Can the 18 volt batterey used on the grass trimmer be used on other ryobi cordless tools such as the drill or sawsall?

whichever work best for you

YOU CAN GO TO GOOGLE AND DOWN LOAD THE MANUEL.JUST TYPE IN INSTRUCTION MANUEL ROYOBI CORDLESS DRILL 1048-67479CS LITHIUM BATATERY CB 120L YOU SHOULD BE ABLE TO DOWNLOAD THE MANUEL

When looking at the head, you will see to two gray rectangle pieces. Press them both at the same time between your thumb and forefinger and then pull. It should come unlatched and reveal the line spool on the inside

replce it cost to much to fix

The motor is failing. Most likely the brushes have failed, but this is all sealed and they cannot be replaced alone.

This WEB site show how to install the nylon string.
http://www.ehow.com/how_6326062_string-ryobi-trimmer-plus.html

the wiring maybe disconnected, either at the switch or down near the head of the trimmer

take a hammer, and apply force. These batteries dont last forever... but i am serious about the hammer... you may be able to free up whats inside (if you know how a batterey works.)... also apply heat.. try to hit it from all angles than slam it down on the back and leave it sit... than try. I hope this works for you my dewalt batteries over the years have don this, and it works.. but BTW. it wont hold a charge for very long anymore.. the battery is pretty much garbage.