I have the opposite problem with my 18 month old Mini 9 running XP.
Having left the netbook for 2 months unused, when powering backup I noticed that the battery wasn't taking any charge, but XP noticing the AC was connected as the screen brightness was altering.
Thinking the OE battery was finally failing, I've installed a replacement battery, which is giving the exact same symptoms.
It would seem if I boot the Mini with the battery disconnected, and AC connected, then attach the battery then it starts charging successfully.
This is all with running BIOS A05 and XP sp3;