@robl45 I imagine that using one of those
portable jumpstart battery would provide a little bit of juice to your car,
and sufficient enough to perform some diagnostic, in particular
to check the voltage of your battery.
If the voltage is around 12 to 12.5 V, the inboard charger is not charging.
If the voltage is around 14 to 14.5 V, the inboard charger charging the 12 V battery.
I installed a
Bluetooth 12 V Battery Monitor so
I can check when the 12 V battery get recharged
with the internal DC/DC converter even when the car is locked, using
Bluetooth and my phone APP.
When I am not using my car, and keep the car locked for few days,
I noticed that
the 12 V battery get recharged with the DC/DC converter during about 2 hours, about every 36 hours .
I have a
two year old battery, I live in Northern California, and I park at night in a garage.
You can see that the battery lost (13.18 - 12.62) = 0.56 V in 36 hours while sleeping, or about 15 mV an hour.
View attachment 604379
@robl45 in the case of your used battery, I suspect that the DC/DC converter tried to charge your battery but noticed that
after stopping charging the battery,
the voltage was going down very fast as
the battery was not able to keep the charge.
So the DC/DC converter (may be) decided not to continue charging the 12 V battery to avoid dumping too much energy
from the high voltage propulsion battery.
I'm surprised that
Tesla didn't send an early warning to inform about the 12 V battery charging issue?
It is a pity that Tesla doesn't display the 12 V voltage of the battery on the central display,
and don't give a battery status information for the 12 V battery by checking the discharge rate.
It is strange that Tesla can calculate with great precision the energy needed to reach a Supercharger without been stranded,
but cannot send a warning diagnostic of the 12 V battery degradation to avoid the kind of bad experienced the OP encountered.
Note: I would strongly recommend to anyone, as first accessory, to get one of those
digital voltmeters connected to the Auxiliary plug,
so this would allow to
check if the battery get charged, which happen every time you unlock your car, or access remotely your car,
and
check the default battery voltage when the battery is not charged by the DC/DC converter.