The 12V sealed lead battery in a piece of equipment has been
left connected to a circuit long after after the mains charging was
switched off. It's got almost 0V across it and is poked. I'd like to
add a low battery switch so this doesn't happen again. There are
battery supervisor ICs around but I think a bypass transistor with a
zener in there somewhere could disconnect the battery at say 11V.
Nothing fancy, as long as the cut-off is a reasonable margin above
point-of-no-return for the battery (which I think is ~10V for a sealed
lead). Some help please ?

The requirements are that the battery is in circuit while either the
mains charging is on and/or the battery V is > cut-off. In the event
of a power cut, the battery should stay connected until it reaches
the cut-off V, and the V monitor should use as little battery power
as possible. On resumption off power, the battery is reconnected
so probably a relay is needed to do this as well as the low V switch
to disconnect. I'm redesigning the unit's PCB so anything will be
considered, doesn't have to be a retro-fit or even cheap as dirt,
as long as it works

Example via Google

http://www.homepower.com/files/lvdhp60.pdf

TIA

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