  7. CooCooCaChoo

    CooCooCaChoo Active Member

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    Putting it in "P" also allows the engine to come on and charge when necessary. If you just leave it in "N", the batteries will not charge and will drain.
     
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  8. The Professor

    The Professor Senior Member

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    True, but the OP is referring to the 12V battery, not the traction battery. The 12V battery charges constantly when the car is in Ready mode, no matter what was selected with the gear selector. The traction battery only charges in P (via ICE), D (via regen and ICE), R (via ICE), and B (by ICE and at a reduced rate via regen, with the remainder of the potentially recoverable energy instead being wasted as heat via Adiabatic heating within the engine).
     
    #8 The Professor, May 3, 2020
    Last edited: May 3, 2020
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