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

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.

 

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.

 

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.

 

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!".

 

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.

 

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).