From 'ON' mode to 'Accessory' mode directly

I thought it was better to leave it in ready cause acc. mode drains the battery I thought. Correct me if I'm wrong.

 

You're better off leaving it/putting it in READY mode. I wouldn't leave it in ACC nor IG-ON for long. At least on Gen 2, IG-ON appears to use more power due to the voltage dropping from ACC to IG-ON.

Yes, leaving it in READY will cause the engine to restart if the HV battery is too low. I'd rather have that than flattening the 12 volt.

 

That's what I've understood as well.

If you leave it in ready mode, the 12v battery will draw power from the traction battery to keep itself topped up as you listen to the radio. If outside temps are warm and your not using AC or the heater, fan off, you should be able to sit in ready mode for quite awhile before the engine will need to run to charge the traction battery or warm itself up.

So IMO there's not much point in using accessory mode to save gas unless its really cold outside(below 32f), and in that case you probably wouldn't want to be sitting in a cold car very long anyway so go ahead and use the accessory mode for a few minutes while you wait.

 

Where did you read that the 12 v battery charges from the traction battery?

Normally a 12 volt auto battery can power accessories for a long time without signs of discharge.

 

I just assumed that was the case.

How else would the 12v battery recharge itself? Prius does not have an alternator/generator as do conventional automobiles.
I assumed the 12v battery gets its charge from some kind of step down connection from the traction battery.

Imagine how a 12v battery is used to power accessories in a Toyota Corolla. That car has an alternator to charge the 12v battery while the car is running. When you shut the car off you can listen to the radio for quite awhile with no worries of the battery going dead.

Now replace the word "alternator" with "traction battery" and "Corolla" with "Prius". That's how I understand it.

To further clear up the misunderstanding, I completely agree, the 12v battery used to run accessories like the radio will last for awhile just sitting in "accessory mode". I never said otherwise.

 

Looks like you're right. Here's something I found on it:

Automobile Repair/Toyota/Prius - Wikibooks, open books for an open world

 

I did a quick and lazy google search earlier, to try to find something to back up my claims but never saw this one. Good find.

 

The module for lowering the DC voltage is called a DC/DC converter (Step-down is an AC term). The manual does address the issue of leaving the car in ACCESSORY too long (page 178):

If the vehicle is left in ACCESSORY mode for more than 20 minutes or ON
mode (the hybrid system is not operating) for more than an hour with the
shift position in P, the “POWER” switch will automatically turn off. However,
this function cannot entirely prevent the 12-volt battery from discharging. Do
not leave the vehicle with the “POWER” switch in ACCESSORY or ON mode
for long periods of time when the hybrid system is not operating.

 

I just monitor the voltage on my Scangauge II and when it begins to drop I put the car on "READY". Only happened once, while waiting for my wife, who was shopping - natch.

 

Leave it in Ready mode, heat/AC off and press the EV button. The ICE will only start if the traction battery gets low.

 

OK. Here's my view on the matter, as a Gen 2 owner.

1. Yes, in Acc mode, the 12v battery can become depleted slowly, as it will not be recharged from the traction battery. I regularly use the radio in Acc mode, for 15-20 minutes, with no problems.

There is a diagnostic mode that you can enable if you want, that shows the actual voltage of the 12v battery, so you could monitor this if you wanted.

2. However, if you are in Ready mode, the car will definitely use a LOT more power, powering the inverter etc. You can watch the power bars for the traction battery and you will see that they go down fairly quickly.

So although you will not flatten the 12v battery while in Ready mode, you will take a hit on your MPG if you sit in Ready mode for any period of time. Even if the ICE doesn't start, the ICE will have to recharge the lost traction power when you start to drive.

Also, it doesn't seem the best use of a Super Low Emissions Vehicle to be pumping out CO2 and exhaust gasses just to power your in-car entertainment, to me, to be honest.

 

This is all well and good when your 12V battery is healthy but several on here have been blind-sided by the voltage dropping to the point of disabling the vehicle after doing just what you describe (listening to the radio for 20 minutes). Admittedly, sometimes there's user error involved (leaving the headlights on by accident).
If you feel lucky, go ahead and sit in accessory mode for however long you want but be aware that some day you might end up with a problem.

 

No need to trust to luck! Get the 12v battery checked out (or check it out yourself),to make sure it's healthy, and as I said, use the inbuilt 12v battery voltage monitoring mode while you are in Acc mode.

I replaced my 12v battery a couple of years ago when it started to fail, but before it became a problem. It was apparent to me that it wasn't right, and the dealer confirmed this. So I did have some warning.

Doing that saves you money on fuel and saves you pumping out CO2 unnecessarily, in a vehicle specifically designed to minimise that.

Leaving the headlights on is another issue altogether.