APC UPS BACK UPS XS 1500 external battery recognition?

Hi all...I have an APC XS 1500 with the port for an external battery pack. I
have installed a couple of SLA's in series across the plus and minus
sockets, the ups seems to accept them ok and current is being drawn from
them during testing, but the software..powerchute personal editon...does not
increase the estimate of runtime over internal batteries.
If the actual runtime is increased by the additional battery pack, then no
problem, but would be nice to have the software recognize my non oem
attachment.
What does the third line out on the external battery pack use for
signaling? The oem's internal battery pack third line is connected directly
to the plus lead of the battery pack, as far as I can determine.
Thanks in advance..