diagram link

billymc:

Here's the diagram that wouldn't display correctly on the fieldlines
discussion board:

Batt+
|
|
+--------------+------------------------+---,
| | | |
| | relay ) -
/ / coil ) ^ 1N400X
\ \ ) |
/ / ,--/\/\/\--, +---'
\ \ | | |
/ | | |\ | |D
\ <--/\/\/\/\-------+---|+\ | |--
/ | | >---+-----|| n-channel
\ +--------|-/ |-- mosfet
/ | |/ |S
| | |
| / gnd
gnd /_ 5.6 v zener
|
gnd

Use the contacts of the relay (not shown) to connect the battery to a
dump load.
This way you won't have any heating of transistors and such.
Note that hysteresis (the feedback resistor to the op-amp pos input) is
necessary with this circuit
to prevent chattering of the relay.
The dump load will turn on at a certain voltage and hold on until the
battery
drops to a certain lower voltage, determined by the values of the
resistors on the input of the op-amp.
You can look up op-amp hysteresis or similar terms in Google, and you
will find the formula
to calculate resistor values that will give you the exact voltage
window you want.