>What I'm after is something that will when the voltage droops to a
>set point basically short the cap and de-energise the system.
>Physically small is a constraint. An easy to use IC would be nice.

I would be very tempted to use a switching regulator to supply the 
electronics, with some form of analogue comparator on the battery to signal 
the low voltage condition to the electronics. Having the switching regulator 
will ensure that the electronics has enough volts present the whole time to 
make the thing go safe in a controlled manner. 

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