plug in charging

Yes and no. When the traction battery is accepting a charge the 12v battery will be maintained but when the charging cycle ends no charge goes to the 12v battery.

 

The manual warns against leaving the car plugged in for extended periods as it will drain the 12 volt battery. Even when not charging, the system seems to monitor the connection, and that puts an additional drain on the 12 volt.

 

Hmm, I have experienced when the traction battery was charged and the 12vdc battery was dead. This has happened to me a couple of times.

When the hatchback latch did not fully lock, so overnight the vehicle thought it was open and left the cabin lights on. That drained the 12vdc battery, even though the traction battery was plugged in and charging. We could not access the 12vdc battery because it is required to un-latch the trunk. It was a major pain each time.

 

On the Prime the 12v is under the front hood. Even if totally dead you should be apple to open the driver’s door with the FOB’s mechanical key and then pop the hood’s mechanical latch.

 

See this thread for your final answer.
Hopefully, ultimate 12v battery charging system discussion on PP

There are three different "mode" the EVSE operate while it is plagged in. Of three, #2 Maintainer mode is the most frequent. I have not found exactly what situation causes the other two modes to operate, but it is most likely to be related to the 12v battery SoC at the time when the plug is inserted.

1. EVSE Charging: charges the 12v battery at ~13.5v constant charging voltage which occurs only during the time the traction battery is actively being charged. This charging mode does not happen every time the EVSE is plugged in.
   
2. EVSE Maintaining: maintaines the 12v battery at ~12.7v constant charging voltage to keep the 12v battery at near a full level without actively bringing the SoC any higher. This too only happens during the time the traction battery is actively being charged. This is the most often seen mode of all three.
   
3. EVSE Checking: checks the voltage of the 12v battery and charging at 13.5v starts very briefly, but immediately stops and switches back to the maintainer mode at ~12.7v. I am not exactly sure what causes this to happen.

In any case, the EVSE charging only occurs during the time the traction battery is also being charged. Leaving the EVSE for prolonged time will not increase the 12v battery but drains it quicker as already mentioned by @Trollbait.

 

Opps you are right. Sorry.

We have a Gen-3 and a plugin.

We have experienced dead 12vdc batteries in both vehicles. I remember the nightmare of getting to the 12vdc battery in the Gen-3. But you are correct accessing the battery in the plugin is much easier.