Is it even possible to use the seat heaters without the car being fully on? I haven't tried it, but I hope not.
....
Actually, I found a way to run the seat heaters off the 12V battery alone.
This is a bad idea to do, but I could see someone doing it by mistake. Perhaps that is what Honda is cautioning about in the owners manual.
For background: if keys are in the car and you push POWER
without your foot on the brake, you enter ACC mode. This powers the radio, wipers, and a few other things.
If you push it again,
without your foot on the brake, it enters what looks like a "full on" mode: dash engine-warning and battery-warning light turn on a
nd stay on, and climate control, headlights, seat warmers, etc. all work. Of course it can't be driven, and a message that "to start car, foot on brake and push ON" appears. This would be the equivalent of ON/RUN in an older car with a key ignition. I will call it ON even though the car is not ready to drive.
Of course,
if you have your foot on the brake and power the car, it skips all of these and enters READY TO DRIVE mode; the dashboard warning lights come on and then go off.
Voltage across the 12V battery terminals during each of these:
-OFF: 12.3 V
-ACC: 12.2 V
-ON: 12.2 V. Seat heater, headlights comes on. I can find 30A flowing in/out of the battery with a clamp on ammeter. Since the voltage starts to drop, I presume out.
-READY TO DRIVE: 14.4 V.
I interpret this as meaning: when in ON mode, all circuits are live but the main (traction, high-voltage) battery is not engaged. You can merrily drain the 12V battery to the point where car won't start. I could see someone waiting in the car at a cell phone lot, etc. doing this. Maybe someone in the passenger seat, car, keys still in car, getting cold waiting for the driver to return. This is a familiar situation from full-ICE cars, as well.
If you are in READY TO DRIVE mode, no worries: the high-voltage battery will support the seat heaters.
An obscure circumstance. I assume the ON mode exists primarily for maintenance on the car (where you might want most things powered up, but the high-voltage battery to NOT involved even if stepped down by way of the dc-dc- converter).