IBM Thinkpad X31 with battery problem

[NetScorpium]

Hello,
I have a strange problem with an IBM Thinkpad X31! When I'm working with AC power connected and for some reason I disconnect the AC power, the power status (I use the Windows XP built in power manager) stay the same "Corrent power source: AC power", but the "Total battery power remaining" decreases!
If I stay with AC cable unplugged, the battery power decreases, but if I plug it again the battery don't charge! I think there are some problem switching the status when the notebook is with AC power cable plugged and unplugged or/and a problem charging the battery. Any suggestions to solve this problem?

PS: The battery is new (original from IBM) and work's fine in other notebook IBM X31.