First of all, hello!, I'm new in this forum. I've lurked it several times and decided to sign up.
While doing my monthly backup, after checking the .sav with an emulator, to my amusement the save was blank. Started sweating... Thought it might have been a bug in the download app, tried again, and again. Nope. The internal cart battery had leaked, like, the day before (it was still wet) 
I have two carts, same production date, and the one I use more, sometimes as long as 5 hour sessions, was the one that failed. These are the ones I'm talking about:
Which got me making hypothesis: Was it that I left the cart plugged? Was it because of more heat for leaving it inside a bag? Was it that I had just changed the gameboy batteries and an overvoltage did that? Was it shipped with an old cell? Was that black cat I crossed? The mirror I broke last year?
When the gameboy is turned on, the cart shuts off the internal battery to use the 5 volts provided by the handheld. My guess from looking at the circuit is that it uses a MOSFET for that purpose. But I measured and it doesn't seem to shut the internal battery quite off.
On my CGB, without a cart battery (cart case open) I measured 0.01 volts between the terminals of the (missing) battery while the CGB was on. Then I tried to check for leakage of current with the battery plugged but my cheap multimeter couldn't register any.
Apparently, from what I've been reading, lithium batteries are very sensitive to reverse voltages or short circuits. They will leak and might even explode.
I think this tiny current leak, for several days and hours, might have been the reason for the leakage.
If any of you have any better ideas about this, please let me know.
For now I can recommend to change the battery to a brand you know, with an expiration date you know as soon as you get the cart!. I read cell metals start dissolving during their shelf life, even if unused, and promote leaks.
Best regards