I use 12vdc SLA batteries to power an unattended video recording
system. The load is about 1.5-1.75 A, and the recording sessions are
from 4 to 24 hours.  For short sessions, I'll use a single 22ah
battery, and for longer recording times I'll use up to 4 of these
connected in parallel.  The system is controlled by an external
timerthat turns the power on and off at certain times, but very often
we cannot use this feature because some of the digital recorders we
use cannot be powered up except through a key-press.  Stupid, but
true.  So sometimes I have to turn the system on and let it run until
someone can return to the site and physically shut the system off.

We've had trouble with short battery life, and the vendor says it is
because I allow the batteries to run down below 10.5 volts, which is a
red line for these batteries. So what I need is a design for a circuit
that will cut off power when it declines to a reference voltage of,
say, 11.0  volts, I don't need to control the upper range, because a
fully charged battery drops about 12.6 v under load, and our equipment
operates normally within the range of 11.5-13.0 v.  Also, it is
important to not introduce too much extra resistance in the circuit
because this would mean I need to add more batteries power, and that
would create significant logistical problems.