It's not as bad as it sounds

you should change the battery as soon as possible (or as soon as it is convenient

). I don't remember the size of the battery that is used, but it is the round flat type - like in watches (but bigger). Easy to change if you know how to open the computer; should be explained in the manual.
If you do not change the battery, eventually computer will lose important information that is being used when booting it up. So do change it
