APC back-ups pro 1500 new battery not giving back up runtime

[Anonymous user]

Hey guys, I own APC Back-ups pro 1500 and recently I replaced the batteries. Now it's not giving any backup. Previously when battery was at its prime it use to give backup runtime upto 20 minutes while playing heavy games and upto 40 minutes while idle or surfing intense but now with new battery even when I'm not gaming it's only showing runtime of 5 to 6 mins and while gaming only 1 or 2 mins. Now I was told that you need to charge the batteries for at least 16 hrs but the cell indicator shows battery is full. Is this a problem or is it really just that the battery is not charged. One more thing I wanted to ask is will my UPS charge while its off but it's plugged in or do I need to keep ups on to charge it.