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Actually the 12 volt battery only is charged by the traction battery when in READY mode. The ICE will not start if the car is not in READY mode.

So if you want unlimited 12 volt power, you have to leave the car in Ready mode. As such, the ICE will come on as needed to maintain adequate charge in the traction battery.

 

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I guess what I need to know is this: does the every two months recommendation ONLY apply when the car is 'in storage', or does it applly all the time even when the car is being driven? If it's the latter that sounds just plain bizarre, and I wonder if it is true for most other recent model cars.

The 12 volt battery is being charged when the car is driven. It should not need as much charging as a standard ICE car because there is no starter load being placed on that battery. There is a small amount of power that is used when the car is parked to run some features such as the communication system (DCM) used to communicate with the app, the ability to use the key fob, the alarm system, etc. For very long storage times, this small load may deplete the 12 volt battery. However, a month should not be enough to deplete the battery. As long as you do not put the car into accessory mode too long or ignition on mode (without being in ready mode), the battery should stay charged enough even with short drives.

 

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My 2021 Rav4 prime when sitting in the ignition on, or ready mode, not driving, a message on the dash comes on and says the vehicle will turn off if left on for 10 minutes. How would charging continue if it turns off.

This message is probably the message that happens if you have not been driving for almost an hour. You get a similar message if you park and open the driver door without turning the car off. This is to ensure that you do not unintentionally leave the car on where the engine might come on and cause carbon monoxide to accumulate.

The 12 volt battery is always being maintained when the car is in READY mode. This means that no matter whether the engine is on or off, EV mode or HV mode, the battery is being maintained. 12.5 volts when off is perfectly normal. The textbook voltage of a fully charged 12 volt lead acid battery is 12.6 volts. However, there is some battery drain that happens after the car is shut off. This will lower the battery voltage lower than the textbook value. In addition, the battery voltage will go down with age.

Calcium battery - have no idea what this means. The battery is a standard flooded lead-acid battery. No need to buy the imported Toyota battery.

 

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Accessory mode always shuts off after a short period of time. This does not indicate the battery is low. The Prime typically does not have a problem with low 12 volt battery because in addition to being maintained any time the car is in READY mode, it is also maintained when the traction battery is being actively charged using an EVSE. Accessory mode should not be used for any length of time. It is better to just put the car in READY mode and turn off as many accessories as you can. The highest usage will come from the HVAC system.

In looking into calcium battery, it appears that calcium is added to a lead-acid battery to help make it give off less gasses. This is apparently typical of most "maintenance free" lead acid batteries that are used in cars today. I think the manual specifies this because the battery is located in the passenger compartment. However, I would guess that almost any battery that is currently available (possibly not including deep cycle batteries) are "maintenance free", indicating they probably use calcium.

 

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Yes, I know it shuts off over time in Accessory, but not usually in 5 minutes. Then I gave it a minute and and restarted the accessory mode and it shut of in one minute. Both times a message on the dash said: turning off the vehicle to preserve battery power. I had not seen that message before and had used accessory for at least 10 minutes in the past.

That message comes up when accessory mode is automatically shut off. I am not sure how the car decides to use different times to shut off.

 

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2021 Prime. I'd like to find out more about the battery warranty but can't find a Yuasa site that tells me anything.

BY mention of Yuasa, I assume you are speaking of the 12 volt battery warranty. I believe that the 12 volt battery is included in the basic 3 year/36K mile Toyota warranty. The 12 volt battery is not warranted by the battery manufacturer.