CMM1200 not holding a charge

I have a B&D cmm1200 that is not charging. My unit is 10 years old and i just assumed that the batteries were old and needing to be replaced. i purchased the new ones and installed them into the unit. it seemed to work great for the first couple of time that I mowed the lawn; however the last couple of time it will run out of power before i get done with 1/2 of the front yard. I plugged it into the charge and the charge light immediately goes from red to green. I have left it plugged in all week and now the same problem. Any direction at this point would be a great help.

Kevin