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Yes, these have definitely released the schmoo! My best guess is that they were discharged and then left in that state for quite some time.
These batteries are in a cheap LED lantern, which is nice enough to save, but I'd probably put more effort into the clean up if this was something more expensive, like a computer mouse or a remote control.
While the schmoo that comes out of batteries isn't toxic, it's not smart to get it on your skin, in your eyes, or to be breathing it in, so I recommend taking precautions (even if, as you might notice from the photos, I don't!).
I find having a brush handy -- a toothbrush that isn't going to be used again on teeth is good -- along with a small scraper, and a few cotton swabs to dip into water. Distilled water is best, but tap water also works.
Some people recommend acids like vinegar or lemon juice, but I'm not a fan because they encourage corrosion and can make the problem worse.
While it's far from a requirement, I use a fiberglass scratch brush for scrubbing the contacts. It works really well!
Again, not a must-have, but since I come across a lot of exploded batteries, I find it worthwhile to have a can of Deoxit D5 handy. This stuff is the best contact cleaner out there, and does a fantastic job of cleaning really bad corrosion.
It's also great for resurrecting circuit boards that have been damaged by water and corrosion.
Another thing that's not required, but following a battery leak clean up I always use a tiny smear of dielectric grease or silicone paste on the contacts to help inhibit any future corrosion. This also helps to limit the damage that a battery leak causes.
You only need a tiny smear, so don't go wild with it!
For electronics that live outdoors, I find that a small spray of silicone into the battery compartment also helps to inhibit corrosion, as well as limiting the damage that a battery leak causes.
First, remove the batteries and clean off the worst of the mess using a brush and paper towels. I like to do this outdoors or over something that will catch the mess.
A small metal scraper makes short work of the corrosion. This is how you will remove a good 90 percent of the mess, and if you wanted, you could leave the clean up right here.
The fiberglass scratch brush does an amazing job of cleaning off the remainder. If the contacts have corroded you'll never get them back to pristine condition because it's likely that the corrosion damaged the plating, but you can still get them very clean.
The cleaner you get them, the better your chances of inhibiting future corrosion issues.
A quick spray of Deoxit D5 helps loosen off the last of the corrosion, and helps prevent future rusting.
That's too much! I could have gotten away with half of that.
Just work it over the contacts and the springs. Don't worry, the batteries fit tight enough to move the silicone out of the way to make an electrical contact.
Don't forget this bit!
I get asked a lot about preventing corrosion on car battery terminals, especially persistent corrosion where it's back a few weeks down the line even after a thorough cleaning.
For car battery terminals I use an all-in-one terminal cleaner/protector/lube. This one I'm using even changes color from green to red in the presence of acid, which is a handy indicator that something might be wrong with the battery.
Here are my top tips for preventing batteries from leaking: