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[satchboogiemonster]

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[Ole]

You should, as a thumbrule, never leave batteries in anything you don't use.

 

[Sustainerplayer]

So I opened my Axe Fx. There was no battery lid so had to use a can opener. I found no batteries though. Spooky.

 

[paulasbell]

This warning about leaking 9 V batteries is spot on.
Recently, I've been selling off a pretty extensive collection of stomp boxes. Much to my chagrin, one or two of them had batteries which went dead, leaked a bit out of the connectors, and corroded onto the adapter from the stomp box itself. Had to replace a couple of adapters, as a result. A couple of them presented unusually tricky soldering challenges.
The moral of the story… be careful about keeping dead batteries in those stomp boxes!

 

[Geezerjohn]

Old batteries are like old musicians. They never die, they just decompose. Humans excrete when they die. So do batteries.

 

[mr_fender]

9V batteries are sometimes not quite as bad about leaking because they are actually six separate 1.5V cells chained inside the rectangular casing. The inner cells might leak but it doesn't always make it out of the outer case. They still do leak eventually though, so it's still definitely better to remove them when not in use.

 

[aleclee]

Man, it's probably been 15 years or more since I actually put batteries in a stomp box. After you're using 2-3 pedals, it gets tedious (and expensive) to deal with batteries.