Today I hit the 12V battery drain corner case

This morning I finally hit the "plug the car in when fully charged drains the 12V battery" corner case.

In doing so I managed to kill two car batteries. Let me explain...its too much..I'll sum up.

This past weekend we went camping and took along our Escape (pulled it on a dolly behind our motorhome). For the 10 hour drive home I forgot to start the Escape whenever we stopped. This resulted in car battery #1 dead. The FFE was sitting home for this trip all fully charged. I backed it out to use it to jump the Escape (which worked) pulling the FFE back into the garage and plugged it in again (thereby initiating the corner case: The FFE was fully charged and I plugged it in).

I awoke this morning to 35+ notifications from MFM stating that the 12V battery was low. Using the handy cigarette lighter meter I measured my 12V battery at 8V--low enough to not be able to start the car. I unplugged the FFE from the EVSE and replugged it in which woke everything back up (at this point the meter read 13V). From this point on the car behaved normally--I remote started the FFE and let it run for 30 minutes before proceeding to work.

At work I checked the car at lunch and before leaving; the 12V battery was showing 12.4V at lunch and 12.1V before I started the car to go home. Thus I don't think I did much if any damage to the 12V battery... (These are about the same voltages that I would have read from the car before this incident.)

I found this situation quite humorous: Here is the car screaming that the 12V battery is almost dead and it has two avenues to correct the situation (the HVB is fully charged *and* its plugged in). "Help me I'm drowning!" "um take your head out of the water!!"

I don't have access to it now to paste, but when plugging in to charge the SOBDM reads the 12V SOC and decides how much to charge it while charging the HVB. If the HVB is full and you unplug & plug it back in the car doesn't monitor the 12V SOC and instead lets it go dead. The workshop manual describes this better than I did...

Yep its ridiculous that the FFE software can't always monitor the 12v battery level and when needed charge it from the internal HV battery or from the charger if plugged in.
The 12v battery is the weakest link in the whole system.
This is why I've been proactive, always monitoring the 12v battery level with my little accessory voltmeter plugged in the 12v accessory port and keeping a small Lithium-Ion rechargeable battery device that can jumpstart a 12v car battery.
I don't leave the car plugged in unless I'm charging or just before a set Go time, so I haven't had the low 12v battery condition since I've been doing that for 9 months straight now.

eBay has a bunch of vendors selling 12v charger/meters. Very good to buy one and keep handy. Good for charging devices from the backset via th e12V outlet and for checking on 12V level. I use an inverter on our car when we camp to pull power so I always keep an eye on the voltage. Never had a problem before.