dead battery

I have an acoustic electric and the 9V keeps dying on me. So I try to change the battery with the strings but I have heard that the battery should last longer than 2 weeks if not in use. I don't play plugged in very often but like to when there is a need. I hate using a mic on the guitar and I don't want to buy any new gear. Any Ideas on how to fix this??

don't know why this posted twice