DIYbanter - computer battery problem...

I did a search for 'computer repair' and read in a news article that
seemed to say this group welcomed such inquiries.

I have a Toshiba laptop (my third) that used to run without the
battery pack. The computer is probably 10 years old. I use it in my
vehicle to look at maps and also run a timer program that keeps track
of where you are on any interstate freeway in the US. The computer is
continuously plugged into a 120 volt power converter. Just a few days
ago it started shutting down with a low battery message (I think that
is what it is saying - it shuts down too fast to read the message)
whenever the monitor is out of the screen saver for more than about a
minute. Since the computer will usually run without the battery pack
installed, it seems that there may be an internal battery that has gone
bad - just guessing here. The computer shuts down as above even with
the battery pack installed and the battery charger plugged in.
Obviously, the monitor is draining too much juice, causing the computer
to croak.

This same thing happened to my 1st Toshiba laptop.

Any suggestion on how this can be repaired will be appreciated!

Thanks - John