This is interesting. So the 12V keeps getting charged while in charging state while plugged in...but not if Charged/Left plugged in.
Is the 12v battery always recharged when you Hold Break and press +Power Button? I haven't heard the engine kick on once except when I manually turned on HV on the highway so I was assuming that the 12v is always being charged for lights, radio etc while driving.
My other question is this....could the 12v faults when the car is parked for several weeks, come from the fact that there a LTE radio inside the car? We know a low LTE signal technically uses more battery power on a cell phone because it's constantly searching for cell towers... is this why these cars "die" so easily if parked for several weeks in a garage never turned on?
When you hold the brake pedal and press the power button this is known as READY mode, which it says on the instrument panel just below the shift indicator symbol (P, D, N, R). Whenever the car is in READY mode the HV battery charges the 12V battery. The gas engine has nothing to do with charging the 12V battery, at least not directly.
The LTE Telematics unit runs off of 12V and will use a little bit of power as it "listens" for an incoming call from Honda. A call which will likely never come if your car is sitting undriven, nevertheless it keeps listening 24/7 waiting for a possible remote climate request to come in. It probably uses a similar amount of power as a smartphone sitting idle with the screen turned off and no calls or text messages coming in.
The smartkey system also uses a little bit of power as it listens for the keyfob. My Prius had a way to turn that off when you went out of town. Clarity doesn't seem to have that, although it uses so little power it probably makes little difference.
My car makes odd sounds sometimes while sitting in the garage even if not plugged in. Some people think it's the battery management system, maybe but I suppose it could be other things also. Whatever it is might be running off of the 12V not the HV battery, but I don't know. As far as I know the HV system is only active while charging, when in READY mode, and while remote climate is running.
Some people have said they didn't use or charge their Clarity for several weeks and it started right up. However based on the number of people who have returned home to dead batteries after a week or two, I would not be surprised if Clarity uses more 12V power than is typical. However even so I suspect that their 12V battery was already pretty weak to die in that short of time, and the slightly higher drain of the Clarity probably exacerbates the situation.
I also suspect that people who go three weeks and say "the 12V battery was fine", well what they are really saying is just that the car started. It's possible that their 12V was in fact drained down pretty far, and they may have been closer to being discharged than they realized, but they would have no way to know that since it takes very little power to start the car (i.e. get it into READY mode). Unlike a gas car where the 12V battery has to spin the starter motor and the engine, so you will usually get a sense of the battery getting weak. With the Clarity (any hybrid actually) if you have difficulty getting the car into READY mode then the 12V battery is probably pretty close to being dead and should be fully charged before trying again to start the car, otherwise you run the risk of permanently killing the 12V, or at least greatly shortening its life.
One way to charge the 12V in that situation if you don't have a battery charger but you do have jumper cables or a jumpstart battery, is to connect the jumper cables, then plug in the charge cable and get charging started. Once charging is started you can then disconnect the jumper cables, and let it charge for a couple of hours before trying to start the car again.