I've got 2 of these units and 4 different cells I know to be good. Used, and currently low, but not bad batteries. On the better set of two they read 9 and 6 v, because they are low. When grouped, the expected 15v is displayed on a meter.
Neither UPS will charge them because they instead report either battery fault (if a test is done) or "no battery" after a while or before testing.
I don't have a means to just externally charge batteries. I have left it connected overnight for 2 or 3 nights now.