Windows 7 - Back-UPS ES 750

My 4 year 2 month old APC Back-UPS ES 750 using Powerchute 2.0 Firmware revision: 841.I2 .D will only charge to 98% and after being on for about 30 mins goes down to 91% and always says charging (see notification area in attached shots I posted in APC forum). Windows 7 Home Premium. I have un/reinstalled the software but I did not let the battery run down. Can this be a false reading? or Do I need a new battery? I have read in other posts that I may have to recalibrate the unit, I will do this if I have to but if a false reading I will probably just ignore. Also if it's always saying charging is that affecting my electric bill with it using more? THE APC website says my model # is invalid, I do see other APC 750's but is the Back-UPS ES 750 just a old model? I can get an entire new unit for $80 on Amazon.