I left my car 2018 clarity sitting in Southern California for about 3 months. Ive got exactly 24,000 miles on the car. When I left there was about 5 bars or the battery and 3/4 tank of gas. When I came back, I couldn't even open my car with the key fob. The car was extremely dead. I connected my 12v battery to another car with jumper cables and let it charge for about 30 minutes or so. When doing this, i got what seemed to be every code in the book and my HV battery was completely drained. 0 bars. When I disconnect the jumper cables from the clarity it won't stay on for more than 30 seconds. The car is so dead, it go into drive even with jumper cables attached. I'm going to have it towed to the dealership tomorrow. I hope it's a similar problem as above where that 8 year HV warranty covers it.
When the jumper cables are connected, instead of trying to turn on the car, try charging. If you plug in the charger and get a steady green light then it should be charging the HV battery. While charging is occurring it will also be charging the 12V battery, so once you see the green light you should be able to remove the jumper cables and the green light should stay on, if it does then let it keep charging.
I recommend don't stop charging and don't try and turn on the car until it has charged for at least two hours, because every time you turn on the car there is a momentary drain on the 12V battery while the HV system is being activated. So it's better to let the 12V battery get fully charged before trying to turn on the car. In your case the 12V is completely depleted so that's why I recommend two hours to try and get it back to full, assuming that the 12V battery will accept a charge and is not completely dead, but the only way to know is to try charging it for a couple of hours. And either way during those two hours you will be charging your HV battery so it's not a waste of time.
While it's charging you can monitor the SOC using the HondaLink app. Note that the app only updates SOC at intervals of 15 percent, even if you press refresh on the app screen you will notice that the "Last updated at" time does not change and neither does the SOC. But there is a trick to get the charge status to update, go into Schedule Charge and set a timer schedule. Doesn't matter what schedule you set up as the schedule won't take effect while the car is already charging. But when you save the schedule, then on the main screen the "Last Updated at" time should now be the current time, although it sometimes take a couple of minutes to display it, if you don't want to wait this is one time that Refresh is actually useful. To later check the SOC again, go into Schedule Charge, press Modify Schedule, on the next screen you don't need to change the charge times, just press the check mark to accept the schedule, after the schedule is accepted press X to go back to the main screen and it should show updated SOC and current time (if not press Refresh).
If you can't get the HondaLink app to work, an alternative to check the SOC is to get into the car while it is still charging and press the power button twice, don't bother pressing the brake pedal as the car won't start while you are charging. This should bring up the main display and you can check the HV battery bars. As long as you do this while the car is still charging it won't put a drain on the 12V battery.
As for the error messages you are seeing, if they are about ABS and TPMS etc then that is normal messages to see whenever 12V power is lost even momentarily. Those messages won't go away until you have driven about a mile, so until you get fully charged and are able to drive the car you will continue to see those messages.
If after two hours of charging you still can't get the car into READY mode and into D, then your next step would be to replace the 12V battery and hopefully that fixes it. Although as a reminder even with a new 12V battery you will still see the ABS, TPMS type of warning messages for the first mile of driving.
