And does it always happen at the same end?
And is it true having a battery go nasty inside an electronic device ruins the device?
I know from experience that it doesn’t necessarily ruin the device, although if it leaks long enough, or bad enough, I’m sure it can. It does create a mess that I usually try to clean off before trying to reinstall new batteries.
Why does it happen? I assume it is because the battery over a long period of time leaks acidic electrolytic fluid that interacts with the metal contacts and causes corrosion.
Which end leaks seem to depend on how the batteries are oriented in the device when it is stored. Gravity would cause the fluid to puddle at the bottom of the device versus the top. Of course, batteries can be in any direction relative to the top or bottom of the device, so I think it varies.
dolphiboy pretty much nailed it.
My only addition is that most batteries from the negative end to very close to the positive end are one continuous metal case. So no leaks are possible except if punctured.
Up near the positive end there is some sealing method to isolate the negative case material from the positive end structure. That is where the battery will seep out nasty chemicals when it fails.
Oh! It requires a mechanical failure/flaw in the case? I sorta assumed it was something that just happened, as part of the battery being ‘used up’ after some stretch of just sitting around.
Which then makes sense. I was thinking that if it happened for some ‘electrical’ reason, the crustiness would always happen at the positive or negative end.
Not that I know which end is which. I’m mostly familiar with AA or AAA batteries, which have ends that I think of as ‘smooth’ or ‘bumps’. (Can you tell I don’t really grok electricity at all?)
Aha, so the bumps are the positive end? Good to know, maybe it’ll come up on Jeopardy some day. 
And it makes sense that it’s the sealing stuff that actually fails, rather than the metal.
BTW, this was inspired because I was cleaning out a closet and found a flashlight that was not just dead, but had one of the batteries practically welded into place by that crusty goo. Which reminded me of my mother always telling me I was supposed to take the batteries out of things before I put them away into storage to prevent this… But who knows when something is going to turn out to be the ‘last time’ you will use it before a long hiatus? Maybe I’d have needed that flashlight again a week later to explore under the sink or something?
Anyway, my thanks to both of you for enlightening me a bit.
I find it doesn’t happen in devices I use at least once or twice a month, but if I have a battery-powered device stashed in the back of my closet for 5 years it’s almost guaranteed to have a leaky battery. Sometimes it’s so bad you can’t really clean it and you have to toss the device out. That’s really rare, but it happens.
I must have bad luck. It almost always ruins my devices. Even when I clean out the goo, the electrical contact is often bad enough or enough other damage has been done that I have to replace the device. I do have a headlamp that is the rare exception. It still works despite once hosting corroded batteries.
Old-style carbon-zinc batteries, which are labeled as “heavy duty” these days, actively consume the zinc casing of the battery as they age and through use, eventually causing leaks. The electrolyte in these is acidic.
Alkaline batteries don’t do this, but they can still have external corrosion on the steel casing, or they can build up internal pressure and break the seals and leak that way. The leaking electrolyte in these is typically potassium hydroxide, which is alkaline (duh!).
So how a given device reacts is going to depend on what kind of battery, and what the electrolyte leaks onto.
That explains why when I was a kid battery leakage was very common and ruined many a toy.
In recent memory the only battery leakage I have seen has been with house-brand batteries (specifically from BJ’s Wholesale Club and Walgreen’s). Haven’t had a leaky Duracell or Energizer battery in years. I’ve stopped buying house-brand batteries.
I’d better check my radiation detector. I haven’t turned that thing on in at least 3 years. I think I’ll change the batteries, too, just in case.
I just changed the AAA alkaline Energizer batteries in my tv remote. One of them had gone crusty and the bottom part of the outer case came off like a ring.
Dumped the crusty stuff out and wiped my finger around the spring and the new batteries worked just fine.
If you can get ti the contacts, you can lightly file the corroded area. And I’ve used this stuff with some success and the price is right.
https://www.sciplus.com/corrosion-be-gone-battery-contact-cleaner-45986-p
But is that a function of use or time? That is, the battery device you use regularly, presumably you’ll have to change the battery at some point before five years. So the unused battery is likely older than your more used batteries.
(I’m actually wondering – I don’t know the answer to this.)
As for ruining the device, I’ve never had (to my memory) a device that was completely ruined by a leaky battery. It’s possible I either have good luck and/or never let it get corroded enough to matter.
Thanks! You answered my question.
Thanks! I’ll probably give it a try. I’ve tried cleaning and polishing contacts before with few positive results. Maybe this stuff will work. With my luck, I’ll wind up with a leaking jar of clean-up goo. 
I’ve used sandpaper and a rubber pencil eraser to clean contacts. The Energizer Battery site says
The best way to remove alkaline leakage from the device is to neutralize by carefully dabbing with a few drops of a mild acid like white vinegar or lemon juice. For stubborn leaks, an old toothbrush dipped in vinegar or lemon juice gets the job done. After the leakage has been neutralized (fizzing stops), carefully dry the area. After the area has dried thoroughly, the metal device contacts can be scraped to remove any residue and then polished using a pencil eraser.
If heavy-duty (non-alkaline) dry cells leak, use some baking soda and water paste to clean the device. Rinse well with plain water.
Since I can’t regale 9th grade physical science students with pedantry anymore, I will point out here that it takes more than one AA or AAA cell to make a battery.
Yeah, with covid restrictions over the last few years lots of my handheld battery powered devices haven’t gotten much use (and I’ve forgotten to check periodically) so now I’m finding lots of flashlights, cameras, etc with bad batteries. About 2/3 of the devices survived the ordeal.
NM