Paul wrote:
"Sloppy", would be running it down to zero, pushing it
into the garage, shutting the door, coming along next
spring and noticing "gee, the charger refuses to charge it.
Broken charger ???".

It probably shuts down at some low but not zero point. So it's likely OK to
do that if you don't leave it for years such that it self-discharges into
the danger zone (Li-ion self discharge is small but non zero). Similarly
it's OK to store fully charged, but a bit more stressful on the battery.
Better would be to fully charge it and then run for a few minutes to make it
a bit more relaxed, best would be to store at 40% SoC which is the best spot
for a storage charge.

To give an example of science, a twit down the
street left their Prius in the drive for six months,
no apparent "engine turnover and charge" or anything else.
Is the battery pack in a Prius, impervious to such
treatment ? Is it lithium ? Or NiMH ? It just seemed
strange that they left the Prius sitting there, and
drove some other gas guzzler instead. Why would you own
a Prius, if you didn't plan to take it out every couple
days and allow it to maintain itself ?

Depends on the Prius - the Plug In ones are lithium, the others are NiMH.
But they all have a 12V lead acid which is used to start the car and run
accessories, the alarm etc. 'Starting' means opening the contactors to
connect the HV NiMH battery to the inverter, and running the brake servo,
which takes a gulp of current (~20A for a few seconds).

If your 12V battery is flat because the alarm ran it down it won't 'start'
but the NiMH battery is sitting there disconnected perfectly fine. The 12V
is just a regular lead acid car battery (although AGM).

With a flat 12V battery I can 'start' my Prius with a tiny 12V lithium
jumpstart pack which is enough to get the contactors open and the HV system
up and running. If the HV battery is a bit low it'll fire the engine to
recharge it. I don't know what happens if the HV is so flat it can't fire
the engine, but it doesn't seem to suffer much from self-discharge. It's
possible there's a bypass route to run the inverter directly off the 12V but
I doubt it.

When I was looking at them to buy, often dealer cars would have a flat 12V
battery - but happily jumpstarted.

You would think they would rotate their two vehicles
and keep them all happy.

It's what I've done, but driving a few miles every couple of weeks hasn't
been enough. I've had to externally charge the 12V on a few occasions (or
jumpstart and leave the car powered up where it charges the 12V from the HV
battery - it will run the engine periodically when the HV gets low).

Theo