I was playing with my Gameboy (12 years and still cookin!) and I heard a “POP!” sound, sorta like a small firecracker. I could tell the origin was inside my Gameboy.
I took it into the bathroom, and opened the back. I noticed a brownish liquid, and a hissing sound. The seal on the negative side of one of the middle two batteries had been slightly compromised, as the bottom of the battery had been pushed out ever so slightly.
I know that the last time I loaded it, I had only three batteries, so I left one of the old deadish ones in, and it was one of the middle batteries. Can’t say for sure that it’s the one that was damaged, but could be.
So, what exactly happened that pushed the bottom hard enough to pop part of it out of the seal?
It is. For future refence, this is a Bad idea™. What happens is the other, good batteries push current through the dead one, overheating it. In this case, the heating was sufficient to cause enough pressure to build to burst the cell open, shich can be very bad for the device being powered, since the corrosive electrolyte can cause unintended short circuits, and eat away circuit traces and cause other damage due to corrosion over the long term. Never mix old and new batteries, or batteries of different types (for example, never mix alkalines with NiCd or NiMH types).
Efectively what happens when you mix batteries is that the new batteries try to even things out by re-charging the old battery. Not only may the old battery not be designed to be recharged, the recharging also happens at an uncontrolled rate.
When batteries charge or discharge they generate heat (in what is, at the root of it, a chemical reaction). As it heats the contents of the battery, which are confined within a metal case, expand. The result is a rupture of the battery and venting of unpleasant, hot chemicals. Not good for your Gameboy or you.
Usually your batteries are not draining at the same rate. So one of your batteries will go dead before the others. So if you then change their relative positions, you kind of have the same problem as mixing new and old batteries.