I am experiencing a very weird problem with my 2010... (by 2 users)

hi i have a problem with my panasonic rechargeable li-lon battery the battery pack: 258-4S4400-F1P1. The problem is when the battery is at 100% if i unplug the rechargeing cord i can play games but if the battery is at 100% and the recharging cord is pluged in it will only let me play the game for about 5min some times a little longer i was wondering if you guys have any idea what the problem might be i all ready have a pc docter and nothing seems to be working i would apreciat if yould e-mail me and tell me what the problem might be. I will not be able to take it to a profeshional because i'm curently deployed to afganistan and will not be back to the states for another 4 months thank you for your time you can e-mail me at WWW.death_reaper@hotmail.com all lower cases thanks again bye.