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I guess the practical application is if you've left the car in the garage for a couple of months okay to open the garage door, 'start' it (put it in Ready mode), and leave for 30min and call it good.
That should work. From what I've read the ICE will only start if the TB is below some threshold. Otherwise, the car should just charge the 12V battery from the TB when put into ready mode.
 

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From what I've read the ICE will only start if the TB is below some threshold.
The Owner's Manual states otherwise:

2021 Toyota RAV4 Prime Owner's Manual said:
EV driving may be canceled automatically in the following circumstances:
  • When vehicle speed is more than approximately 84 mph (135 km/h).
  • When power is needed temporarily, for example when the accelerator pedal is depressed firmly or when accelerating suddenly.
  • When the temperature of the hybrid system is high. The vehicle has been left in the sun, driven on a hill, driven at high speeds, etc.
  • When the temperature of the hybrid system is low.
  • When the heater is switched on when the outside temperature is below about 14°F (-10°C).
  • When the windshield defogger switch is pressed.
  • When the system determines that the gasoline engine needs to be started.
Those cover basically every scenario possible, especially that last condition. The engine (when in READY mode) could start at any point.
 

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That is true but I was referring specifically to when the car is sitting idle, not in READY mode, but just sitting (as in a garage for 6 months).
I thought I read somewhere that the ICE can come on without any user intervention (ie not in READY mode) if the HV battery voltage gets too low. I'll try to find where I saw that.

Edit to add: I couldn't find anything in the manual but when I read this it seemed legit. Now that I think about it though.....it seems like this couldn't possibly be accurate since there would be nothing to stop the car from starting the ICE inside a garage and that can't be something Toyota would ever do. I'd guess the low TB voltage causing the ICE to start falls under the category of
  • When the system determines that the gasoline engine needs to be started.
and it must only be possible when the car is in READY mode. Sorry for any confusion caused.
 

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That is true but I was referring specifically to when the car is sitting idle, not in READY mode, but just sitting (as in a garage for 6 months).
I thought I read somewhere that the ICE can come on without any user intervention (ie not in READY mode) if the HV battery voltage gets too low. I'll try to find where I saw that.

Edit to add: I couldn't find anything in the manual but when I read this it seemed legit. Now that I think about it though.....it seems like this couldn't possibly be accurate since there would be nothing to stop the car from starting the ICE inside a garage and that can't be something Toyota would ever do. I'd guess the low TB voltage causing the ICE to start falls under the category of
  • When the system determines that the gasoline engine needs to be started.
and it must only be possible when the car is in READY mode. Sorry for any confusion caused.
Right. If the engine started when the car was parked and not in Ready mode, people could die.
 

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I wonder if a solar trickle charger would work to help out with 12v depletion - I have one in the Taco, but the only live 12v without hard wiring is the ODB2 port.
 

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Looking at a 2021 RAV4 Prime XSE, and about to buy. One thing bothers me: hybrid 12V batteries are pretty notorious for depleting quickly if you use accessories (radio, 120V outlet in cargo area, etc.) if the car is not being driven, or is not on in Ready mode when parked.

The dealership is telling me: "With the RAV4 Prime, you don't have to worry about that -- you can just park the car, use accessories all you want, and the ICE engine will automatically kick in to recharge the 12V as needed to avoid depletion. (And without having to turn the vehicle on to Ready mode.)"

Anyone know if that is true or false?
Well Toyota is not so high technology company and designing R4P they tried to mimic any other ICE car. Then you car is off, 12 volt battery feeds some electronics in your car. Without using your car for more than 30 days, your 12 volt battery will be depleted. When you start you car, it uses traction battery to charge 12 volts. The traction battery can never be depleted, because when its remaining actual charge is 13%, your ICE kicks in and you drive in HV mode. The only way to deplete the traction battery to not use your car for a very long time. Lithium Ion batteries keep charge for a long time but not forever. If it happens you may permanently damage it.
For 120 volt outlet it uses 12 volt battery. However to activate 120v outlet you must turn on your car. So your 12 volt battery feeds DC/AC inverter and the traction battery charges your 12 volt battery at the same time.
Personally I think Toyota could do a better job. Tesla's 12 volts battery is always maintained by high voltage battery. Because of that Tesla recommends to leave their cars plugged in if they not used for extended period of time. In other hand because R4P uses computer to manage charging, it takes too much juice from 12 volt battery. Leaving a car like this may deplete the 12 volt battery in few days.
 

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For 120 volt outlet it uses 12 volt battery. However to activate 120v outlet you must turn on your car. So your 12 volt battery feeds DC/AC inverter and the traction battery charges your 12 volt battery at the same time.
Are you sure about that? Using the 12V to supply 1500W would require 125A, which is a whole lot of current. Using the high-voltage battery accomplishes the same task with only around 5A. There's a reason that ICE vehicles without a high-voltage battery limit their 120V outlet to around 100W.
 

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Are you sure about that? Using the 12V to supply 1500W would require 125A, which is a whole lot of current. Using the high-voltage battery accomplishes the same task with only around 5A. There's a reason that ICE vehicles without a high-voltage battery limit their 120V outlet to around 100W.
Unlikely, I believe it uses DC-DC converter line that charges 12 volt battery. I know Toyota makes 12 to 120 volts 400W inverter DC-AC Inverters | Toyota Industries Corporation . Though the dont specify voltage for 1500 watts only stating "using the electrified vehicle's high-capacity battery". Since, mostly all inverters are designed for 12/24 volts, I assume 1500watt inverter is designed for 12 volts but maybe you are right and Toyota uses high voltage DC current for that.
 

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Unlikely, I believe it uses DC-DC converter line that charges 12 volt battery. I know Toyota makes 12 to 120 volts 400W inverter DC-AC Inverters | Toyota Industries Corporation . Though the dont specify voltage for 1500 watts only stating "using the electrified vehicle's high-capacity battery". Since, mostly all inverters are designed for 12/24 volts, I assume 1500watt inverter is designed for 12 volts but maybe you are right and Toyota uses high voltage DC current for that.
The owner's manual suggested that it uses the traction battery.
Toyota RAV4 Owner's Manual page 423 said:
When the 120 VAC power outlet cannot be used
If the AC 120 V switch is pressed but the switch indicator does not illuminate, the protection circuit may have operated.
In this case, perform the appropriate procedures as follows.
  • […]
  • Check the charge of the hybrid battery (traction battery) (P.165). If the charge is low, shift the shift lever to P, allow the engine to run to charge the hybrid battery (traction battery), and then press the AC 120 V switch again.
  • […]
  • If the vehicle has been stopped in a cold area, to protect the hybrid battery (traction battery), the power outlet may not be able to be used. In this case, drive the vehicle for a while to warm up the hybrid battery (traction battery), and then press the AC 120 V switch again.
 

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I'd say another way to run down the traction battery would be to leave it parked in cold weather at low charge for a long time. There must be battery heaters running off the HV battery to keep the HV battery from freezing. That will draw down the charge level quicker than normal self-discharge. I don't know the exact numbers for 'cold' and 'long time' but if my R4P was going to be outside in sub-zero F weather for more than a few days I'd want to make sure it was parked with a good charge on the HV battery.

I think the Nissan Leaf owner's manual actually has some numbers in it for this situation but I don't know them off the top of my head.

For the 12V computer drain I assume a R4P is about the same as any other car with comparable electronics.. The 12V battery doesn't need the CCA rating that a normal ICE car needs but it probably needs the same Ahr reserve capacity. I just left my RP4 parked for 3 weeks and it started up without issue.

Have Prius owners seen low 12V battery issues when their cars were parked for a month? That seems like a fairly short time for a 12V battery to be drained by a car that should be 'off'.
 
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