Hey James don't glue it. We pitch the old part that is supposed to be stuck to the guitar and install a new one after a naphtha wipe and some time to dry all the way. You can purchase lots of velcro from any supplier of tape for pedal boards.
But before just replacing the tape let's prevent this from happening again. People stick the battery in all manner of places and that's often the issue not just the tape. Once gravity pulls the bag with velcro off the guitar that piece of velcro is finished.
So I place my velcro butted against the upper side of the neck block so for a right handed player this would be the left side of the rim butted to the neck block. This way when the battery is in the bag and it has a little slop it naturally rests on the neck block in the playing position hence reducing the pull on the velcro for the most part. To be more clear the velcro is not on the neck block but next to it and that battery is in the black bag with it's side resting in the neck block.
We have a considerable number of our clients as gigging musicians so this is a very common issue. The fix above has never not worked to our knowledge once installed this way unless it Peter Townshend
