I've noticed that my new car allows draining of the 12 volt battery
through 12 volt sockets even when the ignition is off and the engine
is not running.

I use to leave charging devices attached to the car at night know they
would only charge while I drove around, but now I can't any more as
they will drain the primary battery.

Does anybody know of any devices that help stop current flow when the
source is below a certain level?

I'm also interested in attaching some of these same devices to my
boat, but would like really only like to pull when the alternator is
charging the primary, so perhaps I could set the threshold really high
on the boat.

I know there are delayed relay devices I could do this with, but I was
hoping I could use something that would require no additional wiring
that to the + and - of the source battery.

Many thanks for any help or information.