[chbot] Circuit help please, low voltage cutout

[Carl Ranson]

Ive been looking at this circuit for my bot's battery protector circuit:
http://www.siliconchip.com.au/cms/A_104378/article.html

There  are a couple of things I don't understand about it:
a. Whats the point of ZD1? How does its value effect the cutout voltage 
which I thought was determined by the voltage divider involving VR1?

b. Can Q2 be replaced by a normal transistor and a relay? is the choice 
of a mosfet because it will use less power than a relay?

I know it was mooted that i could do it by using a voltage divider to 
one of the pics ADC pins etc, but i want it to cover all of the 
circuitry on the bot (even if the controller is disconnected).

I will probably still monitor the voltage into the controller to 
determine bot behavior but thats a ways off yet.
Thanks all,
CR