Problem with power management

[Charlie Brune]

I have an old laptop with a completely worn out battery.  It does not 
run at all if the A/C power is removed, but runs just fine if it is 
plugged in.

I'm running Ubuntu 9.04.  The laptop is a Compaq Presario 1800.

My problem started after applying the latest Ubuntu updates this weekend.

The problem:  After I log in, I see a message at the upper right-hand 
side of the screen telling me that the laptop battery power is 
critically low and that the laptop is immediately shutting down.  Then 
the laptop shuts down.

Since the laptop is on A/C power, I don't want it to shut down.

Is there a way for me to log in using "recovery mode" and edit a 
configuration file to change this behavior to allow the laptop to stay on?

Thanks,
Charlie