Yes, when the car is at rest, the high-voltage system is offline. During this period, the systems in the car are powered exclusively by the small AGM 12v battery. Because these systems draw a decent amount of power the little 12v is cycled often and works really hard. Several times a day the Gateway detects the 12v is getting low and brings the high-voltage system on line to charge it back up. You'll hear the contactors make the standard "Clunk Clack" noise when they close and then a while later you'll hear them go back offline with a single "thuck" noise. Whenever the HV system is online the car's DC-DC converter powers all 12v loads and also recharges the 12v battery.
The powertrain, PTC cabin heater and A/C compressor is all high-voltage, everything else is 12v. This includes things like the HVAC fan, seat heaters, power steering, etc.
If the gateway detects that the 12v isn't holding enough charge it will notify both you via way of a message on the IC and Tesla over the cellular connection. In severe cases the gateway will just keep the high-voltage system online constantly until you can replace the 12v.
If the 12v does ever manage to go dead without the gateway noticing in time, the car will be unresponsive and will require a jump or an external charge. This is because there is no manual way to bring the HV system online without first having the 12v system powered.
Because of the high duty-cycle, it seems most 12v batteries last only up to about 2 years or so. (There is plenty of variance based on driving patterns and ambient temperature though.)
If you are not a daily driver and let your car sit unused for more than a few days at a time, you will likely see shorter 12v life. If this bothers you, there are things you can do:
1. Set your charge power as low as possible so the charging takes as long as it can. While the car is charging, the HV system is online and the 12v battery is not being used. This adds life!
2. Add in an external battery charger. I use and recommend this one:
Amazon.com: CTEK (56-864) MUS 4.3 12 Volt Fully Automatic 8 Step Battery Charger: Automotive It has the best algorithm I have evaluated. It even does a better job than Tesla's on-board system of taking care of the 12v battery. It can stay connected indefinitely.
3. If your car is going to be disused for over a month, charge the pack to around 70% (no higher) and then disconnect the 12v battery or make sure it's connected to an external charger with a proper algorithm that will not harm an AGM chemistry. (see above) DO NOT use a "dumb" 12v charger!
