Either you didn't give us most of the information or this isn't your
answer. The standard SmartUPS 1400 uses 4 12 v 7 AH batteries in
a series-parallel combination. I have not had any real success
using higher capacity batteries in these -- they don't charge
properly and seem to have a shorter lifetime.

You didn't mention that you have an XL model and that does use
the 12v 18 AH batteries, and has a different charger.

Michael Kennedy wrote:
Found my answer at

http://home.tekies.net/HowTo/UPS%20Upgrade.htm

Here is the article off APC's Web site.

Smart-UPS XL models are not designed with communication modules to the
external