There is an error message that appears when you boot up (might be disabled by default). It reads:
"The AC Power Adapter type cannot be determined. This will
prevent optimal system performance.
Strike the F3 key (before the F1 or F2 key) if you do not want to see power warning messages again.
Strike the F1 key to continue, F2 to run the setup utility"
I have a Dell Latitude D600 and I get this message due to the power pack being

up. It will run off AC fine but it won't charge the battery.
I hope for your sake it's a different story.