ups 12v 9ah batteries not charging
UPS batteries usually do not last much more than 3 years, it's that easy really.

To check the battery or batteries: As your UPS is a large one I'm assuming that  it is actually has a separate battery compartment which you can access  without dissembling the whole thing. 

Connect as many batteries as you need in series to get 12V and load test it which a common 12V lamp.

Charge the 12V pack with a normal automotive charger. As there most likely are several batteries in that UPS, repeat for all batteries.