Two backup batteries in my system?

["walter" <someone@xxxxxxxxxxxx>]

My ADT Safewatch 3000EN system has two big lead-acid backup batteries type
ca 1240, instead of just one backup battery! I understand this system is
similar to the Vista 15P.

I cancelled my monitoring contract with ADT and I am now on my own. The
system is about 4 years old, all hardwired. I thought of changing the system
backup battery preemptively. Then I found two identical big batteries in the
panel. It also has two wall wart transformers, both for 16.5V AC.

Battery leads (red and black) go from battery A into the Main Circuit Board
without terminals. The leads from Battery B go to (come from) a small
sub-panel marked Altronix AL 624 Power Supply/Charger.  The two batteries do
not seem to be interconnected (such as being in series or parallel).

What is the second battery for and how do I replace both batteries?

http://img854.imageshack.us/img854/87/1000871.jpg
http://img402.imageshack.us/img402/2883/1000874d.jpg

Thanks for any input.

--
www.rationality.net