> What causes a battery terminal to corrode? The one that's on now is copper.

Believe it or not, the older non-sealed batteries emit hydrogen gas
when they charge, often leading to corrosion if they don't get
ventilated away effectively. If you are commonly in the practice 
of running down the battery and then charging it up again (like you
sit there a long time with lights on) you run risk of this. 

When electrons go running through a piece of wire, they attract postively
charged ions into the metal. This makes things like corrosion more
likely. It also makes things like electroplating possible. ;)

> The replacement is heavy-duty lead. Does that matter? 

Different metals have different resistance to current, and different
resistance to corrosion.

> Why didn't the 
> secondary battery kick in? Was it because it needed to go through the 
> primary battery whose contact was bad? 

Depends on how things are wired. It could be that your second battery
is not getting charged or used like you expected.

> How can you tell whether or not 
> the secondary battery is doing its job? 

Take the terminals off the primary battery and see if you can still
run your accessories. Some people set the second battery up to ONLY
run accessories. Some people set it up to be switched over to. Some
people set them up in parallel.

> What is its job? I always figured
> it handled the interior lights and maybe the electric cooler that I never 
> use.

It could very well be, and it could also not be connected up so you can
run the starter from it.

------------
Joshua R. Poulson, Systems Engineering, ADP Dealer Services, Portland, OR
PGP Public Key available upon request