9 volt battery becomes warm

Hi,
I have modded the hell out of a boss effects pedal and to my dismay the 9 volt battery gets warm after about 15-20 minutes of operation. I also added a DPDT slider switch(installed via Craig Anderton style-with ground) to act as the power switch and found that the battery gets warm/ semi hot after about an hour in the "off" position. I added the power switch so I wouldn't have to open and close the enclosure repeatedly. Are there any modders/benders out there who have had this experience and successfully ended this issue?