Why is this battery so hot, and what do I do with it?

It sounds like the battery has shorted all it’s energy is being transformed to heat. As to what to do with it I would dispose of it forthwith according to local laws.

Take it outside and put it on a piece of concrete. Now! Don’t wrap it up as that will retain the heat. Lave it out in the open. It will cool down eventually. Then discard.

@Lightlyseared Thank you. According to local law, it can be tossed in the bin.
@LuckyGuy Thank you. The instructions were followed, and it is cool to the touch now.

New questions: What might have happened if it hadn’t been discovered? Is this what causes corroded batteries? Could it have exploded?

You did not mention the battery technology: Lead Acid, Alkaline, Mercury free alkaline, NiCad, NiMH, Li-Ion, LiPO
Sometimes a battery gets old and piece of corrosion falls down between the plates. That makes an internal short, which makes a hot spot. The hot spot burns a hole which frees up more material that makes more shorts, that make more material….You get the idea. It runs away with itself.
Depending upon the battery technology, you can have a chemical leak, a swelling large enough to break the device, or even a fire.
Usually people see the after-effects but not the actual process. You are very lucky to have witnessed this. Congratulations.

@LuckyGuy is right, leave it out somewhere to cool off then dispose of it according to local laws. What you don’t want to do is throw it in the garbage or wrap it in a flammable material. For some reason the battery had an internal short and is dissipating power as heat. I would also watch the brand of battery that I buy. Some of the ultra cheap Chinese or Indian batteries have low standards and may be dangerous to operate or store.

The battery is an EVEREADY GOLD™ alkaline. It was purchased about two years ago. There was no sign of leakage or swelling. It looked exactly like its counterpart. The only difference is one was cool to the touch and the other was hot. The flashlight only is used when the power goes out due to thunderstorms and tornadoes.

@LuckyGuy Now that the situation is resolved without any damage, it is rather fascinating to have experienced this, albeit by pure luck. It just now makes me wonder how often I need to check the status of the batteries used around the house.

@Ron_C Since it is bin day here, it got tossed in the garbage can after it cooled down. An internet search didn’t provide any local sites that recycle or dispose of batteries other than for vehicles. I do my best to keep battery-supplied power sources to a minimum. I’d purchase a hand-cranked contraption if I were planning to stay here, but I’m in the process of moving.

I’m with @Ron_C . I do not buy the cheap chinese batteries that look like the good ones. Did you notice where it was made?
I see you just replied that is was an Eveready Gold Alkaline. Alkalines supposedly are designed to be disposable. .
Frankly I am surprised. I expected it to be a cheap knock-off.

[mod says] This is our Question of the Day!

Note that even the best brand-name batteries can get damaged. Was that flashlight ever dropped? That could do it right there, as could just age. Metal doesn’t last forever, and chemicals can do “fun” things to metal over time… and by “chemicals”, I include water, like the normal moisture is the air. Stored items require periodic inspection and battery replacement regardless of whether they’ve ever been used for that reason amongst others.

While your normal alkaline batteries rarely do more than get a bit warm or leak a bit of electrolyte, rechargables can do interesting things. I have melted NiCads, and it’s not uncommon for LiPo packs to just self-ignite.

@jerv The flashlight is a Coleman® Armor Light 2D. It has never been dropped. It’s probably about eight years old. It’s rarely used, but kept handy and checked every time there is a thunderstorm or tornado warning.

Yes, the battery could explode, under some circumstances.
Hot batteries are especially dangerous when they are rechargeable. This can cause thermal runaway, in which the battery tries to recharge itself well beyond its capacity.
On the battery you described, the best thing to do would be to put it back into the flashlight case, and turn on the flashlight. The casing would protect you from a sudden burst, and turning it on would drain the battery. You might damage the flashlight, but it is much better than handling it to put it somewhere else.

@filmfann The battery that got hot is now gone. Even if I still had it, I’d prefer not to tempt fate in having it explode.
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To those that talked about the dangers of rechargeable batteries, I appreciate that information as well. The SO and I have been migrating to all rechargeable ones. Are there any tips you can share about recharging them and how to store the back-ups? We have already discovered that there were one or two that stopped recharging, and I wonder if it is due to how they were maintained.

Here are my rules for recharables.
Heat is the number one killer of batteries. When charging make sure to put them in a cool place with plenty of airflow. Put the charger on a smooth surface – not a carpet.
All rechargeables lose some of their charge every day. Do not expect to charge and forget about them for years like you do now with alkalines.
All rechageables have a limited number of charges. Depending upon the technology it might be 200, 300, or 500 charges. Make sure you get the most out of each charge cycle by using them properly and letting them discharge to about 20% before fully recharging.
Read the manual to see if you have a smart or dumb charger and if it is ok to leave the batteries in the charger all the time.
Keep some alkalines handy.

@LuckyGuy…unless they are NiCads. Those things have a hellacious memory effect, so I tend to run them down even more, often with a discharger. I had one NiCad pack last for about 15 years of heavy use that way, and I know of people with the Toyota Rav4 EV that got well over a decade out of their NiMH pack and still had over 60% capacity after >150k miles.

Sadly, LiPo packs degrade with exposure to oxygen, so no matter how you treat them, they don’t last more than a small handful of years.