Update: We went with Custom Arduino-based LVD solution for this project.
The LVD Project is great, but relatively complex -- overkill for some applications. A more static circuit could be made with a zener diode, a signalling LED, and possibly a relay (for actual switching)
This is a zener-based circuit. An interesting reference is at http://pubpages.unh.edu/~aperkins/pdf/Misc-devices/unijunction.pdf --- highly recommended, but a bit technical.
A prototype is about the area of a dime, with only 6 components:
This working model lights up right at 10.7v, indicating sufficient voltage for a specific application (SLA batteries with 15w fluorescent load).
|10v zener diode, 5240
|3904 NPN transistor
|small LED (t1)
|100k Ohm resistor
|10k Ohm resistor
|1k Ohm resistor
Wire the LED-side of the transistor over to an appropriately-sized relay and you get the actual protection circuit. For better usability, green and red LEDs can be added on the open/closed size of the relay as well, to indicate low/high voltage conditions.
For lower-power applications I found a Bestar BS-102B relay -- 12V SPDT, switches 2A, and draws less than 200mW.