Have i done something wrong?

i installed a cd player in my wifes 1997 mitsubishi lancer the cd player works with out a drama . any way after mucking round with the cd player for a while the battery was to flat to start the car so i grabbed another battery out of an old car sitting out the back when i tried starting the lancer again it was still struggeling to start as the battery was weak after a few kicks it started my wife drove it for about 5 minutes.this morning my wife went to go to the shops and the battery was again to flat to start the car .. is there any thing i could have done wrong which would be draining the battery??